Re: [Linux-kernel-mentees] [PATCH v2] fs: gfs2: add validation checks for size of superblock

In gfs2_check_sb(), no validation checks are performed with regards to
the size of the superblock.
syzkaller detected a slab-out-of-bounds bug that was primarily caused
because the block size for a superblock was set to zero.
A valid size for a superblock is a power of 2 between 512 and PAGE_SIZE.
Performing validation checks and ensuring that the size of the superblock
is valid fixes this bug.

Reported-by: syzbot+af90d47a37376844e731@syzkaller.appspotmail.com
Tested-by: syzbot+af90d47a37376844e731@syzkaller.appspotmail.com
Suggested-by: Andrew Price <redacted>
Signed-off-by: Anant Thazhemadam <redacted>
---

Changes in v2:

        * Completely dropped the changes proposed in v1. Instead,
          validity checks for superblock size have been introduced.
          (Suggested by Andrew Price[off-list ref])

        * Addded a "Suggested-by" tag accrediting the patch idea to
          Andrew. If there's any issue with that, please let me know.

        * Changed the commit header and commit message appropriately.

        * Updated "Reported-by" and "Tested-by" tags to the same instance
          of the bug that was detected earlier (non consequential change).


 fs/gfs2/ops_fstype.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
index 6d18d2c91add..f0605fae2c4c 100644
--- a/fs/gfs2/ops_fstype.c
+++ b/fs/gfs2/ops_fstype.c

@@ -169,6 +169,13 @@ static int gfs2_check_sb(struct gfs2_sbd *sdp, int silent)
                return -EINVAL;
        }

+       /* Check if the size of the block is valid - a power of 2 between 512 and  PAGE_SIZE */
+       if (sb->sb_bsize < 512 || sb->sb_bsize > PAGE_SIZE || (sb->sb_bsize & (sb->sb_bsize - 1))) {
+               if (!silent)
+                       pr_warn("Invalid superblock size\n");
+               return -EINVAL;
+       }
+