From mboxrd@z Thu Jan 1 00:00:00 1970 Reply-To: kernel-hardening@lists.openwall.com MIME-Version: 1.0 In-Reply-To: <20160616183303.puhiggbbxza2bxcv@treble> References: <20160616181642.r2bpceuvvffttp7r@treble> <20160616183303.puhiggbbxza2bxcv@treble> From: Andy Lutomirski Date: Thu, 16 Jun 2016 11:37:07 -0700 Message-ID: Content-Type: text/plain; charset=UTF-8 Subject: [kernel-hardening] Re: [PATCH 10/13] x86/dumpstack: Try harder to get a call trace on stack overflow To: Josh Poimboeuf Cc: Andy Lutomirski , "linux-kernel@vger.kernel.org" , X86 ML , Borislav Petkov , Nadav Amit , Kees Cook , Brian Gerst , "kernel-hardening@lists.openwall.com" , Linus Torvalds List-ID: On Thu, Jun 16, 2016 at 11:33 AM, Josh Poimboeuf wrote: > On Thu, Jun 16, 2016 at 11:22:14AM -0700, Andy Lutomirski wrote: >> On Thu, Jun 16, 2016 at 11:16 AM, Josh Poimboeuf 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 >> >> --- >> >> 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? Yes. But hmm. Maybe the right fix is to drop the + 1 and to change the last line of valid_stck_ptr from: return p > t && p < t + THREAD_SIZE - size; to: return p >= t && p < t + THREAD_SIZE - size; The current definition of valid_stack_ptr is certainly nonsensical. It should either be p >= t or p >= t + 1. --Andy