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

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Date:	Thu, 16 Jun 2016 13:33:03 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <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 Thu, Jun 16, 2016 at 11:22:14AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 16, 2016 at 11:16 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > 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...)
> 
> I did mean what I wrote, because I wanted to start at the bottom of
> the validly allocated area.  IOW I wanted to do the minimum possible
> backward jump to make the code display something.

But why the "+ 1"?  Is that a hack to make it pass the valid_stack_ptr()
check?

-- 
Josh