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

Eric Paris
2014-10-23T04:04:03Z

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

--
1.9.3