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[PATCH 13/15] xfs: validity check the directory block leaf entry count

To: xfs@xxxxxxxxxxx
Subject: [PATCH 13/15] xfs: validity check the directory block leaf entry count
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 29 Oct 2013 22:11:56 +1100
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1383045118-31107-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1383045118-31107-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

The directory block format verifier fails to check that the leaf
entry count is in a valid range, and so if it is corrupted then it
can lead to derefencing a pointer outside the block buffer. While we
can't exactly validate the count without first walking the directory
block, we can ensure the count lands in the valid area within the
directory block and hence avoid out-of-block references.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/xfs_dir2_data.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_dir2_data.c b/fs/xfs/xfs_dir2_data.c
index 18e920c..70acff4 100644
--- a/fs/xfs/xfs_dir2_data.c
+++ b/fs/xfs/xfs_dir2_data.c
@@ -65,7 +65,6 @@ __xfs_dir3_data_check(
        const struct xfs_dir_ops *ops;
 
        mp = bp->b_target->bt_mount;
-       hdr = bp->b_addr;
 
        /*
         * We can be passed a null dp here from a verifier, so we need to go the
@@ -73,12 +72,25 @@ __xfs_dir3_data_check(
         */
        ops = xfs_dir_get_ops(mp, dp);
 
+       hdr = bp->b_addr;
+       p = (char *)ops->data_entry_p(hdr);
+
        switch (hdr->magic) {
        case cpu_to_be32(XFS_DIR3_BLOCK_MAGIC):
        case cpu_to_be32(XFS_DIR2_BLOCK_MAGIC):
                btp = xfs_dir2_block_tail_p(mp, hdr);
                lep = xfs_dir2_block_leaf_p(btp);
                endp = (char *)lep;
+
+               /*
+                * The number of leaf entries is limited by the size of the
+                * block and the amount of space used by the data entries.
+                * We don't know how much space is used by the data entries yet,
+                * so just ensure that the count falls somewhere inside the
+                * block right now.
+                */
+               XFS_WANT_CORRUPTED_RETURN(be32_to_cpu(btp->count) <
+                       ((char *)btp - p) / sizeof(struct xfs_dir2_leaf_entry));
                break;
        case cpu_to_be32(XFS_DIR3_DATA_MAGIC):
        case cpu_to_be32(XFS_DIR2_DATA_MAGIC):
@@ -88,13 +100,12 @@ __xfs_dir3_data_check(
                XFS_ERROR_REPORT("Bad Magic", XFS_ERRLEVEL_LOW, mp);
                return EFSCORRUPTED;
        }
-       bf = ops->data_bestfree_p(hdr);
-       p = (char *)ops->data_entry_p(hdr);
 
-       count = lastfree = freeseen = 0;
        /*
         * Account for zero bestfree entries.
         */
+       bf = ops->data_bestfree_p(hdr);
+       count = lastfree = freeseen = 0;
        if (!bf[0].length) {
                XFS_WANT_CORRUPTED_RETURN(!bf[0].offset);
                freeseen |= 1 << 0;
-- 
1.8.4.rc3

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