Description

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

btrfs: fix double free of anonymous device after snapshot creation failure

When creating a snapshot we may do a double free of an anonymous device
in case there's an error committing the transaction. The second free may
result in freeing an anonymous device number that was allocated by some
other subsystem in the kernel or another btrfs filesystem.

The steps that lead to this:

1) At ioctl.c:create_snapshot() we allocate an anonymous device number
and assign it to pending_snapshot->anon_dev;

2) Then we call btrfs_commit_transaction() and end up at
transaction.c:create_pending_snapshot();

3) There we call btrfs_get_new_fs_root() and pass it the anonymous device
number stored in pending_snapshot->anon_dev;

4) btrfs_get_new_fs_root() frees that anonymous device number because
btrfs_lookup_fs_root() returned a root - someone else did a lookup
of the new root already, which could some task doing backref walking;

5) After that some error happens in the transaction commit path, and at
ioctl.c:create_snapshot() we jump to the 'fail' label, and after
that we free again the same anonymous device number, which in the
meanwhile may have been reallocated somewhere else, because
pending_snapshot->anon_dev still has the same value as in step 1.

Recently syzbot ran into this and reported the following trace:

------------[ cut here ]------------
ida_free called for id=51 which is not...

Classification

CVE ID: CVE-2024-26792

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.04% (probability of being exploited)

EPSS Percentile: 5.08% (scored less or equal to compared to others)

EPSS Date: 2025-02-04 (when was this score calculated)

References

https://git.kernel.org/stable/c/c34adc20b91a8e55e048b18d63f4f4ae003ecf8f
https://git.kernel.org/stable/c/eb3441093aad251418921246fc3b224fd1575701
https://git.kernel.org/stable/c/c8ab7521665bd0f8bc4a900244d1d5a7095cc3b9
https://git.kernel.org/stable/c/e2b54eaf28df0c978626c9736b94f003b523b451

Timeline