Re: [PATCH] i386/audit: stop scribbling on the stack frame

Richard Guy Briggs
2014-10-27T02:01:40Z

On 14/10/23, 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 <eparis@redhat.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@redhat.com>
> > >> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > >> > Cc: Ingo Molnar <mingo@redhat.com>
> > >> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > >> > 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? 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)

(Credit to HPA for walking me through some of this... I had to stare at
it for a while...)

The bottom of the stack was dropped to reuse syscall args a1-a3 on the
stack for the call to __audit_syscall_entry while %ebx wasn't changed in
the call. a0 was in the unchanged %ebx and the pushl_cfi directly
restores the value in PT_EBX when it puts a0 back on the stack and
restores the original value of %esp.

> /me anxiously awaits x86 guy to tell me how dumb I am....

("Dumber" tries to explain...)

> $ 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
>
>

- RGB

--
Richard Guy Briggs <rbriggs@redhat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
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