x86/mpx: Add temporary variable to reduce masking

When we allocate a bounds table, we call mmap(), then add a
"valid" bit to the value before storing it in to the bounds
directory.

If we fail along the way, we go and mask that valid bit
_back_ out.  That seems a little silly, and this makes it
much more clear when we have a plain address versus an
actual table _entry_.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave@sr71.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150607183704.3D69D5F4@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index c17fd27579af..4f7fb7c233cc 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -429,6 +429,7 @@ static int allocate_bt(long __user *bd_entry)
         unsigned long expected_old_val = 0;
         unsigned long actual_old_val = 0;
         unsigned long bt_addr;
+        unsigned long bd_new_entry;
         int ret = 0;
 
         /*
@@ -441,7 +442,7 @@ static int allocate_bt(long __user *bd_entry)
         /*
          * Set the valid flag (kinda like _PAGE_PRESENT in a pte)
          */
-        bt_addr = bt_addr | MPX_BD_ENTRY_VALID_FLAG;
+        bd_new_entry = bt_addr | MPX_BD_ENTRY_VALID_FLAG;
 
         /*
          * Go poke the address of the new bounds table in to the
@@ -455,7 +456,7 @@ static int allocate_bt(long __user *bd_entry)
          * of the MPX code that have to pagefault_disable().
          */
         ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry,
-                                           expected_old_val, bt_addr);
+                                           expected_old_val, bd_new_entry);
         if (ret)
                 goto out_unmap;
 
@@ -486,7 +487,7 @@ static int allocate_bt(long __user *bd_entry)
         trace_mpx_new_bounds_table(bt_addr);
         return 0;
 out_unmap:
-        vm_munmap(bt_addr & MPX_BT_ADDR_MASK, MPX_BT_SIZE_BYTES);
+        vm_munmap(bt_addr, MPX_BT_SIZE_BYTES);
         return ret;
 }