From mboxrd@z Thu Jan 1 00:00:00 1970 Reply-To: kernel-hardening@lists.openwall.com Date: Thu, 16 Jun 2016 13:33:03 -0500 From: Josh Poimboeuf Message-ID: <20160616183303.puhiggbbxza2bxcv@treble> References: <20160616181642.r2bpceuvvffttp7r@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Subject: [kernel-hardening] Re: [PATCH 10/13] x86/dumpstack: Try harder to get a call trace on stack overflow To: Andy Lutomirski 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 11:22:14AM -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? -- Josh