[PATCH] x86_32,entry: Do syscall exit work on badsys (CVE-2014-4508)

[Andy Lutomirski]

The bad syscall nr paths are their own incomprehensible route
through the entry control flow.  Rearrange them to work just like
syscalls that return -ENOSYS.

This fixes an OOPS in the audit code when fast-path auditing is
enabled and sysenter gets a bad syscall nr (CVE-2014-4508).

This has probably been broken since Linux 2.6.27:
af0575bba0 i386 syscall audit fast-path

Cc: stable@xxxxxxxxxxxxxxx
Cc: Roland McGrath <roland@xxxxxxxxxx>
Reported-by: Toralf FÃrster <toralf.foerster@xxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---

I realize that the syscall audit fast path and badsys code, on 32-bit
x86 no less, is possibly one of the least fun things in the kernel to
review, but this is still a real security bug and should get fixed :(

So I'm cc-ing a bunch of people and maybe someone will review it.

 arch/x86/kernel/entry_32.S | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index a2a4f46..f4258a5 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -431,9 +431,10 @@ sysenter_past_esp:
 	jnz sysenter_audit
 sysenter_do_call:
 	cmpl $(NR_syscalls), %eax
-	jae syscall_badsys
+	jae sysenter_badsys
 	call *sys_call_table(,%eax,4)
 	movl %eax,PT_EAX(%esp)
+sysenter_after_call:
 	LOCKDEP_SYS_EXIT
 	DISABLE_INTERRUPTS(CLBR_ANY)
 	TRACE_IRQS_OFF
@@ -688,7 +689,12 @@ END(syscall_fault)
 
 syscall_badsys:
 	movl $-ENOSYS,PT_EAX(%esp)
-	jmp resume_userspace
+	jmp syscall_exit
+END(syscall_badsys)
+
+sysenter_badsys:
+	movl $-ENOSYS,PT_EAX(%esp)
+	jmp sysenter_after_call
 END(syscall_badsys)
 	CFI_ENDPROC
 /*
-- 
1.9.3

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