x86/entry/64/compat: Fix bad fast syscall arg failure path

If user code does SYSCALL32 or SYSENTER without a valid stack,
then our attempt to determine the syscall args will result in a
failed uaccess fault. Previously, we would try to recover by
jumping to the syscall exit code, but we'd run the syscall exit
work even though we never made it to the syscall entry work.

Clean it up by treating the failure path as a non-syscall entry
and exit pair.

This fixes strace's output when running the syscall_arg_fault
test. Without this fix, strace would get out of sync and would
fail to associate syscall entries with syscall exits.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/entry/entry_64.S

···
 	testb	$3, CS(%rsp)
 	jz	retint_kernel
 	/* Interrupt came from user space */
-retint_user:
+GLOBAL(retint_user)
 	GET_THREAD_INFO(%rcx)
 
 	/* %rcx: thread info. Interrupts are off. */

arch/x86/entry/entry_64_compat.S

···
 END(entry_SYSCALL_compat)
 
 ia32_badarg:
-	ASM_CLAC
-	movq	$-EFAULT, RAX(%rsp)
+	/*
+	 * So far, we've entered kernel mode, set AC, turned on IRQs, and
+	 * saved C regs except r8-r11.  We haven't done any of the other
+	 * standard entry work, though.  We want to bail, but we shouldn't
+	 * treat this as a syscall entry since we don't even know what the
+	 * args are.  Instead, treat this as a non-syscall entry, finish
+	 * the entry work, and immediately exit after setting AX = -EFAULT.
+	 *
+	 * We're really just being polite here.  Killing the task outright
+	 * would be a reasonable action, too.  Given that the only valid
+	 * way to have gotten here is through the vDSO, and we already know
+	 * that the stack pointer is bad, the task isn't going to survive
+	 * for long no matter what we do.
+	 */
+
+	ASM_CLAC			/* undo STAC */
+	movq	$-EFAULT, RAX(%rsp)	/* return -EFAULT if possible */
+
+	/* Fill in the rest of pt_regs */
+	xorl	%eax, %eax
+	movq	%rax, R11(%rsp)
+	movq	%rax, R10(%rsp)
+	movq	%rax, R9(%rsp)
+	movq	%rax, R8(%rsp)
+	SAVE_EXTRA_REGS
+
+	/* Turn IRQs back off. */
+	DISABLE_INTERRUPTS(CLBR_NONE)
+	TRACE_IRQS_OFF
+
+	/* And exit again. */
+	jmp retint_user
+
 ia32_ret_from_sys_call:
 	xorl	%eax, %eax		/* Do not leak kernel information */
 	movq	%rax, R11(%rsp)