On Tue, Aug 4, 2020 at 3:29 PM Qu Wenruo <[email protected]> wrote:
>
> [BUG]
> When running btrfs/071 with inode_need_compress() removed from
> compress_file_range(), we got the following crash:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000018
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> Workqueue: btrfs-delalloc btrfs_work_helper [btrfs]
> RIP: 0010:compress_file_range+0x476/0x7b0 [btrfs]
> Call Trace:
> ? submit_compressed_extents+0x450/0x450 [btrfs]
> async_cow_start+0x16/0x40 [btrfs]
> btrfs_work_helper+0xf2/0x3e0 [btrfs]
> process_one_work+0x278/0x5e0
> worker_thread+0x55/0x400
> ? process_one_work+0x5e0/0x5e0
> kthread+0x168/0x190
> ? kthread_create_worker_on_cpu+0x70/0x70
> ret_from_fork+0x22/0x30
> ---[ end trace 65faf4eae941fa7d ]---
>
Ping？
The bug occurred while running btrfs/074 days ago. And the fix
looks reasonable.
--
Su
> This is already after the patch "btrfs: inode: fix NULL pointer
> dereference if inode doesn't need compression."
>
> [CAUSE]
> @pages is firstly created by kcalloc() in compress_file_extent():
> pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);
>
> Then passed to btrfs_compress_pages() to be utilized there:
>
> ret = btrfs_compress_pages(...
> pages,
> &nr_pages,
> ...);
>
> btrfs_compress_pages() will initial each pages as output, in
> zlib_compress_pages() we have:
>
> pages[nr_pages] = out_page;
> nr_pages++;
>
> Normally this is completely fine, but there is a special case which
> is in btrfs_compress_pages() itself:
> switch (type) {
> default:
> return -E2BIG;
> }
>
> In this case, we didn't modify @pages nor @out_pages, leaving them
> untouched, then when we cleanup pages, the we can hit NULL pointer
> dereference again:
>
> if (pages) {
> for (i = 0; i < nr_pages; i++) {
> WARN_ON(pages[i]->mapping);
> put_page(pages[i]);
> }
> ...
> }
>
> Since pages[i] are all initialized to zero, and btrfs_compress_pages()
> doesn't change them at all, accessing pages[i]->mapping would lead to
> NULL pointer dereference.
>
> This is not possible for current kernel, as we check
> inode_need_compress() before doing pages allocation.
> But if we're going to remove that inode_need_compress() in
> compress_file_extent(), then it's going to be a problem.
>
> [FIX]
> When btrfs_compress_pages() hits its default case, modify @out_pages to
> 0 to prevent such problem from happening.
>
> Signed-off-by: Qu Wenruo <[email protected]>
> ---
> fs/btrfs/compression.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index 1ab56a734e70..17c27edd804b 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -115,10 +115,14 @@ static int compression_compress_pages(int type, struct
> list_head *ws,
> case BTRFS_COMPRESS_NONE:
> default:
> /*
> - * This can't happen, the type is validated several times
> - * before we get here. As a sane fallback, return what the
> - * callers will understand as 'no compression happened'.
> + * This happens when compression races with remount to no
> + * compress, while caller doesn't call inode_need_compress()
> + * to check if we really need to compress.
> + *
> + * Not a big deal, just need to inform caller that we
> + * haven't allocated any pages yet.
> */
> + *out_pages = 0;
> return -E2BIG;
> }
> }
> --
> 2.28.0
>
