[PATCH] exec: NULL out bprm->argv0 when it is an ERR_PTR

[Kees Cook]

Attempting to free an ERR_PTR will not work. ;)

    process 'syz-executor210' launched '/dev/fd/3' with NULL argv: empty string added
    kernel BUG at arch/x86/mm/physaddr.c:23!

Set bprm->argv0 to NULL if it fails to get a string from userspace so
that bprm_free() will not try to free an invalid pointer when cleaning up.

Reported-by: syzbot+03e1af5c332f7e0eb84b@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/6729d8d1.050a0220.701a.0017.GAE@xxxxxxxxxx
Fixes: 7bdc6fc85c9a ("exec: fix up /proc/pid/comm in the execveat(AT_EMPTY_PATH) case")
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
---
 fs/exec.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 79045c1d1608..65448ea609a2 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1522,8 +1522,12 @@ static int bprm_add_fixup_comm(struct linux_binprm *bprm,
 		return 0;
 
 	bprm->argv0 = strndup_user(p, MAX_ARG_STRLEN);
-	if (IS_ERR(bprm->argv0))
-		return PTR_ERR(bprm->argv0);
+	if (IS_ERR(bprm->argv0)) {
+		int rc = PTR_ERR(bprm->argv0);
+
+		bprm->argv0 = NULL;
+		return rc;
+	}
 
 	return 0;
 }
-- 
2.34.1