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

Eric Paris
2014-10-23T19:15:32Z

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...

> 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.