x86/ptrace: Stop setting TS_COMPAT in ptrace code

Setting TS_COMPAT in ptrace is wrong: if we happen to do it during
syscall entry, then we'll confuse seccomp and audit.  (The former
isn't a security problem: seccomp is currently entirely insecure if a
malicious ptracer is attached.)  As a minimal fix, this patch adds a
new flag TS_I386_REGS_POKED that handles the ptrace special case.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pedro Alves <palves@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/5383ebed38b39fa37462139e337aff7f2314d1ca.1469599803.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/ptrace.c

@@ -923,15 +923,18 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
 
 	case offsetof(struct user32, regs.orig_eax):
 		/*
-		 * A 32-bit debugger setting orig_eax means to restore
-		 * the state of the task restarting a 32-bit syscall.
-		 * Make sure we interpret the -ERESTART* codes correctly
-		 * in case the task is not actually still sitting at the
-		 * exit from a 32-bit syscall with TS_COMPAT still set.
+		 * Warning: bizarre corner case fixup here.  A 32-bit
+		 * debugger setting orig_eax to -1 wants to disable
+		 * syscall restart.  Make sure that the syscall
+		 * restart code sign-extends orig_ax.  Also make sure
+		 * we interpret the -ERESTART* codes correctly if
+		 * loaded into regs->ax in case the task is not
+		 * actually still sitting at the exit from a 32-bit
+		 * syscall with TS_COMPAT still set.
 		 */
 		regs->orig_ax = value;
 		if (syscall_get_nr(child, regs) >= 0)
-			task_thread_info(child)->status |= TS_COMPAT;
+			task_thread_info(child)->status |= TS_I386_REGS_POKED;
 		break;
 
 	case offsetof(struct user32, regs.eflags):