futex/pi: Plug private futex exec() race

The check for private futexes whether the waiter's mm, which is stored in
the futex_key and copied into the pi_state, is the same as the owner's mm
is not sufficient for exec(). exec() has a gap where the mm check fails to
give the correct answer:

  exec()
  ...
    exec_release_mm()
      futex_exec_release()
        tsk::futex::exit_state = EXITING;
        cleanup_robust_list();
1)      tsk::futex::exit_state = OK;
    ...
    old_mm = tsk::mm;
2)  tsk::mm = ->mm;

Between #1 and #2 the check for the mm is wrong as that mm is about to be
swapped out and eventually freed.

Plug this gap by:

  1) Setting tsk::futex::exit_state to FUTEX_STATE_DEAD in
     futex_exec_release()

  2) Setting tsk::futex::exit_state to FUTEX_STATE_OK after
     the mm has been switched.

From a futex point of view the task is dead after it finished the robust
list cleanup up to the point where it sets the state to OK again.

Fixes: 80367ad01d93 ("futex: Add basic infrastructure for local task local hash")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Kyle Zeng <kylebot@openai.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org

1 files changed, 5 insertions, 2 deletions

diff --git a/fs/exec.c b/fs/exec.c
index c7b8f2d6366c..d01523d0d8b4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -30,6 +30,7 @@
 #include <linux/mm.h>
 #include <linux/stat.h>
 #include <linux/fcntl.h>
+#include <linux/futex.h>
 #include <linux/swap.h>
 #include <linux/string.h>
 #include <linux/init.h>
@@ -854,6 +855,7 @@ static int exec_mmap(struct linux_binprm *bprm)
 	/* Notify parent that we're no longer interested in the old VM */
 	tsk = current;
 	old_mm = current->mm;
+	/* Clean up futexes and release the mm */
 	exec_mm_release(tsk, old_mm);
 
 	ret = down_write_killable(&tsk->signal->exec_update_lock);
@@ -902,9 +904,10 @@ static int exec_mmap(struct linux_binprm *bprm)
 		BUG_ON(active_mm != old_mm);
 		/* Defer teardown to setup_new_exec(), outside the exec locks. */
 		bprm->old_mm = old_mm;
-		return 0;
+	} else {
+		mmdrop_lazy_tlb(active_mm);
 	}
-	mmdrop_lazy_tlb(active_mm);
+	futex_exec_done(tsk);
 	return 0;
 }