On Tue, Feb 06, 2024 at 12:23:57PM -0700, Tycho Andersen wrote:
> From: Tycho Andersen <tandersen@xxxxxxxxxxx>
>
> We can get EBADF from __pidfd_fget() if a task is currently exiting, which
> might be confusing. Let's check PF_EXITING, and just report ESRCH if so.
>
> I chose PF_EXITING, because it is set in exit_signals(), which is called
> before exit_files(). Since ->exit_status is mostly set after exit_files()
> in exit_notify(), using that still leaves a window open for the race.
>
> Signed-off-by: Tycho Andersen <tandersen@xxxxxxxxxxx>
> v2: fix a race in the check by putting the check after __pidfd_fget()
> (thanks Oleg)
> ---
> kernel/pid.c | 17 +++++++++-
> .../selftests/pidfd/pidfd_getfd_test.c | 31 ++++++++++++++++++-
> 2 files changed, 46 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index de0bf2f8d18b..a8cd6296ed6d 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -693,8 +693,23 @@ static int pidfd_getfd(struct pid *pid, int fd)
>
> file = __pidfd_fget(task, fd);
> put_task_struct(task);
> - if (IS_ERR(file))
> + if (IS_ERR(file)) {
> + /*
> + * It is possible that the target thread is exiting; it can be
> + * either:
> + * 1. before exit_signals(), which gives a real fd
> + * 2. before exit_files() takes the task_lock() gives a real fd
> + * 3. after exit_files() releases task_lock(), ->files is NULL;
> + * this has PF_EXITING, since it was set in exit_signals(),
> + * __pidfd_fget() returns EBADF.
> + * In case 3 we get EBADF, but that really means ESRCH, since
> + * the task is currently exiting and has freed its files
> + * struct, so we fix it up.
> + */
> + if (task->flags & PF_EXITING && PTR_ERR(file) == -EBADF)
> + return -ESRCH;
Isn't that a potential UAF because we called put_task_struct() above but
this is exiting task->flags afterwards?
