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Message-ID: <20160616183303.puhiggbbxza2bxcv@treble>
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
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