Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932323AbaJWTif (ORCPT ); Thu, 23 Oct 2014 15:38:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60154 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932071AbaJWTid (ORCPT ); Thu, 23 Oct 2014 15:38:33 -0400 Message-ID: <1414093104.2363.14.camel@redhat.com> Subject: Re: [PATCH] i386/audit: stop scribbling on the stack frame From: Eric Paris To: Andy Lutomirski Cc: Richard Guy Briggs , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , X86 ML , "linux-kernel@vger.kernel.org" , linux-audit@redhat.com Date: Thu, 23 Oct 2014 15:38:24 -0400 In-Reply-To: <1414092624.2363.13.camel@redhat.com> References: <1414037043-30647-1-git-send-email-eparis@redhat.com> <54494B74.2080504@amacapital.net> <1414091732.2363.11.camel@redhat.com> <1414092624.2363.13.camel@redhat.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-10-23 at 15:30 -0400, Eric Paris wrote: > On Thu, 2014-10-23 at 12:20 -0700, Andy Lutomirski wrote: > > On Thu, Oct 23, 2014 at 12:15 PM, Eric Paris wrote: > > > On Thu, 2014-10-23 at 11:39 -0700, Andy Lutomirski wrote: > > >> On 10/22/2014 09:04 PM, Eric Paris wrote: > > >> > git commit b4f0d3755c5e9cc86292d5fd78261903b4f23d4a was very very dumb. > > >> > It was writing over %esp/pt_regs semi-randomly on i686 with the expected > > >> > "system can't boot" results. As noted in: > > >> > > > >> > https://bugs.freedesktop.org/show_bug.cgi?id=85277 > > >> > > > >> > This patch stops fscking with pt_regs. Instead it sets up the registers > > >> > for the call to __audit_syscall_entry in the most obvious conceivable > > >> > way. It then does just a tiny tiny touch of magic. We need to get what > > >> > started in PT_EDX into 0(%esp) and PT_ESI into 4(%esp). This is as easy > > >> > as a pair of pushes. > > >> > > > >> > After the call to __audit_syscall_entry all we need to do is get that > > >> > now useless junk off the stack (pair of pops) and reload %eax with the > > >> > original syscall so other stuff can keep going about it's business. > > >> > > > >> > Signed-off-by: Eric Paris > > >> > Cc: Thomas Gleixner > > >> > Cc: Ingo Molnar > > >> > Cc: "H. Peter Anvin" > > >> > Cc: x86@kernel.org > > >> > Cc: linux-kernel@vger.kernel.org > > >> > Cc: linux-audit@redhat.com > > >> > --- > > >> > arch/x86/kernel/entry_32.S | 15 +++++++-------- > > >> > 1 file changed, 7 insertions(+), 8 deletions(-) > > >> > > > >> > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S > > >> > index f9e3fab..fb01d22 100644 > > >> > --- a/arch/x86/kernel/entry_32.S > > >> > +++ b/arch/x86/kernel/entry_32.S > > >> > @@ -447,15 +447,14 @@ sysenter_exit: > > >> > sysenter_audit: > > >> > testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags(%ebp) > > >> > jnz syscall_trace_entry > > >> > - addl $4,%esp > > >> > - CFI_ADJUST_CFA_OFFSET -4 > > >> > - movl %esi,4(%esp) /* 5th arg: 4th syscall arg */ > > >> > - movl %edx,(%esp) /* 4th arg: 3rd syscall arg */ > > >> > - /* %ecx already in %ecx 3rd arg: 2nd syscall arg */ > > >> > - movl %ebx,%edx /* 2nd arg: 1st syscall arg */ > > >> > - /* %eax already in %eax 1st arg: syscall number */ > > >> > + /* movl PT_EAX(%esp), %eax already set, syscall number: 1st arg to audit */ > > >> > + movl PT_EBX(%esp), %edx /* ebx/a0: 2nd arg to audit */ > > >> > + /* movl PT_ECX(%esp), %ecx already set, a1: 3nd arg to audit */ > > >> > + pushl_cfi PT_ESI(%esp) /* a3: 5th arg */ > > >> > + pushl_cfi PT_EDX+4(%esp) /* a2: 4th arg */ > > >> > call __audit_syscall_entry > > >> > - pushl_cfi %ebx > > >> > + popl_cfi %ecx /* get that remapped edx off the stack */ > > >> > + popl_cfi %ecx /* get that remapped esi off the stack */ > > >> > movl PT_EAX(%esp),%eax /* reload syscall number */ > > >> > jmp sysenter_do_call > > >> > > > >> > > > >> > > >> This looks reasonably likely to be correct, but this code is complicated > > >> and now ever slower. > > > > > > I guess I could just use push/pop and do the CFI_ADJUST_CFA_OFFSET by > > > hand. But I figured this was reasonable enough... > > > > > > > I'm not complaining about your new assembly in particular. There's > > just too much assembly in there in general. > > > > But I feel like I'm missing something in the new code. Aren't you > > corrupting ecx with those popl_cfi insns? > > After the call __audit_syscall_entry aren't they already polluted? > Isn't that the reason we need to reload EAX? Well, I guess EAX is special... > You can verify this leaves > things in a similar state (although slightly differently polluted) than > before it got screwed up. Here is diff between before the breakage and > what I propose we do now. > > (I admit I don't understand how the pushl_cfi %ebx wasn't messing up > PT_EBX) > > /me anxiously awaits x86 guy to tell me how dumb I am.... > > $ git diff a17c8b54dc738c4fda31e8be0302cd131a04c19f -- arch/x86/kernel/entry_32.S > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S > index 0d0c9d4..fb01d22 100644 > --- a/arch/x86/kernel/entry_32.S > +++ b/arch/x86/kernel/entry_32.S > @@ -447,16 +447,14 @@ sysenter_exit: > sysenter_audit: > testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags(%ebp) > jnz syscall_trace_entry > - addl $4,%esp > - CFI_ADJUST_CFA_OFFSET -4 > - /* %esi already in 8(%esp) 6th arg: 4th syscall arg */ > - /* %edx already in 4(%esp) 5th arg: 3rd syscall arg */ > - /* %ecx already in 0(%esp) 4th arg: 2nd syscall arg */ > - movl %ebx,%ecx /* 3rd arg: 1st syscall arg */ > - movl %eax,%edx /* 2nd arg: syscall number */ > - movl $AUDIT_ARCH_I386,%eax /* 1st arg: audit arch */ > + /* movl PT_EAX(%esp), %eax already set, syscall number: 1st arg to audit */ > + movl PT_EBX(%esp), %edx /* ebx/a0: 2nd arg to audit */ > + /* movl PT_ECX(%esp), %ecx already set, a1: 3nd arg to audit */ > + pushl_cfi PT_ESI(%esp) /* a3: 5th arg */ > + pushl_cfi PT_EDX+4(%esp) /* a2: 4th arg */ > call __audit_syscall_entry > - pushl_cfi %ebx > + popl_cfi %ecx /* get that remapped edx off the stack */ > + popl_cfi %ecx /* get that remapped esi off the stack */ > movl PT_EAX(%esp),%eax /* reload syscall number */ > jmp sysenter_do_call > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/