On Wed, May 29, 2019 at 01:31:57PM +0200, Jann Horn wrote:
> Restore the read memory barrier in __ptrace_may_access() that was deleted
> a couple years ago. Also add comments on this barrier and the one it pairs
> with to explain why they're there (as far as I understand).
>
> Fixes: bfedb589252c ("mm: Add a user_ns owner to mm_struct and fix ptrace
> permission checks")
> Cc: sta...@vger.kernel.org
> Signed-off-by: Jann Horn <ja...@google.com>
Ah, thanks! Nice find.
Acked-by: Kees Cook <keesc...@chromium.org>
(And, yeah, Eric, I say snag it if you've got stuff queued up...)
-Kees
> ---
> (I have no clue whatsoever what the relevant tree for this is, but I
> guess Oleg is the relevant maintainer?)
>
> kernel/cred.c | 9 +++++++++
> kernel/ptrace.c | 10 ++++++++++
> 2 files changed, 19 insertions(+)
>
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 45d77284aed0..07e069d00696 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -450,6 +450,15 @@ int commit_creds(struct cred *new)
> if (task->mm)
> set_dumpable(task->mm, suid_dumpable);
> task->pdeath_signal = 0;
> + /*
> + * If a task drops privileges and becomes nondumpable,
> + * the dumpability change must become visible before
> + * the credential change; otherwise, a __ptrace_may_access()
> + * racing with this change may be able to attach to a task it
> + * shouldn't be able to attach to (as if the task had dropped
> + * privileges without becoming nondumpable).
> + * Pairs with a read barrier in __ptrace_may_access().
> + */
> smp_wmb();
> }
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 5710d07e67cf..e54452c2954b 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -324,6 +324,16 @@ static int __ptrace_may_access(struct task_struct *task,
> unsigned int mode)
> return -EPERM;
> ok:
> rcu_read_unlock();
> + /*
> + * If a task drops privileges and becomes nondumpable (through a syscall
> + * like setresuid()) while we are trying to access it, we must ensure
> + * that the dumpability is read after the credentials; otherwise,
> + * we may be able to attach to a task that we shouldn't be able to
> + * attach to (as if the task had dropped privileges without becoming
> + * nondumpable).
> + * Pairs with a write barrier in commit_creds().
> + */
> + smp_rmb();
> mm = task->mm;
> if (mm &&
> ((get_dumpable(mm) != SUID_DUMP_USER) &&
> --
> 2.22.0.rc1.257.g3120a18244-goog
>
--
Kees Cook
