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/entry/common.c

@@ -270,8 +270,12 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
 	 * handling, because syscall restart has a fixup for compat
 	 * syscalls.  The fixup is exercised by the ptrace_syscall_32
 	 * selftest.
+	 *
+	 * We also need to clear TS_REGS_POKED_I386: the 32-bit tracer
+	 * special case only applies after poking regs and before the
+	 * very next return to user mode.
 	 */
-	ti->status &= ~TS_COMPAT;
+	ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
 #endif
 
 	user_enter_irqoff();

arch/x86/include/asm/syscall.h

@@ -60,7 +60,7 @@ static inline long syscall_get_error(struct task_struct *task,
 	 * TS_COMPAT is set for 32-bit syscall entries and then
 	 * remains set until we return to user mode.
 	 */
-	if (task_thread_info(task)->status & TS_COMPAT)
+	if (task_thread_info(task)->status & (TS_COMPAT|TS_I386_REGS_POKED))
 		/*
 		 * Sign-extend the value so (int)-EFOO becomes (long)-EFOO
 		 * and will match correctly in comparisons.
@@ -239,9 +239,6 @@ static inline int syscall_get_arch(void)
 	 * TS_COMPAT is set for 32-bit syscall entry and then
 	 * remains set until we return to user mode.
 	 *
-	 * TIF_IA32 tasks should always have TS_COMPAT set at
-	 * system call time.
-	 *
 	 * x32 tasks should be considered AUDIT_ARCH_X86_64.
 	 */
 	if (task_thread_info(current)->status & TS_COMPAT)

arch/x86/include/asm/thread_info.h

@@ -219,6 +219,9 @@ static inline unsigned long current_stack_pointer(void)
  * have to worry about atomic accesses.
  */
 #define TS_COMPAT		0x0002	/* 32bit syscall active (64BIT)*/
+#ifdef CONFIG_COMPAT
+#define TS_I386_REGS_POKED	0x0004	/* regs poked by 32-bit ptracer */
+#endif
 #define TS_RESTORE_SIGMASK	0x0008	/* restore signal mask in do_signal() */
 
 #ifndef __ASSEMBLY__

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):

arch/x86/kernel/signal.c

@@ -760,8 +760,30 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 
 static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
 {
-#ifdef CONFIG_X86_64
-	if (in_ia32_syscall())
+	/*
+	 * This function is fundamentally broken as currently
+	 * implemented.
+	 *
+	 * The idea is that we want to trigger a call to the
+	 * restart_block() syscall and that we want in_ia32_syscall(),
+	 * in_x32_syscall(), etc. to match whatever they were in the
+	 * syscall being restarted.  We assume that the syscall
+	 * instruction at (regs->ip - 2) matches whatever syscall
+	 * instruction we used to enter in the first place.
+	 *
+	 * The problem is that we can get here when ptrace pokes
+	 * syscall-like values into regs even if we're not in a syscall
+	 * at all.
+	 *
+	 * For now, we maintain historical behavior and guess based on
+	 * stored state.  We could do better by saving the actual
+	 * syscall arch in restart_block or (with caveats on x32) by
+	 * checking if regs->ip points to 'int $0x80'.  The current
+	 * behavior is incorrect if a tracer has a different bitness
+	 * than the tracee.
+	 */
+#ifdef CONFIG_IA32_EMULATION
+	if (current_thread_info()->status & (TS_COMPAT|TS_I386_REGS_POKED))
 		return __NR_ia32_restart_syscall;
 #endif
 #ifdef CONFIG_X86_X32_ABI