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

On Thu, Oct 23, 2014 at 12:15 PM, Eric Paris <eparis(a)redhat.com&gt; 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(a)redhat.com&gt; >> > Cc: Thomas Gleixner <tglx(a)linutronix.de&gt; >> > Cc: Ingo Molnar <mingo(a)redhat.com&gt; >> > Cc: "H. Peter Anvin" <hpa(a)zytor.com&gt; >> > Cc: x86(a)kernel.org >> > Cc: linux-kernel(a)vger.kernel.org >> > Cc: linux-audit(a)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?