Commit 349fa7d6 authored Apr 12, 2018 by Eric Biggers Committed by Theodore Ts'o Apr 12, 2018

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: 331573fe ("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>

parent e40ff213

fs/ext4/extents.c

+11 −5

Original line number	Diff line number	Diff line
		@@ -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;
		}
		}