CVE-2024-26794

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix race between ordered extent completion and fiemap

For fiemap we recently stopped locking the target extent range for the
whole duration of the fiemap call, in order to avoid a deadlock in a
scenario where the fiemap buffer happens to be a memory mapped range of
the same file. This use case is very unlikely to be useful in practice but
it may be triggered by fuzz testing (syzbot, etc).

However by not locking the target extent range for the whole duration of
the fiemap call we can race with an ordered extent. This happens like
this:

1. The fiemap task finishes processing a file extent item that covers
   the file range [512K, 1M[, and that file extent item is the last item
   in the leaf currently being processed;

2. And ordered extent for the file range [768K, 2M[, in COW mode,
   completes (btrfs_finish_one_ordered()) and the file extent item
   covering the range [512K, 1M[ is trimmed to cover the range
   [512K, 768K[ and then a new file extent item for the range [768K, 2M[
   is inserted in the inode’s subvolume tree;

3. The fiemap task calls fiemap_next_leaf_item(), which then calls
   btrfs_next_leaf() to find the next leaf / item. This finds that the
   the next key following the one we previously processed (its type is
   BTRFS_EXTENT_DATA_KEY and its offset is 512K), is the key corresponding
   to the new file extent item inserted by the ordered extent, which has
   a type of BTRFS_EXTENT_DATA_KEY and an offset of 768K;

4. Later the fiemap code ends up at emit_fiemap_extent() and triggers
   the warning:

   if (cache->offset + cache->len > offset) {
   WARN_ON(1);
   return -EINVAL;
   }

   Since we get 1M > 768K, because the previously emitted entry for the
   old extent covering the file range [512K, 1M[ ends at an offset that
   is greater than the new extent’s start offset (768K). This makes fiemap
   fail with -EINVAL besides triggering the warning that produces a stack
   trace like the following:

   [1621.677651] ------------[ cut here ]------------
   [1621.677656] WARNING: CPU: 1 PID: 204366 at fs/btrfs/extent_io.c:2492 emit_fiemap_extent+0x84/0x90 [btrfs]
   [1621.677899] Modules linked in: btrfs blake2b_generic (…)
   [1621.677951] CPU: 1 PID: 204366 Comm: pool Not tainted 6.8.0-rc5-btrfs-next-151+ #1
   [1621.677954] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
   [1621.677956] RIP: 0010:emit_fiemap_extent+0x84/0x90 [btrfs]
   [1621.678033] Code: 2b 4c 89 63 (…)
   [1621.678035] RSP: 0018:ffffab16089ffd20 EFLAGS: 00010206
   [1621.678037] RAX: 00000000004fa000 RBX: ffffab16089ffe08 RCX: 0000000000009000
   [1621.678039] RDX: 00000000004f9000 RSI: 00000000004f1000 RDI: ffffab16089ffe90
   [1621.678040] RBP: 00000000004f9000 R08: 0000000000001000 R09: 0000000000000000
   [1621.678041] R10: 0000000000000000 R11: 0000000000001000 R12: 0000000041d78000
   [1621.678043] R13: 0000000000001000 R14: 0000000000000000 R15: ffff9434f0b17850
   [1621.678044] FS: 00007fa6e20006c0(0000) GS:ffff943bdfa40000(0000) knlGS:0000000000000000
   [1621.678046] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
   [1621.678048] CR2: 00007fa6b0801000 CR3: 000000012d404002 CR4: 0000000000370ef0
   [1621.678053] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
   [1621.678055] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
   [1621.678056] Call Trace:
   [1621.678074] <TASK>
   [1621.678076] ? __warn+0x80/0x130
   [1621.678082] ? emit_fiemap_extent+0x84/0x90 [btrfs]
   [1621.678159] ? report_bug+0x1f4/0x200
   [1621.678164] ? handle_bug+0x42/0x70
   [1621.678167] ? exc_invalid_op+0x14/0x70
   [1621.678170] ? asm_exc_invalid_op+0x16/0x20
   [1621.678178] ? emit_fiemap_extent+0x84/0x90 [btrfs]
   [1621.678253] extent_fiemap+0x766
   —truncated—