CVE-2022-50067

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

btrfs: unset reloc control if transaction commit fails in prepare_to_relocate()

In btrfs_relocate_block_group(), the rc is allocated. Then btrfs_relocate_block_group() calls

relocate_block_group() prepare_to_relocate() set_reloc_control()

that assigns rc to the variable fs_info->reloc_ctl. When prepare_to_relocate() returns, it calls

btrfs_commit_transaction() btrfs_start_dirty_block_groups() btrfs_alloc_path() kmem_cache_zalloc()

which may fail for example (or other errors could happen). When the failure occurs, btrfs_relocate_block_group() detects the error and frees rc and doesnt set fs_info->reloc_ctl to NULL. After that, in btrfs_init_reloc_root(), rc is retrieved from fs_info->reloc_ctl and then used, which may cause a use-after-free bug.

This possible bug can be triggered by calling btrfs_ioctl_balance() before calling btrfs_ioctl_defrag().

To fix this possible bug, in prepare_to_relocate(), check if btrfs_commit_transaction() fails. If the failure occurs, unset_reloc_control() is called to set fs_info->reloc_ctl to NULL.

The error log in our fault-injection testing is shown as follows:

[ 58.751070] BUG: KASAN: use-after-free in btrfs_init_reloc_root+0x7ca/0x920 [btrfs] … [ 58.753577] Call Trace: … [ 58.755800] kasan_report+0x45/0x60 [ 58.756066] btrfs_init_reloc_root+0x7ca/0x920 [btrfs] [ 58.757304] record_root_in_trans+0x792/0xa10 [btrfs] [ 58.757748] btrfs_record_root_in_trans+0x463/0x4f0 [btrfs] [ 58.758231] start_transaction+0x896/0x2950 [btrfs] [ 58.758661] btrfs_defrag_root+0x250/0xc00 [btrfs] [ 58.759083] btrfs_ioctl_defrag+0x467/0xa00 [btrfs] [ 58.759513] btrfs_ioctl+0x3c95/0x114e0 [btrfs] … [ 58.768510] Allocated by task 23683: [ 58.768777] ____kasan_kmalloc+0xb5/0xf0 [ 58.769069] __kmalloc+0x227/0x3d0 [ 58.769325] alloc_reloc_control+0x10a/0x3d0 [btrfs] [ 58.769755] btrfs_relocate_block_group+0x7aa/0x1e20 [btrfs] [ 58.770228] btrfs_relocate_chunk+0xf1/0x760 [btrfs] [ 58.770655] __btrfs_balance+0x1326/0x1f10 [btrfs] [ 58.771071] btrfs_balance+0x3150/0x3d30 [btrfs] [ 58.771472] btrfs_ioctl_balance+0xd84/0x1410 [btrfs] [ 58.771902] btrfs_ioctl+0x4caa/0x114e0 [btrfs] … [ 58.773337] Freed by task 23683: … [ 58.774815] kfree+0xda/0x2b0 [ 58.775038] free_reloc_control+0x1d6/0x220 [btrfs] [ 58.775465] btrfs_relocate_block_group+0x115c/0x1e20 [btrfs] [ 58.775944] btrfs_relocate_chunk+0xf1/0x760 [btrfs] [ 58.776369] __btrfs_balance+0x1326/0x1f10 [btrfs] [ 58.776784] btrfs_balance+0x3150/0x3d30 [btrfs] [ 58.777185] btrfs_ioctl_balance+0xd84/0x1410 [btrfs] [ 58.777621] btrfs_ioctl+0x4caa/0x114e0 [btrfs] …

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/5d741afed0bac206640cc64d77b97853283cf719
• https://git.kernel.org/stable/c/78f8c2370e3d33e35f23bdc648653d779aeacb6e
• https://git.kernel.org/stable/c/85f02d6c856b9f3a0acf5219de6e32f58b9778eb
• https://git.kernel.org/stable/c/8e546674031fc1576da501e27a8fd165222e5a37
• https://git.kernel.org/stable/c/b60e862e133f646f19023ece1d476d630a660de1
• https://git.kernel.org/stable/c/dcb11fe0a0a9cca2b7425191b9bf30dc29f2ad0f
• https://git.kernel.org/stable/c/ff0e8ed8dfb584575cffc1561f17a1d094e8565b