Re: [PATCH v2] pidfd: getfd should always report ESRCH if a task is exiting

On Wed, Feb 07, 2024 at 10:15:25AM +0100, Christian Brauner wrote:

On Tue, Feb 06, 2024 at 12:23:57PM -0700, Tycho Andersen wrote:

quoted

From: Tycho Andersen <redacted>

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 <redacted>
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?

s/exiting/accessing/