x86/fault: Don't run fixups for SMAP violations

A SMAP-violating kernel access is not a recoverable condition.  Imagine
kernel code that, outside of a uaccess region, dereferences a pointer to
the user range by accident.  If SMAP is on, this will reliably generate
as an intentional user access.  This makes it easy for bugs to be
overlooked if code is inadequately tested both with and without SMAP.

This was discovered because BPF can generate invalid accesses to user
memory, but those warnings only got printed if SMAP was off. Make it so
that this type of error will be discovered with SMAP on as well.

 [ bp: Massage commit message. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/66a02343624b1ff46f02a838c497fc05c1a871b3.1612924255.git.luto@kernel.org

arch/x86/mm/fault.c

@@ -1279,9 +1279,12 @@ void do_user_addr_fault(struct pt_regs *regs,
 	 */
 	if (unlikely(cpu_feature_enabled(X86_FEATURE_SMAP) &&
 		     !(error_code & X86_PF_USER) &&
-		     !(regs->flags & X86_EFLAGS_AC)))
-	{
-		bad_area_nosemaphore(regs, error_code, address);
+		     !(regs->flags & X86_EFLAGS_AC))) {
+		/*
+		 * No extable entry here.  This was a kernel access to an
+		 * invalid pointer.  get_kernel_nofault() will not get here.
+		 */
+		page_fault_oops(regs, error_code, address);
 		return;
 	}