[PATCH 5.11 056/120] pstore: Fix warning in pstore_kill_sb()

[Greg Kroah-Hartman]

From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

commit 9c7d83ae6ba67d6c6199cce24573983db3b56332 upstream.

syzbot is hitting WARN_ON(pstore_sb != sb) at pstore_kill_sb() [1], for the
assumption that pstore_sb != NULL is wrong because pstore_fill_super() will
not assign pstore_sb = sb when new_inode() for d_make_root() returned NULL
(due to memory allocation fault injection).

Since mount_single() calls pstore_kill_sb() when pstore_fill_super()
failed, pstore_kill_sb() needs to be aware of such failure path.

[1] https://syzkaller.appspot.com/bug?id=6abacb8da5137cb47a416f2bef95719ed60508a0

Reported-by: syzbot <syzbot+d0cf0ad6513e9a1da5df@xxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210214031307.57903-1-penguin-kernel@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/pstore/inode.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/pstore/inode.c
+++ b/fs/pstore/inode.c
@@ -467,7 +467,7 @@ static struct dentry *pstore_mount(struc
 static void pstore_kill_sb(struct super_block *sb)
 {
 	mutex_lock(&pstore_sb_lock);
-	WARN_ON(pstore_sb != sb);
+	WARN_ON(pstore_sb && pstore_sb != sb);
 
 	kill_litter_super(sb);
 	pstore_sb = NULL;