Re: [PATCH] user_ns: use correct check for single-threadedness

[Kees Cook]

On Tue, Jul 28, 2015 at 1:55 PM, Ricky Zhou <rickyz@xxxxxxxxxx> wrote:
> On Tue, Jul 28, 2015 at 11:17 AM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
>> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>>
>>> From: Ricky Zhou <rickyz@xxxxxxxxxxxx>
>>>
>>> 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 <rickyz@xxxxxxxxxxxx>
>>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>
>> 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(&current->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(&current->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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