x86/mpx: Introduce new 'directory entry' to 'addr' helper function

Currently, to get from a bounds directory entry to the virtual
address of a bounds table, we simply mask off a few low bits.
However, the set of bits we mask off is different for 32-bit and
64-bit binaries.

This breaks the operation out in to a helper function and also
adds a temporary variable to store the result until we are
sure we are returning one.

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/20150607183704.007686CE@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 34 insertions, 8 deletions

diff --git a/arch/x86/include/asm/mpx.h b/arch/x86/include/asm/mpx.h
index 871e5e5b0499..99d374eb7b08 100644
--- a/arch/x86/include/asm/mpx.h
+++ b/arch/x86/include/asm/mpx.h
@@ -45,7 +45,6 @@
 #define MPX_BNDSTA_TAIL         2
 #define MPX_BNDCFG_TAIL         12
 #define MPX_BNDSTA_ADDR_MASK    (~((1UL<<MPX_BNDSTA_TAIL)-1))
-#define MPX_BT_ADDR_MASK        (~((1UL<<MPX_BD_ENTRY_TAIL)-1))
 
 #define MPX_BNDCFG_ADDR_MASK    (~((1UL<<MPX_BNDCFG_TAIL)-1))
 #define MPX_BNDSTA_ERROR_CODE   0x3
diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 4f7fb7c233cc..8cc793479039 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -576,29 +576,55 @@ static int mpx_resolve_fault(long __user *addr, int write)
         return 0;
 }
 
+static unsigned long mpx_bd_entry_to_bt_addr(struct mm_struct *mm,
+                                             unsigned long bd_entry)
+{
+        unsigned long bt_addr = bd_entry;
+        int align_to_bytes;
+        /*
+         * Bit 0 in a bt_entry is always the valid bit.
+         */
+        bt_addr &= ~MPX_BD_ENTRY_VALID_FLAG;
+        /*
+         * Tables are naturally aligned at 8-byte boundaries
+         * on 64-bit and 4-byte boundaries on 32-bit.  The
+         * documentation makes it appear that the low bits
+         * are ignored by the hardware, so we do the same.
+         */
+        if (is_64bit_mm(mm))
+                align_to_bytes = 8;
+        else
+                align_to_bytes = 4;
+        bt_addr &= ~(align_to_bytes-1);
+        return bt_addr;
+}
+
 /*
  * Get the base of bounds tables pointed by specific bounds
  * directory entry.
  */
 static int get_bt_addr(struct mm_struct *mm,
-                        long __user *bd_entry, unsigned long *bt_addr)
+                        long __user *bd_entry_ptr,
+                        unsigned long *bt_addr_result)
 {
         int ret;
         int valid_bit;
+        unsigned long bd_entry;
+        unsigned long bt_addr;
 
-        if (!access_ok(VERIFY_READ, (bd_entry), sizeof(*bd_entry)))
+        if (!access_ok(VERIFY_READ, (bd_entry_ptr), sizeof(*bd_entry_ptr)))
                 return -EFAULT;
 
         while (1) {
                 int need_write = 0;
 
                 pagefault_disable();
-                ret = get_user(*bt_addr, bd_entry);
+                ret = get_user(bd_entry, bd_entry_ptr);
                 pagefault_enable();
                 if (!ret)
                         break;
                 if (ret == -EFAULT)
-                        ret = mpx_resolve_fault(bd_entry, need_write);
+                        ret = mpx_resolve_fault(bd_entry_ptr, need_write);
                 /*
                  * If we could not resolve the fault, consider it
                  * userspace's fault and error out.
@@ -607,8 +633,8 @@ static int get_bt_addr(struct mm_struct *mm,
                         return ret;
         }
 
-        valid_bit = *bt_addr & MPX_BD_ENTRY_VALID_FLAG;
-        *bt_addr &= MPX_BT_ADDR_MASK;
+        valid_bit = bd_entry & MPX_BD_ENTRY_VALID_FLAG;
+        bt_addr = mpx_bd_entry_to_bt_addr(mm, bd_entry);
 
         /*
          * When the kernel is managing bounds tables, a bounds directory
@@ -617,7 +643,7 @@ static int get_bt_addr(struct mm_struct *mm,
          * data in the address field, we know something is wrong. This
          * -EINVAL return will cause a SIGSEGV.
          */
-        if (!valid_bit && *bt_addr)
+        if (!valid_bit && bt_addr)
                 return -EINVAL;
         /*
          * Do we have an completely zeroed bt entry?  That is OK.  It
@@ -628,6 +654,7 @@ static int get_bt_addr(struct mm_struct *mm,
         if (!valid_bit)
                 return -ENOENT;
 
+        *bt_addr_result = bt_addr;
         return 0;
 }