xfs: don't walk off the end of a directory data block

This adds sanity checks for xfs_dir2_data_unused and xfs_dir2_data_entry
to make sure don't stray beyond valid memory region. Before patching, the
loop simply checks that the start offset of the dup and dep is within the
range. So in a crafted image, if last entry is xfs_dir2_data_unused, we
can change dup->length to dup->length-1 and leave 1 byte of space. In the
next traversal, this space will be considered as dup or dep. We may
encounter an out of bound read when accessing the fixed members.

In the patch, we make sure that the remaining bytes large enough to hold
an unused entry before accessing xfs_dir2_data_unused and
xfs_dir2_data_unused is XFS_DIR2_DATA_ALIGN byte aligned. We also make
sure that the remaining bytes large enough to hold a dirent with a
single-byte name before accessing xfs_dir2_data_entry.

Signed-off-by: lei lu <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Chandan Babu R <[email protected]>

Author: LLfam

fs/xfs/libxfs/xfs_dir2_data.c

@@ -178,6 +178,14 @@ __xfs_dir3_data_check(
 	while (offset < end) {
 		struct xfs_dir2_data_unused	*dup = bp->b_addr + offset;
 		struct xfs_dir2_data_entry	*dep = bp->b_addr + offset;
+		unsigned int	reclen;
+
+		/*
+		 * Are the remaining bytes large enough to hold an
+		 * unused entry?
+		 */
+		if (offset > end - xfs_dir2_data_unusedsize(1))
+			return __this_address;
 
 		/*
 		 * If it's unused, look for the space in the bestfree table.
@@ -187,9 +195,13 @@ __xfs_dir3_data_check(
 		if (be16_to_cpu(dup->freetag) == XFS_DIR2_DATA_FREE_TAG) {
 			xfs_failaddr_t	fa;
 
+			reclen = xfs_dir2_data_unusedsize(
+					be16_to_cpu(dup->length));
 			if (lastfree != 0)
 				return __this_address;
-			if (offset + be16_to_cpu(dup->length) > end)
+			if (be16_to_cpu(dup->length) != reclen)
+				return __this_address;
+			if (offset + reclen > end)
 				return __this_address;
 			if (be16_to_cpu(*xfs_dir2_data_unused_tag_p(dup)) !=
 			    offset)
@@ -207,10 +219,18 @@ __xfs_dir3_data_check(
 				    be16_to_cpu(bf[2].length))
 					return __this_address;
 			}
-			offset += be16_to_cpu(dup->length);
+			offset += reclen;
 			lastfree = 1;
 			continue;
 		}
+
+		/*
+		 * This is not an unused entry. Are the remaining bytes
+		 * large enough for a dirent with a single-byte name?
+		 */
+		if (offset > end - xfs_dir2_data_entsize(mp, 1))
+			return __this_address;
+
 		/*
 		 * It's a real entry.  Validate the fields.
 		 * If this is a block directory then make sure it's
@@ -219,9 +239,10 @@ __xfs_dir3_data_check(
 		 */
 		if (dep->namelen == 0)
 			return __this_address;
-		if (!xfs_verify_dir_ino(mp, be64_to_cpu(dep->inumber)))
+		reclen = xfs_dir2_data_entsize(mp, dep->namelen);
+		if (offset + reclen > end)
 			return __this_address;
-		if (offset + xfs_dir2_data_entsize(mp, dep->namelen) > end)
+		if (!xfs_verify_dir_ino(mp, be64_to_cpu(dep->inumber)))
 			return __this_address;
 		if (be16_to_cpu(*xfs_dir2_data_entry_tag_p(mp, dep)) != offset)
 			return __this_address;
@@ -245,7 +266,7 @@ __xfs_dir3_data_check(
 			if (i >= be32_to_cpu(btp->count))
 				return __this_address;
 		}
-		offset += xfs_dir2_data_entsize(mp, dep->namelen);
+		offset += reclen;
 	}
 	/*
 	 * Need to have seen all the entries and all the bestfree slots.

fs/xfs/libxfs/xfs_dir2_priv.h

@@ -189,6 +189,13 @@ void xfs_dir2_sf_put_ftype(struct xfs_mount *mp,
 extern int xfs_readdir(struct xfs_trans *tp, struct xfs_inode *dp,
 		       struct dir_context *ctx, size_t bufsize);
 
+static inline unsigned int
+xfs_dir2_data_unusedsize(
+	unsigned int	len)
+{
+	return round_up(len, XFS_DIR2_DATA_ALIGN);
+}
+
 static inline unsigned int
 xfs_dir2_data_entsize(
 	struct xfs_mount	*mp,