x86/mm: Add clarifying comments for user addr space

The SMAP and Reserved checking do not have nice comments.  Add
some to clarify and make it match everything else.

Cc: x86@kernel.org
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180928160225.FFD44B8D@viggo.jf.intel.com

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 0d1f5d3..1d83870 100644 (file)
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1276,9 +1276,17 @@ void do_user_addr_fault(struct pt_regs *regs,
        if (unlikely(kprobes_fault(regs)))
                return;
 
+       /*
+        * Reserved bits are never expected to be set on
+        * entries in the user portion of the page tables.
+        */
        if (unlikely(hw_error_code & X86_PF_RSVD))
                pgtable_bad(regs, hw_error_code, address);
 
+       /*
+        * Check for invalid kernel (supervisor) access to user
+        * pages in the user address space.
+        */
        if (unlikely(smap_violation(hw_error_code, regs))) {
                bad_area_nosemaphore(regs, hw_error_code, address, NULL);
                return;