ext4: prevent right-shifting extents beyond EXT_MAX_BLOCKS

During the "insert range" fallocate operation, extents starting at the
range offset are shifted "right" (to a higher file offset) by the range
length.  But, as shown by syzbot, it's not validated that this doesn't
cause extents to be shifted beyond EXT_MAX_BLOCKS.  In that case
->ee_block can wrap around, corrupting the extent tree.

Fix it by returning an error if the space between the end of the last
extent and EXT4_MAX_BLOCKS is smaller than the range being inserted.

This bug can be reproduced by running the following commands when the
current directory is on an ext4 filesystem with a 4k block size:

        fallocate -l 8192 file
        fallocate --keep-size -o 0xfffffffe000 -l 4096 -n file
        fallocate --insert-range -l 8192 file

Then after unmounting the filesystem, e2fsck reports corruption.

Reported-by: syzbot+06c885be0edcdaeab40c@syzkaller.appspotmail.com
Fixes: 331573febb6a ("ext4: Add support FALLOC_FL_INSERT_RANGE for fallocate")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 0a7315961bac6ebbca0c2bfe83cc3aba8fc5c807..c969275ce3ee7469167a6921e8fa22e4c91ed3a2 100644 (file)
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -5329,8 +5329,9 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
        stop = le32_to_cpu(extent->ee_block);
 
        /*
-        * In case of left shift, Don't start shifting extents until we make
-        * sure the hole is big enough to accommodate the shift.
+       * For left shifts, make sure the hole on the left is big enough to
+       * accommodate the shift.  For right shifts, make sure the last extent
+       * won't be shifted beyond EXT_MAX_BLOCKS.
        */
        if (SHIFT == SHIFT_LEFT) {
                path = ext4_find_extent(inode, start - 1, &path,
@@ -5350,9 +5351,14 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 
                if ((start == ex_start && shift > ex_start) ||
                    (shift > start - ex_end)) {
-                       ext4_ext_drop_refs(path);
-                       kfree(path);
-                       return -EINVAL;
+                       ret = -EINVAL;
+                       goto out;
+               }
+       } else {
+               if (shift > EXT_MAX_BLOCKS -
+                   (stop + ext4_ext_get_actual_len(extent))) {
+                       ret = -EINVAL;
+                       goto out;
                }
        }