Re: Review of ptrace Yama ptrace_scope description
Hi Kees,
On 06/28/2016 10:55 PM, Kees Cook wrote:
On Mon, Jun 27, 2016 at 11:11 PM, Michael Kerrisk (man-pages)
wrote:
Hi Jann,
On 06/25/2016 04:30 PM, Jann Horn wrote:
On Sat, Jun 25, 2016 at 09:30:43AM +0200, Michael Kerrisk (man-pages)
wrote:
Hi Kees,
So, last year, I added some documentation to ptrace(2) to describe
the Yama ptrace_scope file. I don't think I asked you for review
at the time, but in the light of other changes to the ptrace(2)
page, it occurred to me that it might be a good idea to ask you
to check the text below to see if anything is missing or could be
improved. Might you have a moment for that?
/proc/sys/kernel/yama/ptrace_scope
On systems with the Yama Linux Security Module (LSM) installed
(i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
the /proc/sys/kernel/yama/ptrace_scope file (available since
Linux 3.4) can be used to restrict the ability to trace a
process with ptrace(2) (and thus also the ability to use tools
such as strace(1) and gdb(1)). The goal of such restrictions
is to prevent attack escalation whereby a compromised process
can ptrace-attach to other sensitive processes (e.g., a GPG
agent or an SSH session) owned by the user in order to gain
additional credentials and thus expand the scope of the attack.
Maybe clarify "additional credentials that may exist in memory only and thus..."
Done.
More precisely, the Yama LSM limits two types of operations:
* Any operation that performs a ptrace access mode
PTRACE_MODE_ATTACH check—for example, ptrace()
PTRACE_ATTACH. (See the "Ptrace access mode checking" dis‐
cussion above.)
* ptrace() PTRACE_TRACEME.
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the follow‐
ing values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the
commoncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
When performing an operation that requires a
PTRACE_MODE_ATTACH check, the calling process must have
a predefined relationship with the target process. By
default, the predefined relationship is that the target
process must be a child of the caller.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target.
See the kernel source file Documentation/secu‐
rity/Yama.txt for further details.
The use of PTRACE_TRACEME is unchanged.
(namespaced) CAP_SYS_PTRACE is also sufficient here.
Both here and in the "admin-only attach" case, it is IMO important to
note that creating a user namespace effectively removes the Yama
protection because the owner of a namespace, when accessing its
contents from outside, is relatively capable.
This means that when a process tries to use namespaces to sandbox
itself, it inadvertently makes itself more accessible.
(This could probably be worked around in the kernel, but such a
workaround would likely not be default, but rather opt-in via a new
flag for clone() and unshare() or so.)
Tanks for catching this!
So I've made that section of text:
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the following
values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the com‐
moncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
Whenperforminganoperation that requires a
PTRACE_MODE_ATTACH check, the calling process must either
have the CAP_SYS_PTRACE capability in the user namespace of
the target process or it have a predefined relationship
with the target process. By default, the predefined rela‐
tionship is that the target process must be a child of the
caller.
More accurately, must be a descendant of the caller (grand child is fine, etc).
Thanks, Fixed.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target. See
Re: Review of ptrace Yama ptrace_scope description
Hi Jann,
...
So I've made that section of text:
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the following
values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the com‐
moncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
Whenperforminganoperation that requires a
PTRACE_MODE_ATTACH check, the calling process must either
have the CAP_SYS_PTRACE capability in the user namespace of
the target process or it have a predefined relationship
with the target process.
Nit: The grammar in this sentence seems wrong to me.
s/or it have/or it must have/?
Yep, thanks for catching that. Fixed now.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
Re: Review of ptrace Yama ptrace_scope description
On Mon, Jun 27, 2016 at 11:11 PM, Michael Kerrisk (man-pages)
wrote:
> Hi Jann,
>
>
> On 06/25/2016 04:30 PM, Jann Horn wrote:
>>
>> On Sat, Jun 25, 2016 at 09:30:43AM +0200, Michael Kerrisk (man-pages)
>> wrote:
>>>
>>> Hi Kees,
>>>
>>> So, last year, I added some documentation to ptrace(2) to describe
>>> the Yama ptrace_scope file. I don't think I asked you for review
>>> at the time, but in the light of other changes to the ptrace(2)
>>> page, it occurred to me that it might be a good idea to ask you
>>> to check the text below to see if anything is missing or could be
>>> improved. Might you have a moment for that?
>>>
>>>/proc/sys/kernel/yama/ptrace_scope
>>>On systems with the Yama Linux Security Module (LSM) installed
>>>(i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
>>>the /proc/sys/kernel/yama/ptrace_scope file (available since
>>>Linux 3.4) can be used to restrict the ability to trace a
>>>process with ptrace(2) (and thus also the ability to use tools
>>>such as strace(1) and gdb(1)). The goal of such restrictions
>>>is to prevent attack escalation whereby a compromised process
>>>can ptrace-attach to other sensitive processes (e.g., a GPG
>>>agent or an SSH session) owned by the user in order to gain
>>>additional credentials and thus expand the scope of the attack.
Maybe clarify "additional credentials that may exist in memory only and thus..."
>>>
>>>More precisely, the Yama LSM limits two types of operations:
>>>
>>>* Any operation that performs a ptrace access mode
>>> PTRACE_MODE_ATTACH check—for example, ptrace()
>>> PTRACE_ATTACH. (See the "Ptrace access mode checking" dis‐
>>> cussion above.)
>>>
>>>* ptrace() PTRACE_TRACEME.
>>>
>>>A process that has the CAP_SYS_PTRACE capability can update the
>>>/proc/sys/kernel/yama/ptrace_scope file with one of the follow‐
>>>ing values:
>>>
>>>0 ("classic ptrace permissions")
>>> No additional restrictions on operations that perform
>>> PTRACE_MODE_ATTACH checks (beyond those imposed by the
>>> commoncap and other LSMs).
>>>
>>> The use of PTRACE_TRACEME is unchanged.
>>>
>>>1 ("restricted ptrace") [default value]
>>> When performing an operation that requires a
>>> PTRACE_MODE_ATTACH check, the calling process must have
>>> a predefined relationship with the target process. By
>>> default, the predefined relationship is that the target
>>> process must be a child of the caller.
>>>
>>> A target process can employ the prctl(2) PR_SET_PTRACER
>>> operation to declare a different PID that is allowed to
>>> perform PTRACE_MODE_ATTACH operations on the target.
>>> See the kernel source file Documentation/secu‐
>>> rity/Yama.txt for further details.
>>>
>>> The use of PTRACE_TRACEME is unchanged.
>>
>>
>> (namespaced) CAP_SYS_PTRACE is also sufficient here.
>>
>>
>> Both here and in the "admin-only attach" case, it is IMO important to
>> note that creating a user namespace effectively removes the Yama
>> protection because the owner of a namespace, when accessing its
>> contents from outside, is relatively capable.
>>
>> This means that when a process tries to use namespaces to sandbox
>> itself, it inadvertently makes itself more accessible.
>>
>> (This could probably be worked around in the kernel, but such a
>> workaround would likely not be default, but rather opt-in via a new
>> flag for clone() and unshare() or so.)
>
>
> Tanks for catching this!
>
> So I've made that section of text:
>
>A process that has the CAP_SYS_PTRACE capability can update the
>/proc/sys/kernel/yama/ptrace_scope file with one of the following
>values:
>
>0 ("classic ptrace permissions")
> No additional restrictions on operations that perform
> PTRACE_MODE_ATTACH checks (beyond those imposed by the com‐
> moncap and other LSMs).
>
> The use of PTRACE_TRACEME is unchanged.
>
>1 ("restricted ptrace") [default value]
> Whenperforminganoperation that requires a
> PTRACE_MODE_ATTACH check, the calling process must either
> have the CAP_SYS_PTRACE capability in the user namespace of
> the target process or it have a predefined relationship
> with the target process. By default, the predefined rela‐
> tionship is that the target process must be a child of the
> caller.
More accurately, must be a descendant of the caller (grand child is fine, etc).
>
>
Re: Review of ptrace Yama ptrace_scope description
On Tue, Jun 28, 2016 at 08:11:36AM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Jann,
>
> On 06/25/2016 04:30 PM, Jann Horn wrote:
> >On Sat, Jun 25, 2016 at 09:30:43AM +0200, Michael Kerrisk (man-pages) wrote:
> >>Hi Kees,
> >>
> >>So, last year, I added some documentation to ptrace(2) to describe
> >>the Yama ptrace_scope file. I don't think I asked you for review
> >>at the time, but in the light of other changes to the ptrace(2)
> >>page, it occurred to me that it might be a good idea to ask you
> >>to check the text below to see if anything is missing or could be
> >>improved. Might you have a moment for that?
> >>
> >> /proc/sys/kernel/yama/ptrace_scope
> >> On systems with the Yama Linux Security Module (LSM) installed
> >> (i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
> >> the /proc/sys/kernel/yama/ptrace_scope file (available since
> >> Linux 3.4) can be used to restrict the ability to trace a
> >> process with ptrace(2) (and thus also the ability to use tools
> >> such as strace(1) and gdb(1)). The goal of such restrictions
> >> is to prevent attack escalation whereby a compromised process
> >> can ptrace-attach to other sensitive processes (e.g., a GPG
> >> agent or an SSH session) owned by the user in order to gain
> >> additional credentials and thus expand the scope of the attack.
> >>
> >> More precisely, the Yama LSM limits two types of operations:
> >>
> >> * Any operation that performs a ptrace access mode
> >> PTRACE_MODE_ATTACH check—for example, ptrace()
> >> PTRACE_ATTACH. (See the "Ptrace access mode checking" dis‐
> >> cussion above.)
> >>
> >> * ptrace() PTRACE_TRACEME.
> >>
> >> A process that has the CAP_SYS_PTRACE capability can update the
> >> /proc/sys/kernel/yama/ptrace_scope file with one of the follow‐
> >> ing values:
> >>
> >> 0 ("classic ptrace permissions")
> >> No additional restrictions on operations that perform
> >> PTRACE_MODE_ATTACH checks (beyond those imposed by the
> >> commoncap and other LSMs).
> >>
> >> The use of PTRACE_TRACEME is unchanged.
> >>
> >> 1 ("restricted ptrace") [default value]
> >> When performing an operation that requires a
> >> PTRACE_MODE_ATTACH check, the calling process must have
> >> a predefined relationship with the target process. By
> >> default, the predefined relationship is that the target
> >> process must be a child of the caller.
> >>
> >> A target process can employ the prctl(2) PR_SET_PTRACER
> >> operation to declare a different PID that is allowed to
> >> perform PTRACE_MODE_ATTACH operations on the target.
> >> See the kernel source file Documentation/secu‐
> >> rity/Yama.txt for further details.
> >>
> >> The use of PTRACE_TRACEME is unchanged.
> >
> >(namespaced) CAP_SYS_PTRACE is also sufficient here.
> >
> >
> >Both here and in the "admin-only attach" case, it is IMO important to
> >note that creating a user namespace effectively removes the Yama
> >protection because the owner of a namespace, when accessing its
> >contents from outside, is relatively capable.
> >
> >This means that when a process tries to use namespaces to sandbox
> >itself, it inadvertently makes itself more accessible.
> >
> >(This could probably be worked around in the kernel, but such a
> >workaround would likely not be default, but rather opt-in via a new
> >flag for clone() and unshare() or so.)
>
> Tanks for catching this!
>
> So I've made that section of text:
>
>A process that has the CAP_SYS_PTRACE capability can update the
>/proc/sys/kernel/yama/ptrace_scope file with one of the following
>values:
>
>0 ("classic ptrace permissions")
> No additional restrictions on operations that perform
> PTRACE_MODE_ATTACH checks (beyond those imposed by the com‐
> moncap and other LSMs).
>
> The use of PTRACE_TRACEME is unchanged.
>
>1 ("restricted ptrace") [default value]
> Whenperforminganoperation that requires a
> PTRACE_MODE_ATTACH check, the calling process must either
> have the CAP_SYS_PTRACE capability in the user namespace of
> the target process or it have a predefined relationship
> with the target process.
Nit: The grammar in this sentence seems wrong to me.
s/or it have/or it must have/?
> By default, the predefined rela‐
> tionship is that the target process must be a child of the
> caller.
>
> A target process can employ the prctl(2) P
Re: Review of ptrace Yama ptrace_scope description
Hi Jann,
On 06/25/2016 04:30 PM, Jann Horn wrote:
On Sat, Jun 25, 2016 at 09:30:43AM +0200, Michael Kerrisk (man-pages) wrote:
Hi Kees,
So, last year, I added some documentation to ptrace(2) to describe
the Yama ptrace_scope file. I don't think I asked you for review
at the time, but in the light of other changes to the ptrace(2)
page, it occurred to me that it might be a good idea to ask you
to check the text below to see if anything is missing or could be
improved. Might you have a moment for that?
/proc/sys/kernel/yama/ptrace_scope
On systems with the Yama Linux Security Module (LSM) installed
(i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
the /proc/sys/kernel/yama/ptrace_scope file (available since
Linux 3.4) can be used to restrict the ability to trace a
process with ptrace(2) (and thus also the ability to use tools
such as strace(1) and gdb(1)). The goal of such restrictions
is to prevent attack escalation whereby a compromised process
can ptrace-attach to other sensitive processes (e.g., a GPG
agent or an SSH session) owned by the user in order to gain
additional credentials and thus expand the scope of the attack.
More precisely, the Yama LSM limits two types of operations:
* Any operation that performs a ptrace access mode
PTRACE_MODE_ATTACH check—for example, ptrace()
PTRACE_ATTACH. (See the "Ptrace access mode checking" dis‐
cussion above.)
* ptrace() PTRACE_TRACEME.
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the follow‐
ing values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the
commoncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
When performing an operation that requires a
PTRACE_MODE_ATTACH check, the calling process must have
a predefined relationship with the target process. By
default, the predefined relationship is that the target
process must be a child of the caller.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target.
See the kernel source file Documentation/secu‐
rity/Yama.txt for further details.
The use of PTRACE_TRACEME is unchanged.
(namespaced) CAP_SYS_PTRACE is also sufficient here.
Both here and in the "admin-only attach" case, it is IMO important to
note that creating a user namespace effectively removes the Yama
protection because the owner of a namespace, when accessing its
contents from outside, is relatively capable.
This means that when a process tries to use namespaces to sandbox
itself, it inadvertently makes itself more accessible.
(This could probably be worked around in the kernel, but such a
workaround would likely not be default, but rather opt-in via a new
flag for clone() and unshare() or so.)
Tanks for catching this!
So I've made that section of text:
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the following
values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the com‐
moncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
Whenperforminganoperation that requires a
PTRACE_MODE_ATTACH check, the calling process must either
have the CAP_SYS_PTRACE capability in the user namespace of
the target process or it have a predefined relationship
with the target process. By default, the predefined rela‐
tionship is that the target process must be a child of the
caller.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target. See
the kernel source file Documentation/security/Yama.txt for
further details.
The use of PTRACE_TRACEME is unchanged.
2 ("admin-only attach")
Only processes with the CAP_SYS_PTRACE capability in the
user namespace of the target process may perfo
Re: Review of ptrace Yama ptrace_scope description
On Sat, Jun 25, 2016 at 09:30:43AM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Kees,
>
> So, last year, I added some documentation to ptrace(2) to describe
> the Yama ptrace_scope file. I don't think I asked you for review
> at the time, but in the light of other changes to the ptrace(2)
> page, it occurred to me that it might be a good idea to ask you
> to check the text below to see if anything is missing or could be
> improved. Might you have a moment for that?
>
>/proc/sys/kernel/yama/ptrace_scope
>On systems with the Yama Linux Security Module (LSM) installed
>(i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
>the /proc/sys/kernel/yama/ptrace_scope file (available since
>Linux 3.4) can be used to restrict the ability to trace a
>process with ptrace(2) (and thus also the ability to use tools
>such as strace(1) and gdb(1)). The goal of such restrictions
>is to prevent attack escalation whereby a compromised process
>can ptrace-attach to other sensitive processes (e.g., a GPG
>agent or an SSH session) owned by the user in order to gain
>additional credentials and thus expand the scope of the attack.
>
>More precisely, the Yama LSM limits two types of operations:
>
>* Any operation that performs a ptrace access mode
> PTRACE_MODE_ATTACH check—for example, ptrace()
> PTRACE_ATTACH. (See the "Ptrace access mode checking" dis‐
> cussion above.)
>
>* ptrace() PTRACE_TRACEME.
>
>A process that has the CAP_SYS_PTRACE capability can update the
>/proc/sys/kernel/yama/ptrace_scope file with one of the follow‐
>ing values:
>
>0 ("classic ptrace permissions")
> No additional restrictions on operations that perform
> PTRACE_MODE_ATTACH checks (beyond those imposed by the
> commoncap and other LSMs).
>
> The use of PTRACE_TRACEME is unchanged.
>
>1 ("restricted ptrace") [default value]
> When performing an operation that requires a
> PTRACE_MODE_ATTACH check, the calling process must have
> a predefined relationship with the target process. By
> default, the predefined relationship is that the target
> process must be a child of the caller.
>
> A target process can employ the prctl(2) PR_SET_PTRACER
> operation to declare a different PID that is allowed to
> perform PTRACE_MODE_ATTACH operations on the target.
> See the kernel source file Documentation/secu‐
> rity/Yama.txt for further details.
>
> The use of PTRACE_TRACEME is unchanged.
(namespaced) CAP_SYS_PTRACE is also sufficient here.
Both here and in the "admin-only attach" case, it is IMO important to
note that creating a user namespace effectively removes the Yama
protection because the owner of a namespace, when accessing its
contents from outside, is relatively capable.
This means that when a process tries to use namespaces to sandbox
itself, it inadvertently makes itself more accessible.
(This could probably be worked around in the kernel, but such a
workaround would likely not be default, but rather opt-in via a new
flag for clone() and unshare() or so.)
>2 ("admin-only attach")
> Only processes with the CAP_SYS_PTRACE capability may
> perform PTRACE_MODE_ATTACH operations or trace children
> that employ PTRACE_TRACEME.
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Description: Digital signature
Review of ptrace Yama ptrace_scope description
Hi Kees,
So, last year, I added some documentation to ptrace(2) to describe
the Yama ptrace_scope file. I don't think I asked you for review
at the time, but in the light of other changes to the ptrace(2)
page, it occurred to me that it might be a good idea to ask you
to check the text below to see if anything is missing or could be
improved. Might you have a moment for that?
/proc/sys/kernel/yama/ptrace_scope
On systems with the Yama Linux Security Module (LSM) installed
(i.e., the kernel was configured with CONFIG_SECURITY_YAMA),
the /proc/sys/kernel/yama/ptrace_scope file (available since
Linux 3.4) can be used to restrict the ability to trace a
process with ptrace(2) (and thus also the ability to use tools
such as strace(1) and gdb(1)). The goal of such restrictions
is to prevent attack escalation whereby a compromised process
can ptrace-attach to other sensitive processes (e.g., a GPG
agent or an SSH session) owned by the user in order to gain
additional credentials and thus expand the scope of the attack.
More precisely, the Yama LSM limits two types of operations:
* Any operation that performs a ptrace access mode
PTRACE_MODE_ATTACH check—for example, ptrace()
PTRACE_ATTACH. (See the "Ptrace access mode checking" dis‐
cussion above.)
* ptrace() PTRACE_TRACEME.
A process that has the CAP_SYS_PTRACE capability can update the
/proc/sys/kernel/yama/ptrace_scope file with one of the follow‐
ing values:
0 ("classic ptrace permissions")
No additional restrictions on operations that perform
PTRACE_MODE_ATTACH checks (beyond those imposed by the
commoncap and other LSMs).
The use of PTRACE_TRACEME is unchanged.
1 ("restricted ptrace") [default value]
When performing an operation that requires a
PTRACE_MODE_ATTACH check, the calling process must have
a predefined relationship with the target process. By
default, the predefined relationship is that the target
process must be a child of the caller.
A target process can employ the prctl(2) PR_SET_PTRACER
operation to declare a different PID that is allowed to
perform PTRACE_MODE_ATTACH operations on the target.
See the kernel source file Documentation/secu‐
rity/Yama.txt for further details.
The use of PTRACE_TRACEME is unchanged.
2 ("admin-only attach")
Only processes with the CAP_SYS_PTRACE capability may
perform PTRACE_MODE_ATTACH operations or trace children
that employ PTRACE_TRACEME.
3 ("no attach")
No process may perform PTRACE_MODE_ATTACH operations or
trace children that employ PTRACE_TRACEME.
Once this value has been written to the file, it cannot
be changed.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
