xfs: fix maxicount division by zero error

From: Darrick J. Wong <[email protected]>

In xfs_ialloc_setup_geometry, it's possible for a malicious/corrupt fs image to set an unreasonably large value for sb_inopblog which will cause ialloc_blks to be zero. If sb_imax_pct is also set, this results in a division by zero error in the second do_div call. Therefore, force maxicount to zero if ialloc_blks is zero.

Note that the kernel metadata verifiers will catch the garbage inopblog value and abort the fs mount long before it tries to set up the inode geometry; this is needed to avoid a crash in xfs_db while setting up the xfs_mount structure.

Found by fuzzing sb_inopblog to 122 in xfs/350.

---
 fs/xfs/libxfs/xfs_ialloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 1a14067aa4d4..5e95648c346c 100644
@@ -2811,7 +2811,7 @@ xfs_ialloc_setup_geometry(
 			inodes);
 
 	/* Set the maximum inode count for this filesystem. */
-	if (sbp->sb_imax_pct) {
+	if (sbp->sb_imax_pct && igeo->ialloc_blks) {
 		/*
 		 * Make sure the maximum inode count is a multiple
 		 * of the units we allocate inodes in.

Hi.

The patch looks good, but maybe a comment is worth so we don't need to check the git log to understand why we need to check ialloc_blks here?

	/*
	 * Set the maximum inode count for this filesystem, being careful
	 * ialloc_blks is not zeroed due a corrupted sb_agblklog
	 */
	if (sbp->sb_imax_pct) {
	if (sbp->sb_imax_pct && igeo->ialloc_blks) {

Carlos