[PATCH 5.7 032/204] btrfs: inode: fix NULL pointer dereference if inode doesnt need compression

Greg Kroah-Hartman
2020-08-20T13:10:50Z

From: Qu Wenruo <wqu@suse.com>

commit 1e6e238c3002ea3611465ce5f32777ddd6a40126 upstream.

[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.

Reported-by: Luciano Chavez <chavez@us.ibm.com>
Fixes: 4d3a800ebb12 ("btrfs: merge nr_pages input and output parameter in compress_pages")
CC: stable@vger.kernel.org # 4.14.x: cecc8d9038d16: btrfs: Move free_pages_out label in inline extent handling branch in compress_file_range
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/inode.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)

--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -650,12 +650,18 @@ cont:
page_error_op |
PAGE_END_WRITEBACK);

- for (i = 0; i < nr_pages; i++) {
- WARN_ON(pages[i]->mapping);
- put_page(pages[i]);
+ /*
+ * Ensure we only free the compressed pages if we have
+ * them allocated, as we can still reach here with
+ * inode_need_compress() == false.
+ */
+ if (pages) {
+ for (i = 0; i < nr_pages; i++) {
+ WARN_ON(pages[i]->mapping);
+ put_page(pages[i]);
+ }
+ kfree(pages);
}
- kfree(pages);
-
return 0;
}
}