[Precise][CVE-2013-1819] xfs: fix _xfs_buf_find oops on blocks beyond the filesystem end

Luis Henriques luis.henriques at canonical.com
Wed Sep 4 08:46:31 UTC 2013

From: Dave Chinner <dchinner at redhat.com>


BugLink: http://bugs.launchpad.net/bugs/1151527

When _xfs_buf_find is passed an out of range address, it will fail
to find a relevant struct xfs_perag and oops with a null
dereference. This can happen when trying to walk a filesystem with a
metadata inode that has a partially corrupted extent map (i.e. the
block number returned is corrupt, but is otherwise intact) and we
try to read from the corrupted block address.

In this case, just fail the lookup. If it is readahead being issued,
it will simply not be done, but if it is real read that fails we
will get an error being reported.  Ideally this case should result
in an EFSCORRUPTED error being reported, but we cannot return an
error through xfs_buf_read() or xfs_buf_get() so this lookup failure
may result in ENOMEM or EIO errors being reported instead.

Signed-off-by: Dave Chinner <dchinner at redhat.com>
Reviewed-by: Brian Foster <bfoster at redhat.com>
Reviewed-by: Ben Myers <bpm at sgi.com>
Signed-off-by: Ben Myers <bpm at sgi.com>
(back ported from commit eb178619f930fa2ba2348de332a1ff1c66a31424)
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 fs/xfs/xfs_buf.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 2f5a8f7..c13fff4 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -434,6 +434,7 @@ _xfs_buf_find(
 	struct rb_node		**rbp;
 	struct rb_node		*parent;
 	xfs_buf_t		*bp;
+	xfs_daddr_t		eofs;
 	range_base = (ioff << BBSHIFT);
 	range_length = (isize << BBSHIFT);
@@ -442,6 +443,23 @@ _xfs_buf_find(
 	ASSERT(!(range_length < (1 << btp->bt_sshift)));
 	ASSERT(!(range_base & (xfs_off_t)btp->bt_smask));
+	/*
+	 * Corrupted block numbers can get through to here, unfortunately, so we
+	 * have to check that the buffer falls within the filesystem bounds.
+	 */
+	eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
+	if (ioff >= eofs) {
+		/*
+		 * XXX (dgc): we should really be returning EFSCORRUPTED here,
+		 * but none of the higher level infrastructure supports
+		 * returning a specific error on buffer lookup failures.
+		 */
+		xfs_alert(btp->bt_mount,
+			  "%s: Block out of range: block 0x%llx, EOFS 0x%llx ",
+			  __func__, ioff, eofs);
+		return NULL;
+	}
 	/* get tree root */
 	pag = xfs_perag_get(btp->bt_mount,
 				xfs_daddr_to_agno(btp->bt_mount, ioff));

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