[patch] i386 seccomp fix for auditing/ptrace

[Andrea Arcangeli <andrea@cpushare.com>]

Hello,

This is the same issue as ppc64 before, when returning to userland we
shouldn't re-compute the seccomp check or the task could be killed
during sigreturn when orig_eax is overwritten by the sigreturn syscall.
This was found by Roland.

This was harmless from a security standpoint, but some i686 users
reported failures with auditing enabled system wide (some distro
surprisingly makes it the default) and I reproduced it too by keeping
the whole workload under strace -f.

Patch is tested and works for me under strace -f.

nobody@athlon:~/cpushare> strace -o /tmp/o -f python seccomp_test.py
make: Nothing to be done for `seccomp_test'.
Starting computing some malicious bytecode
init
load
start
stop
receive_data failure
kill
exit_code 0 signal 9
The malicious bytecode has been killed successfully by seccomp
Starting computing some safe bytecode
init
load
start
stop
174 counts
kill
exit_code 0 signal 0
The seccomp_test.py completed successfully, thank you for testing.

Thanks.

Signed-off-by: Andrea Arcangeli <andrea@cpushare.com>

diff -r 1df7bfbb783f arch/i386/kernel/ptrace.c
--- a/arch/i386/kernel/ptrace.c	Fri Sep  2 09:01:35 2005
+++ b/arch/i386/kernel/ptrace.c	Mon Sep  5 05:30:49 2005
@@ -680,8 +680,9 @@
 __attribute__((regparm(3)))
 void do_syscall_trace(struct pt_regs *regs, int entryexit)
 {
-	/* do the secure computing check first */
-	secure_computing(regs->orig_eax);
+	if (!entryexit)
+		/* do the secure computing check first */
+		secure_computing(regs->orig_eax);
 
 	if (unlikely(current->audit_context) && entryexit)
 		audit_syscall_exit(current, AUDITSC_RESULT(regs->eax), regs->eax);