[PATCH v2 6/6] qnx6: validate sb_blocksize before dividing in qnx6_mmi_fill_super()

[Hui Peng <benquike@gmail.com>]

In qnx6_mmi_fill_super(), QNX6_SUPERBLOCK_AREA /
fs32_to_cpu(sbi, sb1->sb_blocksize) is evaluated before sb_set_blocksize()
validates that sb1->sb_blocksize is a non-zero power of two, risking a
divide-by-zero when sb1->sb_blocksize is 0 on disk. Move the offset
calculation after sb_set_blocksize() (matching qnx6_fill_super() in
fs/qnx6/inode.c).

Tested in QEMU against Linux 7.3.0-rc3 by mounting a crafted QNX6 mmi_fs
image with sb_blocksize = 0 on /dev/loop0 and verifying that
sb_set_blocksize() logs "qnx6: unable to set blocksize" and rejects the
mount with -EINVAL before any division takes place.

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_mmi_fill_super() sb_blocksize division ordering fix into
  its own patch (6/6) and remove Markdown formatting from the commit
  message, as requested by Damien Le Moal.

 fs/qnx6/super_mmi.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/qnx6/super_mmi.c b/fs/qnx6/super_mmi.c
index 18298e273a13..28cb9322278e 100644
--- a/fs/qnx6/super_mmi.c
+++ b/fs/qnx6/super_mmi.c
@@ -63,15 +63,15 @@ struct qnx6_super_block *qnx6_mmi_fill_super(struct super_block *s, int silent)
 		goto out;
 	}
 
-	/* calculate second superblock blocknumber */
-	offset = fs32_to_cpu(sbi, sb1->sb_num_blocks) + QNX6_SUPERBLOCK_AREA /
-					fs32_to_cpu(sbi, sb1->sb_blocksize);
-
 	/* set new blocksize */
 	if (!sb_set_blocksize(s, fs32_to_cpu(sbi, sb1->sb_blocksize))) {
 		pr_err("unable to set blocksize\n");
 		goto out;
 	}
+
+	/* calculate second superblock blocknumber */
+	offset = fs32_to_cpu(sbi, sb1->sb_num_blocks) + QNX6_SUPERBLOCK_AREA /
+					fs32_to_cpu(sbi, sb1->sb_blocksize);
 	/* blocksize invalidates bh - pull it back in */
 	brelse(bh1);
 	bh1 = sb_bread(s, 0);
-- 
2.49.0