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

Andy Lutomirski
2014-10-23T19:20:51Z

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?

>> 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