Re: [PATCH] btrfs: inode: Fix NULL pointer dereference if inode doesn't need compression

David Sterba
2020-08-25T15:04:24Z

On Mon, Aug 03, 2020 at 07:14:22AM +0800, Qu Wenruo wrote:
> On 2020/8/3 上午3:16, Nikolay Borisov wrote:
> > On 28.07.20 г. 11:39 ч., Qu Wenruo wrote:
> >> [BUG]
> >> There is a bug report of NULL pointer dereference caused in
> >> compress_file_extent():
> >>
> >> Oops: Kernel access of bad area, sig: 11 [#1]
> >> LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
> >> Workqueue: btrfs-delalloc btrfs_delalloc_helper [btrfs]
> >> NIP [c008000006dd4d34] compress_file_range.constprop.41+0x75c/0x8a0 [btrfs]
> >> LR [c008000006dd4d1c] compress_file_range.constprop.41+0x744/0x8a0 [btrfs]
> >> Call Trace:
> >> [c000000c69093b00] [c008000006dd4d1c] compress_file_range.constprop.41+0x744/0x8a0 [btrfs] (unreliable)
> >> [c000000c69093bd0] [c008000006dd4ebc] async_cow_start+0x44/0xa0 [btrfs]
> >> [c000000c69093c10] [c008000006e14824] normal_work_helper+0xdc/0x598 [btrfs]
> >> [c000000c69093c80] [c0000000001608c0] process_one_work+0x2c0/0x5b0
> >> [c000000c69093d10] [c000000000160c38] worker_thread+0x88/0x660
> >> [c000000c69093db0] [c00000000016b55c] kthread+0x1ac/0x1c0
> >> [c000000c69093e20] [c00000000000b660] ret_from_kernel_thread+0x5c/0x7c
> >> ---[ end trace f16954aa20d822f6 ]---
> >>
> >> [CAUSE]
> >> For the following execution route of compress_file_range(), it's
> >> possible to hit NULL pointer dereference:
> >>
> >> compress_file_extent()
> >> |- pages = NULL;
> >> |- start = async_chunk->start = 0;
> >> |- end = async_chunk = 4095;
> >> |- nr_pages = 1;
> >> |- inode_need_compress() == false; <<< Possible, see later explanation
> >> | Now, we have nr_pages = 1, pages = NULL
> >> |- cont:
> >> |- ret = cow_file_range_inline();
> >> |- if (ret <= 0) {
> >> |- for (i = 0; i < nr_pages; i++) {
> >> |- WARN_ON(pages[i]->mapping); <<< Crash
> >>
> >> To enter above call execution branch, we need the following race:
> >>
> >> Thread 1 (chattr) | Thread 2 (writeback)
> >> --------------------------+------------------------------
> >> | btrfs_run_delalloc_range
> >> | |- inode_need_compress = true
> >> | |- cow_file_range_async()
> >> btrfs_ioctl_set_flag() |
> >> |- binode_flags |= |
> >> BTRFS_INODE_NOCOMPRESS |
> >> | compress_file_range()
> >> | |- inode_need_compress = false
> >> | |- nr_page = 1 while pages = NULL
> >> | | Then hit the crash
> >>
> >> [FIX]
> >> This patch will fix it by checking @pages before doing accessing it.
> >> This patch is only designed as a hot fix and easy to backport.
> >>
> >> More elegant fix may make btrfs only check inode_need_compress() once to
> >> avoid such race, but that would be another story.
> >
> > So why not do the elegant fix in the first place rather than adding
> > cruft like this hotfix which later has to be cleaned up when the
> > 'proper' fix lands?
>
> For backport purpose.
>
> This is reported from one vendor kernel, not upstream.
> Thus backport is definitely required.

Agreed, minimal fixes are desired when possible. For example this patch
got to from 4.14 up to 5.8, with minimal or no conflicts.