x86/fault: Correct a few user vs kernel checks wrt WRUSS

In general, page fault errors for WRUSS should be just like get_user(),
etc.  Fix three bugs in this area:

There is a comment that says that, if the kernel can't handle a page fault
on a user address due to OOM, the OOM-kill-and-retry logic would be
skipped.  The code checked kernel *privilege*, not kernel mode, so it
missed WRUSS.  This means that the kernel would malfunction if it got OOM
on a WRUSS fault -- this would be a kernel-mode, user-privilege fault, and
the OOM killer would be invoked and the handler would retry the faulting
instruction.

A failed user access from kernel while a fatal signal is pending should
fail even if the instruction in question was WRUSS.

do_sigbus() should not send SIGBUS for WRUSS -- it should handle it like
any other kernel mode failure.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/a7b7bcea730bd4069e6b7e629236bb2cf526c2fb.1612924255.git.luto@kernel.org

arch/x86/mm/fault.c

@@ -945,7 +945,7 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
 	  vm_fault_t fault)
 {
 	/* Kernel mode? Handle exceptions or die: */
-	if (!(error_code & X86_PF_USER)) {
+	if (!user_mode(regs)) {
 		no_context(regs, error_code, address, SIGBUS, BUS_ADRERR);
 		return;
 	}
@@ -1217,7 +1217,14 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
 }
 NOKPROBE_SYMBOL(do_kern_addr_fault);
 
-/* Handle faults in the user portion of the address space */
+/*
+ * Handle faults in the user portion of the address space.  Nothing in here
+ * should check X86_PF_USER without a specific justification: for almost
+ * all purposes, we should treat a normal kernel access to user memory
+ * (e.g. get_user(), put_user(), etc.) the same as the WRUSS instruction.
+ * The one exception is AC flag handling, which is, per the x86
+ * architecture, special for WRUSS.
+ */
 static inline
 void do_user_addr_fault(struct pt_regs *regs,
 			unsigned long error_code,
@@ -1406,14 +1413,14 @@ good_area:
 	if (likely(!(fault & VM_FAULT_ERROR)))
 		return;
 
-	if (fatal_signal_pending(current) && !(error_code & X86_PF_USER)) {
+	if (fatal_signal_pending(current) && !user_mode(regs)) {
 		no_context(regs, error_code, address, 0, 0);
 		return;
 	}
 
 	if (fault & VM_FAULT_OOM) {
 		/* Kernel mode? Handle exceptions or die: */
-		if (!(error_code & X86_PF_USER)) {
+		if (!user_mode(regs)) {
 			no_context(regs, error_code, address,
 				   SIGSEGV, SEGV_MAPERR);
 			return;