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

[David Howells <dhowells@redhat.com>]

Fix pipe_write() to not cache the ring index mask and max_usage as their
values are invalidated by calling pipe_wait() because the latter function
drops the pipe lock, thereby allowing F_SETPIPE_SZ change them.  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

This is not a problem for pipe_read() as the mask is recalculated on each
pass of the loop, after pipe_wait() has been called.

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

 fs/pipe.c |   18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index da782ee251d2..8061b093140d 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -389,7 +389,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 {
 	struct file *filp = iocb->ki_filp;
 	struct pipe_inode_info *pipe = filp->private_data;
-	unsigned int head, max_usage, mask;
+	unsigned int head;
 	ssize_t ret = 0;
 	int do_wakeup = 0;
 	size_t total_len = iov_iter_count(from);
@@ -408,13 +408,12 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	}
 
 	head = pipe->head;
-	max_usage = pipe->max_usage;
-	mask = pipe->ring_size - 1;
 
 	/* We try to merge small writes */
 	chars = total_len & (PAGE_SIZE-1); /* size of the last buffer */
 	if (!pipe_empty(head, pipe->tail) && chars != 0) {
-		struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
+		struct pipe_buffer *buf =
+			&pipe->bufs[(head - 1) & (pipe->ring_size - 1)];
 		int offset = buf->offset + buf->len;
 
 		if (pipe_buf_can_merge(buf) && offset + chars <= PAGE_SIZE) {
@@ -443,8 +442,9 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		}
 
 		head = pipe->head;
-		if (!pipe_full(head, pipe->tail, max_usage)) {
-			struct pipe_buffer *buf = &pipe->bufs[head & mask];
+		if (!pipe_full(head, pipe->tail, pipe->max_usage)) {
+			struct pipe_buffer *buf =
+				&pipe->bufs[head & (pipe->ring_size - 1)];
 			struct page *page = pipe->tmp_page;
 			int copied;
 
@@ -465,7 +465,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			spin_lock_irq(&pipe->wait.lock);
 
 			head = pipe->head;
-			if (pipe_full(head, pipe->tail, max_usage)) {
+			if (pipe_full(head, pipe->tail, pipe->max_usage)) {
 				spin_unlock_irq(&pipe->wait.lock);
 				continue;
 			}
@@ -484,7 +484,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 
 			/* Insert it into the buffer array */
-			buf = &pipe->bufs[head & mask];
+			buf = &pipe->bufs[head & (pipe->ring_size - 1)];
 			buf->page = page;
 			buf->ops = &anon_pipe_buf_ops;
 			buf->offset = 0;
@@ -510,7 +510,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 				break;
 		}
 
-		if (!pipe_full(head, pipe->tail, max_usage))
+		if (!pipe_full(head, pipe->tail, pipe->max_usage))
 			continue;
 
 		/* Wait for buffer space to become available. */