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>

1 files changed, 24 insertions, 2 deletions

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 22cc2f9f8aec..9747a6358949 100644
--- a/arch/x86/kernel/signal.c
+++ b/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