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Date: Thu, 23 Oct 2014 15:30:24 -0400
From: Eric Paris <eparis@...hat.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Richard Guy Briggs <rgb@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
linux-audit@...hat.com
Subject: Re: [PATCH] i386/audit: stop scribbling on the stack frame
On Thu, 2014-10-23 at 12:20 -0700, Andy Lutomirski wrote:
> On Thu, Oct 23, 2014 at 12:15 PM, Eric Paris <eparis@...hat.com> 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 <eparis@...hat.com>
> >> > Cc: Thomas Gleixner <tglx@...utronix.de>
> >> > Cc: Ingo Molnar <mingo@...hat.com>
> >> > Cc: "H. Peter Anvin" <hpa@...or.com>
> >> > Cc: x86@...nel.org
> >> > Cc: linux-kernel@...r.kernel.org
> >> > Cc: linux-audit@...hat.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? 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
--
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