CVE-2022-48733

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

btrfs: fix use-after-free after failure to create a snapshot

At ioctl.c:create_snapshot(), we allocate a pending snapshot structure and then attach it to the transactions list of pending snapshots. After that we call btrfs_commit_transaction(), and if that returns an error we jump to fail label, where we kfree() the pending snapshot structure. This can result in a later use-after-free of the pending snapshot:

1. We allocated the pending snapshot and added it to the transactions list of pending snapshots;

2. We call btrfs_commit_transaction(), and it fails either at the first call to btrfs_run_delayed_refs() or btrfs_start_dirty_block_groups(). In both cases, we dont abort the transaction and we release our transaction handle. We jump to the fail label and free the pending snapshot structure. We return with the pending snapshot still in the transactions list;

3. Another task commits the transaction. This time theres no error at all, and then during the transaction commit it accesses a pointer to the pending snapshot structure that the snapshot creation task has already freed, resulting in a user-after-free.

This issue could actually be detected by smatch, which produced the following warning:

fs/btrfs/ioctl.c:843 create_snapshot() warn: &pending_snapshot->list not removed from list

So fix this by not having the snapshot creation ioctl directly add the pending snapshot to the transactions list. Instead add the pending snapshot to the transaction handle, and then at btrfs_commit_transaction() we add the snapshot to the list only when we can guarantee that any error returned after that point will result in a transaction abort, in which case the ioctl code can safely free the pending snapshot and no one can access it anymore.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/28b21c558a3753171097193b6f6602a94169093a
• https://git.kernel.org/stable/c/7e4c72dbaf62f8978af8321a24dbd35566d3a78a
• https://git.kernel.org/stable/c/9372fa1d73da5f1673921e365d0cd2c27ec7adc2
• https://git.kernel.org/stable/c/a7b717fa15165d3d9245614680bebc48a52ac05d
• https://git.kernel.org/stable/c/28b21c558a3753171097193b6f6602a94169093a
• https://git.kernel.org/stable/c/9372fa1d73da5f1673921e365d0cd2c27ec7adc2
• https://git.kernel.org/stable/c/a7b717fa15165d3d9245614680bebc48a52ac05d