Re: [PATCH v4] f2fs: fix use-after-free of sbi in f2fs_compress_write_end_io()

[Chao Yu <chao@kernel.org>]

On 3/23/26 19:21, George Saad wrote:
> In f2fs_compress_write_end_io(), dec_page_count(sbi, type) can bring
> the F2FS_WB_CP_DATA counter to zero, unblocking
> f2fs_wait_on_all_pages() in f2fs_put_super() on a concurrent unmount
> CPU. The unmount path then proceeds to call
> f2fs_destroy_page_array_cache(sbi), which destroys
> sbi->page_array_slab via kmem_cache_destroy(), and eventually
> kfree(sbi). Meanwhile, the bio completion callback is still executing:
> when it reaches page_array_free(sbi, ...), it dereferences
> sbi->page_array_slab — a destroyed slab cache — to call
> kmem_cache_free(), causing a use-after-free.
> 
> This is the same class of bug as CVE-2026-23234 (which fixed the
> equivalent race in f2fs_write_end_io() in data.c), but in the
> compressed writeback completion path that was not covered by that fix.
> 
> Fix this by moving dec_page_count() to after page_array_free(), so
> that all sbi accesses complete before the counter decrement that can
> unblock unmount. For non-last folios (where atomic_dec_return on
> cic->pending_pages is nonzero), dec_page_count is called immediately
> before returning — page_array_free is not reached on this path, so
> there is no post-decrement sbi access. For the last folio,
> page_array_free runs while the F2FS_WB_CP_DATA counter is still
> nonzero (this folio has not yet decremented it), keeping sbi alive,
> and dec_page_count runs as the final operation.
> 
> Fixes: 4c8ff7095bef ("f2fs: support data compression")
> Cc: stable@vger.kernel.org
> Signed-off-by: George Saad <geoo115@gmail.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,