gfs2: Validate i_depth for exhash directories

A fuzzer test introduced corruption that ends up with a depth of 0 in
dir_e_read(), causing an undefined shift by 32 at:

  index = hash >> (32 - dip->i_depth);

As calculated in an open-coded way in dir_make_exhash(), the minimum
depth for an exhash directory is ilog2(sdp->sd_hash_ptrs) and 0 is
invalid as sdp->sd_hash_ptrs is fixed as sdp->bsize / 16 at mount time.

So we can avoid the undefined behaviour by checking for depth values
lower than the minimum in gfs2_dinode_in(). Values greater than the
maximum are already being checked for there.

Also switch the calculation in dir_make_exhash() to use ilog2() to
clarify how the depth is calculated.

Tested with the syzkaller repro.c and xfstests '-g quick'.

Reported-by: syzbot+4708579bb230a0582a57@syzkaller.appspotmail.com
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: andyprice

fs/gfs2/dir.c

@@ -60,6 +60,7 @@
 #include <linux/crc32.h>
 #include <linux/vmalloc.h>
 #include <linux/bio.h>
+#include <linux/log2.h>
 
 #include "gfs2.h"
 #include "incore.h"
@@ -912,7 +913,6 @@ static int dir_make_exhash(struct inode *inode)
 	struct qstr args;
 	struct buffer_head *bh, *dibh;
 	struct gfs2_leaf *leaf;
-	int y;
 	u32 x;
 	__be64 *lp;
 	u64 bn;
@@ -979,9 +979,7 @@ static int dir_make_exhash(struct inode *inode)
 	i_size_write(inode, sdp->sd_sb.sb_bsize / 2);
 	gfs2_add_inode_blocks(&dip->i_inode, 1);
 	dip->i_diskflags |= GFS2_DIF_EXHASH;
-
-	for (x = sdp->sd_hash_ptrs, y = -1; x; x >>= 1, y++) ;
-	dip->i_depth = y;
+	dip->i_depth = ilog2(sdp->sd_hash_ptrs);
 
 	gfs2_dinode_out(dip, dibh->b_data);
 

fs/gfs2/glops.c

@@ -11,6 +11,7 @@
 #include <linux/bio.h>
 #include <linux/posix_acl.h>
 #include <linux/security.h>
+#include <linux/log2.h>
 
 #include "gfs2.h"
 #include "incore.h"
@@ -450,6 +451,11 @@ static int gfs2_dinode_in(struct gfs2_inode *ip, const void *buf)
 		gfs2_consist_inode(ip);
 		return -EIO;
 	}
+	if ((ip->i_diskflags & GFS2_DIF_EXHASH) &&
+	    depth < ilog2(sdp->sd_hash_ptrs)) {
+		gfs2_consist_inode(ip);
+		return -EIO;
+	}
 	ip->i_depth = (u8)depth;
 	ip->i_entries = be32_to_cpu(str->di_entries);