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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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