libfsimage/xfs: Sanity-check the superblock during mounts

Sanity-check the XFS superblock for wellformedness at the mount handler.
This forces pygrub to abort parsing a potentially malformed filesystem and
ensures the invariants assumed throughout the rest of the code hold.

Also, derive parameters from previously sanitized parameters where possible
(rather than reading them off the superblock)

The code doesn't try to avoid overflowing the end of the disk, because
that's an unlikely and benign error. Parameters used in calculations of
xfs_daddr_t (like the root inode index) aren't in critical need of being
sanitized.

The sanitization of agblklog is basically checking that no obvious
overflows happen on agblklog, and then ensuring agblocks is contained in
the range (2^(sb_agblklog-1), 2^sb_agblklog].

This is part of XSA-443 / CVE-2023-34325

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

tools/libfsimage/xfs/fsys_xfs.c

@@ -18,6 +18,7 @@
  */
 
 #include <stddef.h>
+#include <stdbool.h>
 #include <xenfsimage_grub.h>
 #include "xfs.h"
 
@@ -431,29 +432,56 @@ first_dentry (fsi_file_t *ffi, xfs_ino_t *ino)
 	return next_dentry (ffi, ino);
 }
 
+static bool
+xfs_sb_is_invalid (const xfs_sb_t *super)
+{
+	return (le32(super->sb_magicnum) != XFS_SB_MAGIC)
+	    || ((le16(super->sb_versionnum) & XFS_SB_VERSION_NUMBITS) !=
+	        XFS_SB_VERSION_4)
+	    || (super->sb_inodelog < XFS_SB_INODELOG_MIN)
+	    || (super->sb_inodelog > XFS_SB_INODELOG_MAX)
+	    || (super->sb_blocklog < XFS_SB_BLOCKLOG_MIN)
+	    || (super->sb_blocklog > XFS_SB_BLOCKLOG_MAX)
+	    || (super->sb_blocklog < super->sb_inodelog)
+	    || (super->sb_agblklog > XFS_SB_AGBLKLOG_MAX)
+	    || ((1ull << super->sb_agblklog) < le32(super->sb_agblocks))
+	    || (((1ull << super->sb_agblklog) >> 1) >=
+	        le32(super->sb_agblocks))
+	    || ((super->sb_blocklog + super->sb_dirblklog) >=
+	        XFS_SB_DIRBLK_NUMBITS);
+}
+
 static int
 xfs_mount (fsi_file_t *ffi, const char *options)
 {
 	xfs_sb_t super;
 
 	if (!devread (ffi, 0, 0, sizeof(super), (char *)&super)
-	    || (le32(super.sb_magicnum) != XFS_SB_MAGIC)
-	    || ((le16(super.sb_versionnum) 
-		& XFS_SB_VERSION_NUMBITS) != XFS_SB_VERSION_4) ) {
+	    || xfs_sb_is_invalid(&super)) {
 		return 0;
 	}
 
-	xfs.bsize = le32 (super.sb_blocksize);
-	xfs.blklog = super.sb_blocklog;
-	xfs.bdlog = xfs.blklog - SECTOR_BITS;
+	/*
+	 * Not sanitized. It's exclusively used to generate disk addresses,
+	 * so it's not important from a security standpoint.
+	 */
 	xfs.rootino = le64 (super.sb_rootino);
-	xfs.isize = le16 (super.sb_inodesize);
-	xfs.agblocks = le32 (super.sb_agblocks);
-	xfs.dirbsize = xfs.bsize << super.sb_dirblklog;
 
-	xfs.inopblog = super.sb_inopblog;
+	/*
+	 * Sanitized to be consistent with each other, only used to
+	 * generate disk addresses, so it's safe
+	 */
+	xfs.agblocks = le32 (super.sb_agblocks);
 	xfs.agblklog = super.sb_agblklog;
 
+	/* Derived from sanitized parameters */
+	xfs.bsize = 1 << super.sb_blocklog;
+	xfs.blklog = super.sb_blocklog;
+	xfs.bdlog = super.sb_blocklog - SECTOR_BITS;
+	xfs.isize = 1 << super.sb_inodelog;
+	xfs.dirbsize = 1 << (super.sb_blocklog + super.sb_dirblklog);
+	xfs.inopblog = super.sb_blocklog - super.sb_inodelog;
+
 	xfs.btnode_ptr0_off =
 		((xfs.bsize - sizeof(xfs_btree_block_t)) /
 		(sizeof (xfs_bmbt_key_t) + sizeof (xfs_bmbt_ptr_t)))

tools/libfsimage/xfs/xfs.h

@@ -134,6 +134,18 @@ typedef struct xfs_sb
         xfs_uint8_t       sb_dummy[7];    /* padding */
 } xfs_sb_t;
 
+/* Bound taken from xfs.c in GRUB2. It doesn't exist in the spec */
+#define	XFS_SB_DIRBLK_NUMBITS	27
+/* Implied by the XFS specification. The minimum block size is 512 octets */
+#define	XFS_SB_BLOCKLOG_MIN	9
+/* Implied by the XFS specification. The maximum block size is 65536 octets */
+#define	XFS_SB_BLOCKLOG_MAX	16
+/* Implied by the XFS specification. The minimum inode size is 256 octets */
+#define	XFS_SB_INODELOG_MIN	8
+/* Implied by the XFS specification. The maximum inode size is 2048 octets */
+#define	XFS_SB_INODELOG_MAX	11
+/* High bound for sb_agblklog */
+#define	XFS_SB_AGBLKLOG_MAX	32
 
 /* those are from xfs_btree.h */