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

On Thu, Oct 23, 2014 at 12:30 PM, Eric Paris <eparis(a)redhat.com&gt; 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(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? > > 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) > > /me anxiously awaits x86 guy to tell me how dumb I am.... > hpa, do you have time to figure this out? I don't know the 32-bit ABI well enough, nor will I have time to disassemble things or find and read the spec to figure this out in the next few days. --Andy > $ 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 > >