On Tue, Jul 28, 2015 at 1:55 PM, Ricky Zhou <[email protected]> wrote:
> On Tue, Jul 28, 2015 at 11:17 AM, Eric W. Biederman
> <[email protected]> wrote:
>> Kees Cook <[email protected]> writes:
>>
>>> From: Ricky Zhou <[email protected]>
>>>
>>> Checking mm_users > 1 does not mean a process is multithreaded. For
>>> example, reading /proc/PID/maps temporarily increments mm_users, allowing
>>> other processes to (accidentally) interfere with unshare() calls.
>>>
>>> This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
>>> returning EINVAL if another processes happened to be simultaneously
>>> reading the maps file.
>>>
>>> Signed-off-by: Ricky Zhou <[email protected]>
>>> Signed-off-by: Kees Cook <[email protected]>
>>> Cc: [email protected]
>>
>> This looks like a good fix. Any chance you can drudge up the commit where
>> this hack came in so that Greg & Company know how far to back port this?
>
> userns_install in user_namespace.c (affects setns of a user
> namespace): cde1975bc242f3e1072bde623ef378e547b73f91.
>
> The check in check_unshare_flags is a little more complex. The
> incorrect check was added in
> cf2e340f4249b781b3d2beb41e891d08581f0e10 but I don't think it would
> have triggered under any supported combination of flags at that point.
>
> From 50804fe3737ca6a5942fdc2057a18a8141d00141 until
> 6e556ce209b09528dbf1931cbfd5d323e1345926, the bug affected
> unshare(CLONE_NEWPID).
That's back to v3.8, so this goes quite a way, it seems.
-Kees
>
> From b2e0d98705e60e45bbb3c0032c48824ad7ae0704 onward,
> unshare(CLONE_NEWUSER) is affected.
>
> Thanks,
> Ricky
>>>
>>> ---
>>> kernel/fork.c | 4 ++--
>>> kernel/user_namespace.c | 3 ++-
>>> 2 files changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index dbd9b8d7b7cc..7d138f152dcd 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -75,6 +75,7 @@
>>> #include <linux/aio.h>
>>> #include <linux/compiler.h>
>>> #include <linux/sysctl.h>
>>> +#include <linux/sched.h>
>>>
>>> #include <asm/pgtable.h>
>>> #include <asm/pgalloc.h>
>>> @@ -1876,8 +1877,7 @@ static int check_unshare_flags(unsigned long
>>> unshare_flags)
>>> * needs to unshare vm.
>>> */
>>> if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
>>> - /* FIXME: get_task_mm() increments ->mm_users */
>>> - if (atomic_read(¤t->mm->mm_users) > 1)
>>> + if (!current_is_single_threaded())
>>> return -EINVAL;
>>> }
>>>
>>> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
>>> index 4109f8320684..7e0e021e4304 100644
>>> --- a/kernel/user_namespace.c
>>> +++ b/kernel/user_namespace.c
>>> @@ -22,6 +22,7 @@
>>> #include <linux/ctype.h>
>>> #include <linux/projid.h>
>>> #include <linux/fs_struct.h>
>>> +#include <linux/sched.h>
>>>
>>> static struct kmem_cache *user_ns_cachep __read_mostly;
>>> static DEFINE_MUTEX(userns_state_mutex);
>>> @@ -977,7 +978,7 @@ static int userns_install(struct nsproxy *nsproxy,
>>> struct ns_common *ns)
>>> return -EINVAL;
>>>
>>> /* Threaded processes may not enter a different user namespace */
>>> - if (atomic_read(¤t->mm->mm_users) > 1)
>>> + if (!current_is_single_threaded())
>>> return -EINVAL;
>>>
>>> if (current->fs->users != 1)
>>> --
>>> 1.9.1
--
Kees Cook
Chrome OS Security
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