Re: [libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
On 11/18/2013 08:45 PM, Eric Blake wrote:
> On 11/18/2013 06:28 AM, Pradipta Kr. Banerjee wrote:
>> Creating a qemu VM with /dev/hwrng as backed RNG device throws the
>> following error - "Could not open '/dev/hwrng': Operation not permitted"
>> This patch fixes the issue
>>
>> Signed-off-by: Pradipta Kr. Banerjee
>> ---
>> src/qemu/qemu_cgroup.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> This is probably not the right fix. This says that we are making the
> random device to all possible guests, and that a compromised guest can
> then open() the device and starve it of entropy to the detriment of
> other guests. I think the _real_ fix is to quit exposing /dev/random by
> default (/dev/urandom may be okay), and to instead teach the code for
> RNG devices to add an ACL change for either random device (/dev/random
> or /dev/hwrng) only for the guests that actually use RNG backed by a
> hardware device. That way, guests not using an RNG device cannot starve
> entropy from guests that depend on it.
Right..Thanks for the suggestion Eric.
>
>>
>> diff --git a/src/qemu/qemu_cgroup.c b/src/qemu/qemu_cgroup.c
>> index ace7e35..d9ebb30 100644
>> --- a/src/qemu/qemu_cgroup.c
>> +++ b/src/qemu/qemu_cgroup.c
>> @@ -39,7 +39,7 @@
>>
>> static const char *const defaultDeviceACL[] = {
>> "/dev/null", "/dev/full", "/dev/zero",
>> -"/dev/random", "/dev/urandom",
>> +"/dev/random", "/dev/urandom", "/dev/hwrng",
>> "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
>> "/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
>> NULL,
>>
>
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Re: [libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
On 11/18/2013 09:35 PM, Eric Blake wrote:
> On 11/18/2013 08:20 AM, Daniel P. Berrange wrote:
>
>>> static const char *const defaultDeviceACL[] = {
>>> "/dev/null", "/dev/full", "/dev/zero",
>>> -"/dev/random", "/dev/urandom",
>>> +"/dev/random", "/dev/urandom", "/dev/hwrng",
>>> "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
>>> "/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
>>> NULL,
>>
>> NACK, for any device listed in the XML, we should add it in the per-VM
>> cgroups setup code.
>>
>> The existing /dev/random & /dev/urandom devices are there because they
>> are used for basic crypto libraries unrelated to the XML config.
>
> /dev/urandom, probably. /dev/random - really? Isn't that a DoS
> potential for one guest to starve entropy from other guests, in spite of
> sVirt?
I think you are right ..
>
>> The
>> /dev/hwrng device does not fall into this scenario.
>
> Agreed.
>
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Re: [libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
On Mon, Nov 18, 2013 at 09:05:21AM -0700, Eric Blake wrote:
> On 11/18/2013 08:20 AM, Daniel P. Berrange wrote:
>
> >> static const char *const defaultDeviceACL[] = {
> >> "/dev/null", "/dev/full", "/dev/zero",
> >> -"/dev/random", "/dev/urandom",
> >> +"/dev/random", "/dev/urandom", "/dev/hwrng",
> >> "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
> >> "/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
> >> NULL,
> >
> > NACK, for any device listed in the XML, we should add it in the per-VM
> > cgroups setup code.
> >
> > The existing /dev/random & /dev/urandom devices are there because they
> > are used for basic crypto libraries unrelated to the XML config.
>
> /dev/urandom, probably. /dev/random - really? Isn't that a DoS
> potential for one guest to starve entropy from other guests, in spite of
> sVirt?
It is a tradeoff between providing the crypto libraries a weaker
entropy source vs DOS potential via entropy starvation. Given that
/dev/random is world writable on Linux in general, I think it is
acceptable to allow QEMU access by default. Paranoid admins can
always set the cgroups device ACL in /etc/libvirt/.qemu.conf if
they want to restrict it.
Daniel
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Re: [libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
On 11/18/2013 08:20 AM, Daniel P. Berrange wrote:
>> static const char *const defaultDeviceACL[] = {
>> "/dev/null", "/dev/full", "/dev/zero",
>> -"/dev/random", "/dev/urandom",
>> +"/dev/random", "/dev/urandom", "/dev/hwrng",
>> "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
>> "/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
>> NULL,
>
> NACK, for any device listed in the XML, we should add it in the per-VM
> cgroups setup code.
>
> The existing /dev/random & /dev/urandom devices are there because they
> are used for basic crypto libraries unrelated to the XML config.
/dev/urandom, probably. /dev/random - really? Isn't that a DoS
potential for one guest to starve entropy from other guests, in spite of
sVirt?
> The
> /dev/hwrng device does not fall into this scenario.
Agreed.
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Libvirt virtualization library http://libvirt.org
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Re: [libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
On Mon, Nov 18, 2013 at 06:58:06PM +0530, Pradipta Kr. Banerjee wrote:
> Creating a qemu VM with /dev/hwrng as backed RNG device throws the
> following error - "Could not open '/dev/hwrng': Operation not permitted"
> This patch fixes the issue
>
> Signed-off-by: Pradipta Kr. Banerjee
> ---
> src/qemu/qemu_cgroup.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/qemu/qemu_cgroup.c b/src/qemu/qemu_cgroup.c
> index ace7e35..d9ebb30 100644
> --- a/src/qemu/qemu_cgroup.c
> +++ b/src/qemu/qemu_cgroup.c
> @@ -39,7 +39,7 @@
>
> static const char *const defaultDeviceACL[] = {
> "/dev/null", "/dev/full", "/dev/zero",
> -"/dev/random", "/dev/urandom",
> +"/dev/random", "/dev/urandom", "/dev/hwrng",
> "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
> "/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
> NULL,
NACK, for any device listed in the XML, we should add it in the per-VM
cgroups setup code.
The existing /dev/random & /dev/urandom devices are there because they
are used for basic crypto libraries unrelated to the XML config. The
/dev/hwrng device does not fall into this scenario.
Daniel
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Re: [libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
On 11/18/2013 06:28 AM, Pradipta Kr. Banerjee wrote:
> Creating a qemu VM with /dev/hwrng as backed RNG device throws the
> following error - "Could not open '/dev/hwrng': Operation not permitted"
> This patch fixes the issue
>
> Signed-off-by: Pradipta Kr. Banerjee
> ---
> src/qemu/qemu_cgroup.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
This is probably not the right fix. This says that we are making the
random device to all possible guests, and that a compromised guest can
then open() the device and starve it of entropy to the detriment of
other guests. I think the _real_ fix is to quit exposing /dev/random by
default (/dev/urandom may be okay), and to instead teach the code for
RNG devices to add an ACL change for either random device (/dev/random
or /dev/hwrng) only for the guests that actually use RNG backed by a
hardware device. That way, guests not using an RNG device cannot starve
entropy from guests that depend on it.
>
> diff --git a/src/qemu/qemu_cgroup.c b/src/qemu/qemu_cgroup.c
> index ace7e35..d9ebb30 100644
> --- a/src/qemu/qemu_cgroup.c
> +++ b/src/qemu/qemu_cgroup.c
> @@ -39,7 +39,7 @@
>
> static const char *const defaultDeviceACL[] = {
> "/dev/null", "/dev/full", "/dev/zero",
> -"/dev/random", "/dev/urandom",
> +"/dev/random", "/dev/urandom", "/dev/hwrng",
> "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
> "/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
> NULL,
>
--
Eric Blake eblake redhat com+1-919-301-3266
Libvirt virtualization library http://libvirt.org
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[libvirt] [PATCH] Add hw random number generator (/dev/hwrng) to default qemu cgroup ACL
Creating a qemu VM with /dev/hwrng as backed RNG device throws the
following error - "Could not open '/dev/hwrng': Operation not permitted"
This patch fixes the issue
Signed-off-by: Pradipta Kr. Banerjee
---
src/qemu/qemu_cgroup.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/qemu/qemu_cgroup.c b/src/qemu/qemu_cgroup.c
index ace7e35..d9ebb30 100644
--- a/src/qemu/qemu_cgroup.c
+++ b/src/qemu/qemu_cgroup.c
@@ -39,7 +39,7 @@
static const char *const defaultDeviceACL[] = {
"/dev/null", "/dev/full", "/dev/zero",
-"/dev/random", "/dev/urandom",
+"/dev/random", "/dev/urandom", "/dev/hwrng",
"/dev/ptmx", "/dev/kvm", "/dev/kqemu",
"/dev/rtc", "/dev/hpet", "/dev/vfio/vfio",
NULL,
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