Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932174AbcCJUTi (ORCPT ); Thu, 10 Mar 2016 15:19:38 -0500 Received: from mail-wm0-f51.google.com ([74.125.82.51]:38359 "EHLO mail-wm0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932123AbcCJUTg (ORCPT ); Thu, 10 Mar 2016 15:19:36 -0500 From: Rabin Vincent To: viro@zeniv.linux.org.uk, Steven Rostedt Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Rabin Vincent Subject: [PATCH] splice: handle zero nr_pages in splice_to_pipe() Date: Thu, 10 Mar 2016 21:19:06 +0100 Message-Id: <1457641146-9068-1-git-send-email-rabin@rab.in> X-Mailer: git-send-email 2.7.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2222 Lines: 63 Running the following command: busybox cat /sys/kernel/debug/tracing/trace_pipe > /dev/null with any tracing enabled pretty very quickly leads to various NULL pointer dereferences and VM BUG_ON()s, such as these: BUG: unable to handle kernel NULL pointer dereference at 0000000000000020 IP: [] generic_pipe_buf_release+0xc/0x40 Call Trace: [] splice_direct_to_actor+0x143/0x1e0 [] ? generic_pipe_buf_nosteal+0x10/0x10 [] do_splice_direct+0x8f/0xb0 [] do_sendfile+0x199/0x380 [] SyS_sendfile64+0x90/0xa0 [] entry_SYSCALL_64_fastpath+0x12/0x6d page dumped because: VM_BUG_ON_PAGE(atomic_read(&page->_count) == 0) kernel BUG at include/linux/mm.h:367! invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC RIP: [] generic_pipe_buf_release+0x3c/0x40 Call Trace: [] splice_direct_to_actor+0x143/0x1e0 [] ? generic_pipe_buf_nosteal+0x10/0x10 [] do_splice_direct+0x8f/0xb0 [] do_sendfile+0x199/0x380 [] SyS_sendfile64+0x90/0xa0 [] tracesys_phase2+0x84/0x89 (busybox's cat uses sendfile(2), unlike the coreutils version) This is because tracing_splice_read_pipe() can call splice_to_pipe() with spd->nr_pages == 0. spd_pages underflows in splice_to_pipe() and we fill the page pointers and the other fields of the pipe_buffers with garbage. All other callers of splice_to_pipe() avoid calling it when nr_pages == 0, and we could make tracing_splice_read_pipe() do that too, but it seems reasonable to have splice_to_page() handle this condition gracefully. Cc: stable@vger.kernel.org Signed-off-by: Rabin Vincent --- fs/splice.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/splice.c b/fs/splice.c index 82bc0d64fc38..19e0b103d253 100644 --- a/fs/splice.c +++ b/fs/splice.c @@ -185,6 +185,9 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe, unsigned int spd_pages = spd->nr_pages; int ret, do_wakeup, page_nr; + if (!spd_pages) + return 0; + ret = 0; do_wakeup = 0; page_nr = 0; -- 2.7.0