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 inodes 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 extents 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] [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—

Weakness

The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.

Affected Software

Extended Description

This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider. A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single “x++” statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x). The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/31d07a757c6d3430e03cc22799921569999b9a12
• https://git.kernel.org/stable/c/a1a4a9ca77f143c00fce69c1239887ff8b813bec
• https://git.kernel.org/stable/c/d43f8e58f10a44df8c08e7f7076f3288352cd168
• https://git.kernel.org/stable/c/31d07a757c6d3430e03cc22799921569999b9a12
• https://git.kernel.org/stable/c/a1a4a9ca77f143c00fce69c1239887ff8b813bec
• https://git.kernel.org/stable/c/d43f8e58f10a44df8c08e7f7076f3288352cd168