CVE-2024-26727 Information

Description

In the Linux kernel the following vulnerability has been resolved:

btrfs: do not ASSERT() if the newly created subvolume already got read

[BUG] There is a syzbot crash triggered by the ASSERT() during subvolume creation:

assertion failed: !anon_dev in fs/btrfs/disk-io.c:1319 ————[ cut here ]———— kernel BUG at fs/btrfs/disk-io.c:1319! invalid opcode: 0000 [1] PREEMPT SMP KASAN RIP: 0010:btrfs_get_root_ref.part.0+0x9aa/0xa60 btrfs_get_new_fs_root+0xd3/0xf0 create_subvol+0xd02/0x1650 btrfs_mksubvol+0xe95/0x12b0 __btrfs_ioctl_snap_create+0x2f9/0x4f0 btrfs_ioctl_snap_create+0x16b/0x200 btrfs_ioctl+0x35f0/0x5cf0 __x64_sys_ioctl+0x19d/0x210 do_syscall_64+0x3f/0xe0 entry_SYSCALL_64_after_hwframe+0x63/0x6b —[ end trace 0000000000000000 ]—

[CAUSE] During create_subvol() after inserting root item for the newly created subvolume we would trigger btrfs_get_new_fs_root() to get the btrfs_root of that subvolume.

The idea here is we have preallocated an anonymous device number for the subvolume thus we can assign it to the new subvolume.

But there is really nothing preventing things like backref walk to read the new subvolume. If that happens before we call btrfs_get_new_fs_root() the subvolume would be read out with a new anonymous device number assigned already.

In that case we would trigger ASSERT() as we really expect no one to read out that subvolume (which is not yet accessible from the fs). But things like backref walk is still possible to trigger the read on the subvolume.

Thus our assumption on the ASSERT() is not correct in the first place.

[FIX] Fix it by removing the ASSERT() and just free the @anon_dev reset it to 0 and continue.

If the subvolume tree is read out by something else it should have already get a new anon_dev assigned thus we only need to free the preallocated one.

Reference

https://git.kernel.org/stable/c/3f5d47eb163bceb1b9e613c9003bae5fefc0046f https://git.kernel.org/stable/c/e31546b0f34af21738c4ceac47d662c00ee6382f https://git.kernel.org/stable/c/66b317a2fc45b2ef66527ee3f8fa08fb5beab88d https://git.kernel.org/stable/c/833775656d447c545133a744a0ed1e189ce61430 https://git.kernel.org/stable/c/5a172344bfdabb46458e03708735d7b1a918c468 https://git.kernel.org/stable/c/e03ee2fe873eb68c1f9ba5112fee70303ebf9dfb