xfs: bounds-check buffer log item's dirty bitmap

xlog_recover_do_reg_buffer() replays each dirty region described by a
buffer log item's bitmap into the buffer read for that item:

	memcpy(xfs_buf_offset(bp, (uint)bit << XFS_BLF_SHIFT),
		item->ri_buf[i].iov_base,
		nbits << XFS_BLF_SHIFT);

The destination offset (bit/nbits, from the logged dirty bitmap) and the
buffer size (from the logged blf_len) are both attacker-controlled and
otherwise unrelated, yet the only thing bounding the copy is an ASSERT(),
which compiles away on production kernels. A crafted image logging a
small blf_len together with a bitmap bit past the end of that buffer
drives the memcpy() past the buffer's allocation, corrupting adjacent
kernel heap during mount-time log recovery. This is reachable by anyone
who can get a crafted image mounted -- the malicious-filesystem threat
model XFS already guards against elsewhere.

Turn the ASSERT() into a real XFS_IS_CORRUPT() check that aborts recovery
of the buffer with -EFSCORRUPTED, consistent with the validate-and-fail
idiom already used in xlog_recover_do_inode_buffer() and
xfs_dquot_item_recover.c. xlog_recover_do_reg_buffer() therefore becomes
STATIC int and its three callers propagate the error.

Found and confirmed with KASAN on a CONFIG_XFS_DEBUG=n build: the crafted
image trips a slab-out-of-bounds write before this change and fails
recovery cleanly with -EFSCORRUPTED after it.

Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ibrahim Hashimov <security@auditcode.ai>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/xfs_buf_item_recover.c

@@ -461,7 +461,7 @@ xlog_recover_validate_buf_type(
  * given buffer.  The bitmap in the buf log format structure indicates
  * where to place the logged data.
  */
-STATIC void
+STATIC int
 xlog_recover_do_reg_buffer(
 	struct xfs_mount		*mp,
 	struct xlog_recover_item	*item,
@@ -489,8 +489,24 @@ xlog_recover_do_reg_buffer(
 		ASSERT(nbits > 0);
 		ASSERT(item->ri_buf[i].iov_base != NULL);
 		ASSERT(item->ri_buf[i].iov_len % XFS_BLF_CHUNK == 0);
-		ASSERT(BBTOB(bp->b_length) >=
-		       ((uint)bit << XFS_BLF_SHIFT) + (nbits << XFS_BLF_SHIFT));
+		/*
+		 * The bitmap is only trustworthy to the extent that it
+		 * describes a region that actually fits inside the buffer we
+		 * read in based on the (attacker-controlled) blf_len.  Do not
+		 * rely on an ASSERT() for this -- it compiles away entirely on
+		 * non-DEBUG kernels, which is exactly where this matters, so
+		 * validate it for real and abort recovery of this buffer rather
+		 * than copying past the end of it.
+		 */
+		if (XFS_IS_CORRUPT(mp, BBTOB(bp->b_length) <
+				((uint)bit << XFS_BLF_SHIFT) +
+				(nbits << XFS_BLF_SHIFT))) {
+			xfs_alert(mp,
+	"Bad buffer log item dirty bitmap (bit %d, nbits %d) for %d-byte buffer at daddr 0x%llx.",
+				bit, nbits, BBTOB(bp->b_length),
+				xfs_buf_daddr(bp));
+			return -EFSCORRUPTED;
+		}
 
 		/*
 		 * The dirty regions logged in the buffer, even though
@@ -544,6 +560,7 @@ xlog_recover_do_reg_buffer(
 	ASSERT(i == item->ri_total);
 
 	xlog_recover_validate_buf_type(mp, bp, buf_f, current_lsn);
+	return 0;
 }
 
 /*
@@ -552,10 +569,10 @@ xlog_recover_do_reg_buffer(
  * (ie. USR or GRP), then just toss this buffer away; don't recover it.
  * Else, treat it as a regular buffer and do recovery.
  *
- * Return false if the buffer was tossed and true if we recovered the buffer to
- * indicate to the caller if the buffer needs writing.
+ * Return 0 if the buffer was not recovered (tossed), 1 if it was recovered and
+ * needs writing, or a negative errno if recovery of the buffer failed.
  */
-STATIC bool
+STATIC int
 xlog_recover_do_dquot_buffer(
 	struct xfs_mount		*mp,
 	struct xlog			*log,
@@ -564,6 +581,7 @@ xlog_recover_do_dquot_buffer(
 	struct xfs_buf_log_format	*buf_f)
 {
 	uint			type;
+	int			error;
 
 	trace_xfs_log_recover_buf_dquot_buf(log, buf_f);
 
@@ -571,7 +589,7 @@ xlog_recover_do_dquot_buffer(
 	 * Filesystems are required to send in quota flags at mount time.
 	 */
 	if (!mp->m_qflags)
-		return false;
+		return 0;
 
 	type = 0;
 	if (buf_f->blf_flags & XFS_BLF_UDQUOT_BUF)
@@ -584,10 +602,12 @@ xlog_recover_do_dquot_buffer(
 	 * This type of quotas was turned off, so ignore this buffer
 	 */
 	if (log->l_quotaoffs_flag & type)
-		return false;
+		return 0;
 
-	xlog_recover_do_reg_buffer(mp, item, bp, buf_f, NULLCOMMITLSN);
-	return true;
+	error = xlog_recover_do_reg_buffer(mp, item, bp, buf_f, NULLCOMMITLSN);
+	if (error)
+		return error;
+	return 1;
 }
 
 /*
@@ -724,7 +744,9 @@ xlog_recover_do_primary_sb_buffer(
 	xfs_rgnumber_t			orig_rgcount = mp->m_sb.sb_rgcount;
 	int				error;
 
-	xlog_recover_do_reg_buffer(mp, item, bp, buf_f, current_lsn);
+	error = xlog_recover_do_reg_buffer(mp, item, bp, buf_f, current_lsn);
+	if (error)
+		return error;
 
 	if (orig_agcount == 0) {
 		xfs_alert(mp, "Trying to grow file system without AGs");
@@ -1081,11 +1103,11 @@ xlog_recover_buf_commit_pass2(
 			goto out_release;
 	} else if (buf_f->blf_flags &
 		  (XFS_BLF_UDQUOT_BUF|XFS_BLF_PDQUOT_BUF|XFS_BLF_GDQUOT_BUF)) {
-		bool	dirty;
-
-		dirty = xlog_recover_do_dquot_buffer(mp, log, item, bp, buf_f);
-		if (!dirty)
+		error = xlog_recover_do_dquot_buffer(mp, log, item, bp, buf_f);
+		if (error <= 0)
 			goto out_release;
+		/* write dirty buffer */
+		error = 0;
 	} else if ((xfs_blft_from_flags(buf_f) & XFS_BLFT_SB_BUF) &&
 			xfs_buf_daddr(bp) == 0) {
 		error = xlog_recover_do_primary_sb_buffer(mp, item, bp, buf_f,
@@ -1105,7 +1127,10 @@ xlog_recover_buf_commit_pass2(
 			xfs_buf_relse(rtsb_bp);
 		}
 	} else {
-		xlog_recover_do_reg_buffer(mp, item, bp, buf_f, current_lsn);
+		error = xlog_recover_do_reg_buffer(mp, item, bp, buf_f,
+						   current_lsn);
+		if (error)
+			goto out_release;
 	}
 
 	/*