[PATCH 2/2] pipe: Fix missing mask update after pipe_wait()

[David Howells]

Fix pipe_write() to regenerate the ring index mask and update max_usage
after calling pipe_wait().

This is necessary as the latter function drops the pipe lock, thereby
allowing F_SETPIPE_SZ change it.  Without this, pipe_write() may
subsequently miscalculate the array indices and pipe fullness, leading to
an oops like the following:

 BUG: KASAN: slab-out-of-bounds in pipe_write+0xc25/0xe10 fs/pipe.c:481
 Write of size 8 at addr ffff8880771167a8 by task syz-executor.3/7987
 ...
 CPU: 1 PID: 7987 Comm: syz-executor.3 Not tainted 5.4.0-rc2-syzkaller #0
 ...
 Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x113/0x167 lib/dump_stack.c:113
  print_address_description.constprop.8.cold.10+0x9/0x31d mm/kasan/report.c:374
  __kasan_report.cold.11+0x1b/0x3a mm/kasan/report.c:506
  kasan_report+0x12/0x20 mm/kasan/common.c:634
  __asan_report_store8_noabort+0x17/0x20 mm/kasan/generic_report.c:137
  pipe_write+0xc25/0xe10 fs/pipe.c:481
  call_write_iter include/linux/fs.h:1895 [inline]
  new_sync_write+0x3fd/0x7e0 fs/read_write.c:483
  __vfs_write+0x94/0x110 fs/read_write.c:496
  vfs_write+0x18a/0x520 fs/read_write.c:558
  ksys_write+0x105/0x220 fs/read_write.c:611
  __do_sys_write fs/read_write.c:623 [inline]
  __se_sys_write fs/read_write.c:620 [inline]
  __x64_sys_write+0x6e/0xb0 fs/read_write.c:620
  do_syscall_64+0xca/0x5d0 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 8cefc107ca54 ("pipe: Use head and tail pointers for the ring, not cursor and length")
Reported-by: syzbot+838eb0878ffd51f27c41@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Eric Biggers <ebiggers@xxxxxxxxxx>
---

 fs/pipe.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/pipe.c b/fs/pipe.c
index 5f89f73d4366..4d2a7bbc5d31 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -526,6 +526,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		}
 		pipe->waiting_writers++;
 		pipe_wait(pipe);
+		mask = pipe->ring_size - 1;
+		max_usage = pipe->max_usage;
 		pipe->waiting_writers--;
 	}
 out: