[PATCH v2 3/6] qnx6: avoid double brelse() on error path in qnx6_fill_super()

[Hui Peng <benquike@gmail.com>]

In qnx6_fill_super(), when selecting the active superblock, the inactive
superblock buffer_head (bh2 when superblock #1 is active, or bh1 when
superblock #2 is active) is released immediately via brelse() without
clearing the local pointer to NULL. If a subsequent mount step fails and
jumps to out, out1, out2, or out3, qnx6_fill_super() unconditionally calls
brelse(bh1) and brelse(bh2), causing a second brelse() on the already
released buffer_head. Because the extra brelse() drops the per-CPU bh_lru
reference to 0, freeing the buffer_head while it remains in bh_lru, this
triggers a KASAN slab-use-after-free and VFS buffer refcount warning:

  qnx6: superblock #1 active
  qnx6: error reading root directory.
  BUG: KASAN: slab-use-after-free in invalidate_bh_lrus_cpu+0x9f/0x120
  Read of size 4 at addr ffff88800151e658 by task kworker/1:1/69
  ...
  VFS: brelse: Trying to free [already] free buffer
  WARNING: fs/buffer.c:1051 at invalidate_bh_lru+0x4c/0x160, CPU#0: init/1

Clear bh2/bh1 to NULL immediately after releasing the inactive buffer_head.

Tested in QEMU against Linux 7.3.0-rc3 by mounting a crafted QNX6 image on
/dev/loop0 that activates superblock #1 and then fails root directory
validation: on the unfixed kernel, mount failure triggers the double-brelse
VFS warning, WARNING at fs/buffer.c:1051, and KASAN slab-use-after-free in
invalidate_bh_lrus_cpu(), whereas on the fixed kernel mount fails cleanly
with 0 warnings or KASAN faults.

Fixes: 5d026c724220 ("fs: initial qnx6fs addition")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split the qnx6_fill_super() double-brelse fix into its own patch (3/6)
  separate from the mmi_fs sb_buf leak fix (4/6), and remove Markdown
  formatting from the commit message, as requested by Damien Le Moal.

 fs/qnx6/inode.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/qnx6/inode.c b/fs/qnx6/inode.c
index 9b624829520e..c4a35d81e67c 100644
--- a/fs/qnx6/inode.c
+++ b/fs/qnx6/inode.c
@@ -399,12 +399,14 @@ static int qnx6_fill_super(struct super_block *s, struct fs_context *fc)
 		sbi->sb_buf = bh1;
 		sbi->sb = (struct qnx6_super_block *)bh1->b_data;
 		brelse(bh2);
+		bh2 = NULL;
 		pr_info("superblock #1 active\n");
 	} else {
 		/* superblock #2 active */
 		sbi->sb_buf = bh2;
 		sbi->sb = (struct qnx6_super_block *)bh2->b_data;
 		brelse(bh1);
+		bh1 = NULL;
 		pr_info("superblock #2 active\n");
 	}
 mmi_success:
-- 
2.49.0