On Tue, Mar 10, 2020 at 06:45:32PM +0100, Bernd Edlinger wrote:
> This changes lock_trace to use the new exec_update_mutex
> instead of cred_guard_mutex.
>
> This fixes possible deadlocks when the trace is accessing
> /proc/$pid/stack for instance.
>
> This should be safe, as the credentials are only used for reading,
> and task->mm is updated on execve under the new exec_update_mutex.
>
> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
I have the same question here as in 3/4. I should probably rescind my
Reviewed-by until I'm convinced about the security-safety of this -- why
is this not a race against cred changes?
-Kees
> ---
> fs/proc/base.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ebea950..4fdfe4f 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -403,11 +403,11 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>
> static int lock_trace(struct task_struct *task)
> {
> - int err = mutex_lock_killable(&task->signal->cred_guard_mutex);
> + int err = mutex_lock_killable(&task->signal->exec_update_mutex);
> if (err)
> return err;
> if (!ptrace_may_access(task, PTRACE_MODE_ATTACH_FSCREDS)) {
> - mutex_unlock(&task->signal->cred_guard_mutex);
> + mutex_unlock(&task->signal->exec_update_mutex);
> return -EPERM;
> }
> return 0;
> @@ -415,7 +415,7 @@ static int lock_trace(struct task_struct *task)
>
> static void unlock_trace(struct task_struct *task)
> {
> - mutex_unlock(&task->signal->cred_guard_mutex);
> + mutex_unlock(&task->signal->exec_update_mutex);
> }
>
> #ifdef CONFIG_STACKTRACE
> --
> 1.9.1
--
Kees Cook
