linux-kernel - Re: [PATCH 10/13] x86/dumpstack: Try harder to get a call trace on stack overflow

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Message-ID: <20160616181642.r2bpceuvvffttp7r@treble>
Date:	Thu, 16 Jun 2016 13:16:42 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	x86@...nel.org, Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 10/13] x86/dumpstack: Try harder to get a call trace on
 stack overflow

On Wed, Jun 15, 2016 at 05:28:32PM -0700, Andy Lutomirski wrote:
> If we overflow the stack, print_context_stack will abort.  Detect
> this case and rewind back into the valid part of the stack so that
> we can trace it.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/kernel/dumpstack.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> index d4d085e27d04..400a2e17c1d1 100644
> --- a/arch/x86/kernel/dumpstack.c
> +++ b/arch/x86/kernel/dumpstack.c
> @@ -100,6 +100,13 @@ print_context_stack(struct thread_info *tinfo,
>  {
>  	struct stack_frame *frame = (struct stack_frame *)bp;
>  
> +	/*
> +	 * If we overflowed the stack into a guard page, jump back to the
> +	 * bottom of the usable stack.
> +	 */
> +	if ((unsigned long)tinfo - (unsigned long)stack < PAGE_SIZE)
> +		stack = (unsigned long *)tinfo + 1;

That will start walking the stack in the middle of the thread_info
struct.

I think you meant:

		stack = (unsigned long *)(tinfo + 1)

However, thread_info will have been overwritten anyway.  So maybe it
should just be:

		stack = tinfo;

(Though that still wouldn't quite work because the valid_stack_ptr()
check would fail...)

> +
>  	while (valid_stack_ptr(tinfo, stack, sizeof(*stack), end)) {
>  		unsigned long addr;

-- 
Josh