pipe: Fix missing mask update after pipe_wait()

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:
    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: 8cefc107 ("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>
[ Changed it to use a temporary variable 'mask' to avoid long lines -Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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);
@@ -408,12 +408,11 @@ 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) {
+		unsigned int mask = pipe->ring_size - 1;
 		struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
 		int offset = buf->offset + buf->len;
 
@@ -443,7 +442,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		}
 
 		head = pipe->head;
-		if (!pipe_full(head, pipe->tail, max_usage)) {
+		if (!pipe_full(head, pipe->tail, pipe->max_usage)) {
+			unsigned int mask = pipe->ring_size - 1;
 			struct pipe_buffer *buf = &pipe->bufs[head & mask];
 			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;
 			}
@@ -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. */