[PATCH] exec: do_close_on_exec() before taking exec_update_lock

[Jann Horn <jannh@google.com>]

do_close_on_exec() currently happens while holding the exec_update_lock,
which is used in a lot of places that access process state to
synchronize access checks.
I recently added another such use of exec_update_lock, causing a
regression.

do_close_on_exec() can block waiting for a reply from a filesystem.
That means a hung filesystem can block codepaths that use
exec_update_lock; and it also means that a FUSE filesystem which
attempts to inspect the calling process can deadlock.

To avoid such problems, move do_close_on_exec() before the
exec_update_lock is taken, but after the FD table has been copied if
necessary.

I have looked through all the calls between the old and new position of
the do_close_on_exec() call; there seems to be no file descriptor table
access in between.

Reported-by: Benjamin Peterson <benjamin@locrian.net>
Closes: https://lore.kernel.org/r/f5e8166a-88be-46c5-8939-1e5227ffe4c2@app.fastmail.com
Fixes: 6650527444da ("proc: protect ptrace_may_access() with exec_update_lock (part 1)")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
 fs/exec.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 745f6eb5279e..b51e5d7e4536 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1164,6 +1164,20 @@ int begin_new_exec(struct linux_binprm * bprm)
 	if (retval)
 		goto out;
 
+	/*
+	 * We have to apply CLOEXEC before we change whether the process is
+	 * dumpable (in setup_new_exec) to avoid a race with a process in userspace
+	 * trying to access the should-be-closed file descriptors of a process
+	 * undergoing exec(2).
+	 *
+	 * This can block on filesystem ->flush() handlers, including waiting
+	 * for FUSE daemons, so do it before exec_mmap takes the
+	 * exec_update_lock.
+	 * This must happen after the point of no return, and after unsharing
+	 * the FD table.
+	 */
+	do_close_on_exec(me->files);
+
 	/*
 	 * Must be called _before_ exec_mmap() as bprm->mm is
 	 * not visible until then. Doing it here also ensures
@@ -1214,14 +1228,6 @@ int begin_new_exec(struct linux_binprm * bprm)
 
 	clear_syscall_work_syscall_user_dispatch(me);
 
-	/*
-	 * We have to apply CLOEXEC before we change whether the process is
-	 * dumpable (in setup_new_exec) to avoid a race with a process in userspace
-	 * trying to access the should-be-closed file descriptors of a process
-	 * undergoing exec(2).
-	 */
-	do_close_on_exec(me->files);
-
 	if (bprm->secureexec) {
 		/* Make sure parent cannot signal privileged process. */
 		me->pdeath_signal = 0;

---
base-commit: 73ae59e975966d24e32926247ddb45a537ebe184
change-id: 20260907-cloexec-before-exec-update-lock-972ad108510c

Best regards,
--  
Jann Horn <jannh@google.com>