[PATCH 2/2] btrfs: prevent copying too big compressed lzo segment

["Dāvis Mosāns" <davispuh@gmail.com>]

Compressed length can be corrupted to be a lot larger than memory
we have allocated for buffer.
This will cause memcpy in copy_compressed_segment to write outside
of allocated memory.

This mostly results in stuck read syscall but sometimes when using
btrfs send can get #GP

kernel: general protection fault, probably for non-canonical address 0x841551d5c1000: 0000 [#1] PREEMPT SMP NOPTI
kernel: CPU: 17 PID: 264 Comm: kworker/u256:7 Tainted: P           OE     5.17.0-rc2-1 #12
kernel: Workqueue: btrfs-endio btrfs_work_helper [btrfs]
kernel: RIP: 0010:lzo_decompress_bio (./include/linux/fortify-string.h:225 fs/btrfs/lzo.c:322 fs/btrfs/lzo.c:394) btrfs
Code starting with the faulting instruction
===========================================
   0:*  48 8b 06                mov    (%rsi),%rax              <-- trapping instruction
   3:   48 8d 79 08             lea    0x8(%rcx),%rdi
   7:   48 83 e7 f8             and    $0xfffffffffffffff8,%rdi
   b:   48 89 01                mov    %rax,(%rcx)
   e:   44 89 f0                mov    %r14d,%eax
  11:   48 8b 54 06 f8          mov    -0x8(%rsi,%rax,1),%rdx
kernel: RSP: 0018:ffffb110812efd50 EFLAGS: 00010212
kernel: RAX: 0000000000001000 RBX: 000000009ca264c8 RCX: ffff98996e6d8ff8
kernel: RDX: 0000000000000064 RSI: 000841551d5c1000 RDI: ffffffff9500435d
kernel: RBP: ffff989a3be856c0 R08: 0000000000000000 R09: 0000000000000000
kernel: R10: 0000000000000000 R11: 0000000000001000 R12: ffff98996e6d8000
kernel: R13: 0000000000000008 R14: 0000000000001000 R15: 000841551d5c1000
kernel: FS:  0000000000000000(0000) GS:ffff98a09d640000(0000) knlGS:0000000000000000
kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: CR2: 00001e9f984d9ea8 CR3: 000000014971a000 CR4: 00000000003506e0
kernel: Call Trace:
kernel:  <TASK>
kernel: end_compressed_bio_read (fs/btrfs/compression.c:104 fs/btrfs/compression.c:1363 fs/btrfs/compression.c:323) btrfs
kernel: end_workqueue_fn (fs/btrfs/disk-io.c:1923) btrfs
kernel: btrfs_work_helper (fs/btrfs/async-thread.c:326) btrfs
kernel: process_one_work (./arch/x86/include/asm/jump_label.h:27 ./include/linux/jump_label.h:212 ./include/trace/events/workqueue.h:108 kernel/workqueue.c:2312)
kernel: worker_thread (./include/linux/list.h:292 kernel/workqueue.c:2455)
kernel: ? process_one_work (kernel/workqueue.c:2397)
kernel: kthread (kernel/kthread.c:377)
kernel: ? kthread_complete_and_exit (kernel/kthread.c:332)
kernel: ret_from_fork (arch/x86/entry/entry_64.S:301)
kernel:  </TASK>

Signed-off-by: Dāvis Mosāns <davispuh@gmail.com>
---
 fs/btrfs/lzo.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
index 31319dfcc9fb..ebaa5083f2ae 100644
--- a/fs/btrfs/lzo.c
+++ b/fs/btrfs/lzo.c
@@ -383,6 +383,13 @@ int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 		kunmap(cur_page);
 		cur_in += LZO_LEN;
 
+		if (seg_len > WORKSPACE_CBUF_LENGTH) {
+			// seg_len shouldn't be larger than we have allocated for workspace->cbuf
+			btrfs_err(fs_info, "unexpectedly large lzo segment len %u", seg_len);
+			ret = -EUCLEAN;
+			goto out;
+		}
+
 		/* Copy the compressed segment payload into workspace */
 		copy_compressed_segment(cb, workspace->cbuf, seg_len, &cur_in);
 
-- 
2.35.1