x86/fault: Fold smap_violation() into do_user_addr_fault()

smap_violation() has a single caller, and the contents are a bit
nonsensical.  I'm going to fix it, but first let's fold it into its
caller for ease of comprehension.

In this particular case, the user_mode(regs) check is incorrect --
it will cause false positives in the case of a user-initiated
kernel-privileged access.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/806c366f6ca861152398ce2c01744d59d9aceb6d.1542667307.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/fault.c

@@ -1148,20 +1148,6 @@ static int fault_in_kernel_space(unsigned long address)
 	return address >= TASK_SIZE_MAX;
 }
 
-static inline bool smap_violation(int error_code, struct pt_regs *regs)
-{
-	if (!cpu_feature_enabled(X86_FEATURE_SMAP))
-		return false;
-
-	if (error_code & X86_PF_USER)
-		return false;
-
-	if (!user_mode(regs) && (regs->flags & X86_EFLAGS_AC))
-		return false;
-
-	return true;
-}
-
 /*
  * Called for all faults where 'address' is part of the kernel address
  * space.  Might get called for faults that originate from *code* that
@@ -1249,10 +1235,13 @@ void do_user_addr_fault(struct pt_regs *regs,
 		pgtable_bad(regs, hw_error_code, address);
 
 	/*
-	 * Check for invalid kernel (supervisor) access to user
-	 * pages in the user address space.
+	 * If SMAP is on, check for invalid kernel (supervisor)
+	 * access to user pages in the user address space.
 	 */
-	if (unlikely(smap_violation(hw_error_code, regs))) {
+	if (unlikely(cpu_feature_enabled(X86_FEATURE_SMAP) &&
+		     !(hw_error_code & X86_PF_USER) &&
+		     (user_mode(regs) || !(regs->flags & X86_EFLAGS_AC))))
+	{
 		bad_area_nosemaphore(regs, hw_error_code, address);
 		return;
 	}