i386/audit: stop scribbling on the stack frame

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.

Reported-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Link: http://lkml.kernel.org/r/1414037043-30647-1-git-send-email-eparis@redhat.com
Cc: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 7 insertions, 8 deletions

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index b553ed89e5f5..344b63f18d14 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