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Date:	Thu, 23 Oct 2014 12:20:51 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Eric Paris <eparis@...hat.com>
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, 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?

>> How hard would it be to just delete it and replace it with a
>> straightforward two-phase trace invocation a la x86_64?
>
> For me?  Hard.
>

I can try it eventually, but my track record with the 32-bit entry
code isn't so good.  I agree that this would be pretty nasty for
urgent, but at least neither that or your fix would need to go to
-stable.

--Andy
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