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

[Ricky Zhou]

(apologies for the dup, forgot to reply all)

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).

>From b2e0d98705e60e45bbb3c0032c48824ad7ae0704 onward,
unshare(CLONE_NEWUSER) is affected.

Thanks,
Ricky

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?
>
>> ---
>>  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
--
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