linux-kernel - Re: [PATCH] splice: handle zero nr_pages in splice_to

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Date:	Fri, 18 Mar 2016 15:42:58 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Rabin Vincent <rabin@....in>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] splice: handle zero nr_pages in splice_to_pipe()

On Thu, 10 Mar 2016 21:19:06 +0100
Rabin Vincent <rabin@....in> wrote:

> 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: [<ffffffff8119df6c>] generic_pipe_buf_release+0xc/0x40
>  Call Trace:
>   [<ffffffff811c48a3>] splice_direct_to_actor+0x143/0x1e0
>   [<ffffffff811c42e0>] ? generic_pipe_buf_nosteal+0x10/0x10
>   [<ffffffff811c49cf>] do_splice_direct+0x8f/0xb0
>   [<ffffffff81196869>] do_sendfile+0x199/0x380
>   [<ffffffff81197600>] SyS_sendfile64+0x90/0xa0
>   [<ffffffff8192cbee>] 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: [<ffffffff8119df9c>] generic_pipe_buf_release+0x3c/0x40
>  Call Trace:
>   [<ffffffff811c48a3>] splice_direct_to_actor+0x143/0x1e0
>   [<ffffffff811c42e0>] ? generic_pipe_buf_nosteal+0x10/0x10
>   [<ffffffff811c49cf>] do_splice_direct+0x8f/0xb0
>   [<ffffffff81196869>] do_sendfile+0x199/0x380
>   [<ffffffff81197600>] SyS_sendfile64+0x90/0xa0
>   [<ffffffff8192cd1e>] tracesys_phase2+0x84/0x89
> 
> (busybox's cat uses sendfile(2), unlike the coreutils version)

Ouch!

> 
> 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@...r.kernel.org
> Signed-off-by: Rabin Vincent <rabin@....in>
> ---

I think this should definitely be applied, but it's not my code to
apply it. I'll add this to make sure that trace_pipe is not the cause
anymore:

-- Steve

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 0ae46048f724..cb2b708e4ea7 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4954,7 +4954,10 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
 
 	spd.nr_pages = i;
 
-	ret = splice_to_pipe(pipe, &spd);
+	if (i)
+		ret = splice_to_pipe(pipe, &spd);
+	else
+		ret = 0;
 out:
 	splice_shrink_spd(&spd);
 	return ret;