From e780259b54e618ceb4763fbc21314acf3565e813 Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Mon, 7 Sep 2026 23:26:32 +0200 Subject: [PATCH] exec: do_close_on_exec() before taking exec_update_lock 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 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 Link: https://patch.msgid.link/20260907-cloexec-before-exec-update-lock-v1-1-8018c201a7df@google.com Tested-by: Benjamin Peterson Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) --- fs/exec.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index 263b1f67f1f8..f419a512de63 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1161,6 +1161,20 @@ 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 @@ -1211,14 +1225,6 @@ 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;