Re: [PATCH v2] erofs: fix unsafe pagevec reuse of hooked pclusters

[Chao Yu]

On 2021/11/4 2:20, Gao Xiang wrote:
> There are pclusters in runtime marked with Z_EROFS_PCLUSTER_TAIL
> before actual I/O submission. Thus, the decompression chain can be
> extended if the following pcluster chain hooks such tail pcluster.
>
> As the related comment mentioned, if some page is made of a hooked
> pcluster and another followed pcluster, it can be reused for in-place
> I/O (since I/O should be submitted anyway):
>   _______________________________________________________________
> |  tail (partial) page |          head (partial) page           |
> |_____PRIMARY_HOOKED___|____________PRIMARY_FOLLOWED____________|
>
> However, it's by no means safe to reuse as pagevec since if such
> PRIMARY_HOOKED pclusters finally move into bypass chain without I/O
> submission. It's somewhat hard to reproduce with LZ4 and I just found
> it (general protection fault) by ro_fsstressing a LZMA image for long
> time.
>
> I'm going to actively clean up related code together with multi-page
> folio adaption in the next few months. Let's address it directly for
> easier backporting for now.
>
> Call trace for reference:
>    z_erofs_decompress_pcluster+0x10a/0x8a0 [erofs]
>    z_erofs_decompress_queue.isra.36+0x3c/0x60 [erofs]
>    z_erofs_runqueue+0x5f3/0x840 [erofs]
>    z_erofs_readahead+0x1e8/0x320 [erofs]
>    read_pages+0x91/0x270
>    page_cache_ra_unbounded+0x18b/0x240
>    filemap_get_pages+0x10a/0x5f0
>    filemap_read+0xa9/0x330
>    new_sync_read+0x11b/0x1a0
>    vfs_read+0xf1/0x190
>
> Fixes: 3883a79abd02 ("staging: erofs: introduce VLE decompression support")
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.19+
> Signed-off-by: Gao Xiang <xiang@xxxxxxxxxx>

Reviewed-by: Chao Yu <chao@xxxxxxxxxx>

Thanks,