x86/fault: Check user_mode(regs) when avoiding an mmap_sem deadlock

The fault-handling code that takes mmap_sem needs to avoid a
deadlock that could occur if the kernel took a bad (OOPS-worthy)
page fault on a user address while holding mmap_sem.  This can only
happen if the faulting instruction was in the kernel
(i.e. user_mode(regs)).  Rather than checking the sw_error_code
(which will have the USER bit set if the fault was a USER-permission
access *or* if user_mode(regs)), just check user_mode(regs)
directly.

The old code would have malfunctioned if the kernel executed a bogus
WRUSS instruction while holding mmap_sem.  Fortunately, that is
extremely unlikely in current kernels, which don't use WRUSS.
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/4b89b542e8ceba9bd6abde2f386afed6d99244a9.1542667307.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/fault.c

@@ -1344,13 +1344,10 @@ void do_user_addr_fault(struct pt_regs *regs,
 	 * Only do the expensive exception table search when we might be at
 	 * risk of a deadlock.  This happens if we
 	 * 1. Failed to acquire mmap_sem, and
-	 * 2. The access did not originate in userspace.  Note: either the
-	 *    hardware or earlier page fault code may set X86_PF_USER
-	 *    in sw_error_code.
+	 * 2. The access did not originate in userspace.
 	 */
 	if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
-		if (!(sw_error_code & X86_PF_USER) &&
-		    !search_exception_tables(regs->ip)) {
+		if (!user_mode(regs) && !search_exception_tables(regs->ip)) {
 			/*
 			 * Fault from code in kernel from
 			 * which we do not expect faults.