[PATCH 2/4] xfs: fix maxicount division by zero error

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

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.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/xfs/libxfs/xfs_ialloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)


diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
index 04377ab75863..aa190a502326 100644
--- a/fs/xfs/libxfs/xfs_ialloc.c
+++ b/fs/xfs/libxfs/xfs_ialloc.c
@@ -2788,7 +2788,7 @@ 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.