Re: [REVIEW][PATCH 1/3] ptrace: Capture the ptracer's creds not PT_PTRACE_CAP

[Eric W. Biederman]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:

> On Thu, Nov 17, 2016 at 9:05 AM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
>>
>> When the flag PT_PTRACE_CAP was added the PTRACE_TRACEME path was
>> overlooked.  This can result in incorrect behavior when an application
>> like strace traces an exec of a setuid executable.
>>
>> Further PT_PTRACE_CAP does not have enough information for making good
>> security decisions as it does not report which user namespace the
>> capability is in.  This has already allowed one mistake through
>> insufficient granulariy.
>>
>> I found this issue when I was testing another corner case of exec and
>> discovered that I could not get strace to set PT_PTRACE_CAP even when
>> running strace as root with a full set of caps.
>>
>> This change fixes the above issue with strace allowing stracing as
>> root a setuid executable without disabling setuid.  More fundamentaly
>> this change allows what is allowable at all times, by using the correct
>> information in it's decision.
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Fixes: 4214e42f96d4 ("v2.4.9.11 -> v2.4.9.12")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>> ---
>>  fs/exec.c                  |  2 +-
>>  include/linux/capability.h |  1 +
>>  include/linux/ptrace.h     |  1 -
>>  include/linux/sched.h      |  1 +
>>  kernel/capability.c        | 20 ++++++++++++++++++++
>>  kernel/ptrace.c            | 12 +++++++-----
>>  6 files changed, 30 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 6fcfb3f7b137..fdec760bfac3 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1401,7 +1401,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
>>         unsigned n_fs;
>>
>>         if (p->ptrace) {
>> -               if (p->ptrace & PT_PTRACE_CAP)
>> +               if (ptracer_capable(p, current_user_ns()))
>
> IIRC PT_PTRACE_CAP was added to prevent TOCTOU races.  What prevents
> that type of race now?  For that matter, what guarantees that we've
> already switched to new creds here and will continue to do so in the
> future?

Because instead of capturing a single bit we now capture tracers
entire credentials in tsk->ptracer_cred.   As such tsk->ptracer_cred
never changes except when ptracing begins or ends, and we remain
safe for TOCTOU races.

We do hold cred_guard_mutex here so that guarantees we get a new
ptracer.  So the worst that can happen here is our tracer detaches
and ptracer_capable will uncondintionally return true.

Eric