[PATCH bpf-next v4 2/7] exec: Clear bprm->mm before dropping its reference

[Anastasios Papagiannis <tasos.papagiannnis@gmail.com>]

Once mmput() drops the final reference to bprm->mm, the pointer must no
longer remain accessible through struct linux_binprm.

The successful exec path and the bprm initialization error path already
clear bprm->mm when ownership is transferred or released. Do the same in
free_bprm() before calling mmput().

This is required for BPF kfuncs where bprm->mm is either NULL or points
to a live mm_struct to ensure safe access.

Signed-off-by: Anastasios Papagiannis <tasos.papagiannnis@gmail.com>
Reviewed-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 fs/exec.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index a14f28b15607..4b4d8f0627a7 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1453,9 +1453,12 @@ void bprm_drop_loader(struct linux_binprm *bprm)
 
 static void free_bprm(struct linux_binprm *bprm)
 {
-	if (bprm->mm) {
+	struct mm_struct *mm = bprm->mm;
+
+	if (mm) {
 		acct_arg_size(bprm, 0);
-		mmput(bprm->mm);
+		bprm->mm = NULL;
+		mmput(mm);
 	}
 	if (bprm->user_ns)
 		put_user_ns(bprm->user_ns);
-- 
2.55.0