[xen stable-4.16] libfsimage/xfs: Sanity-check the superblock during mounts

[patchbot@xxxxxxx]

commit 2de503f8fd0d07401e92abed1097ceb5fd1801f6
Author:     Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
AuthorDate: Thu Sep 14 13:22:52 2023 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Sep 27 16:29:50 2023 +0100

    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@xxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 620500dd1baf33347dfde5e7fde7cf7fe347da5c)
---
 tools/libfsimage/xfs/fsys_xfs.c | 48 ++++++++++++++++++++++++++++++++---------
 tools/libfsimage/xfs/xfs.h      | 12 +++++++++++
 2 files changed, 50 insertions(+), 10 deletions(-)

diff --git a/tools/libfsimage/xfs/fsys_xfs.c b/tools/libfsimage/xfs/fsys_xfs.c
index 4720bb4505..e4eb7e1ee2 100644
--- a/tools/libfsimage/xfs/fsys_xfs.c
+++ b/tools/libfsimage/xfs/fsys_xfs.c
@@ -17,6 +17,7 @@
  *  along with this program; If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <stdbool.h>
 #include <xenfsimage_grub.h>
 #include "xfs.h"
 
@@ -433,29 +434,56 @@ 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)))
diff --git a/tools/libfsimage/xfs/xfs.h b/tools/libfsimage/xfs/xfs.h
index 40699281e4..b87e37d3d7 100644
--- a/tools/libfsimage/xfs/xfs.h
+++ b/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 */
 
--
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