fix a fencepost error in pipe_advance()

The logics in pipe_advance() used to release all buffers past the new
position failed in cases when the number of buffers to release was equal
to pipe->buffers.  If that happened, none of them had been released,
leaving pipe full.  Worse, it was trivial to trigger and we end up with
pipe full of uninitialized pages.  IOW, it's an infoleak.

Cc: stable@vger.kernel.org # v4.9
Reported-by: "Alan J. Wylie" <alan@wylie.me.uk>
Tested-by: "Alan J. Wylie" <alan@wylie.me.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -730,43 +730,50 @@ size_t iov_iter_copy_from_user_atomic(struct page *page,
 }
 EXPORT_SYMBOL(iov_iter_copy_from_user_atomic);
 
-static void pipe_advance(struct iov_iter *i, size_t size)
+static inline void pipe_truncate(struct iov_iter *i)
 {
 	struct pipe_inode_info *pipe = i->pipe;
-	struct pipe_buffer *buf;
+	if (pipe->nrbufs) {
+		size_t off = i->iov_offset;
 		int idx = i->idx;
-	size_t off = i->iov_offset, orig_sz;
+		int nrbufs = (idx - pipe->curbuf) & (pipe->buffers - 1);
+		if (off) {
+			pipe->bufs[idx].len = off - pipe->bufs[idx].offset;
+			idx = next_idx(idx, pipe);
+			nrbufs++;
+		}
+		while (pipe->nrbufs > nrbufs) {
+			pipe_buf_release(pipe, &pipe->bufs[idx]);
+			idx = next_idx(idx, pipe);
+			pipe->nrbufs--;
+		}
+	}
+}
 
+static void pipe_advance(struct iov_iter *i, size_t size)
+{
+	struct pipe_inode_info *pipe = i->pipe;
 	if (unlikely(i->count < size))
 		size = i->count;
-	orig_sz = size;
-
 	if (size) {
+		struct pipe_buffer *buf;
+		size_t off = i->iov_offset, left = size;
+		int idx = i->idx;
 		if (off) /* make it relative to the beginning of buffer */
-			size += off - pipe->bufs[idx].offset;
+			left += off - pipe->bufs[idx].offset;
 		while (1) {
 			buf = &pipe->bufs[idx];
-			if (size <= buf->len)
+			if (left <= buf->len)
 				break;
-			size -= buf->len;
+			left -= buf->len;
 			idx = next_idx(idx, pipe);
 		}
-		buf->len = size;
 		i->idx = idx;
-		off = i->iov_offset = buf->offset + size;
-	}
-	if (off)
-		idx = next_idx(idx, pipe);
-	if (pipe->nrbufs) {
-		int unused = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);
-		/* [curbuf,unused) is in use.  Free [idx,unused) */
-		while (idx != unused) {
-			pipe_buf_release(pipe, &pipe->bufs[idx]);
-			idx = next_idx(idx, pipe);
-			pipe->nrbufs--;
-		}
+		i->iov_offset = buf->offset + left;
 	}
-	i->count -= orig_sz;
+	i->count -= size;
+	/* ... and discard everything past that point */
+	pipe_truncate(i);
 }
 
 void iov_iter_advance(struct iov_iter *i, size_t size)
@@ -826,6 +833,7 @@ void iov_iter_pipe(struct iov_iter *i, int direction,
 			size_t count)
 {
 	BUG_ON(direction != ITER_PIPE);
+	WARN_ON(pipe->nrbufs == pipe->buffers);
 	i->type = direction;
 	i->pipe = pipe;
 	i->idx = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);