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

H. Peter Anvin
2014-10-23T19:13:31Z

Well, probably not something for stable/urgent...

On October 23, 2014 11:39:48 AM PDT, Andy Lutomirski <luto@amacapital.net> 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.
>
>How hard would it be to just delete it and replace it with a
>straightforward two-phase trace invocation a la x86_64?
>
>--Andy

--
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