x86/mpx: Fix 32-bit address space calculation

I received a bug report that running 32-bit MPX binaries on
64-bit kernels was broken.  I traced it down to this little code
snippet.  We were switching our "number of bounds directory
entries" calculation correctly.  But, we didn't switch the other
side of the calculation: the virtual space size.

This meant that we were calculating an absurd size for
bd_entry_virt_space() on 32-bit because we used the 64-bit
virt_space.

This was _also_ broken for 32-bit kernels running on 64-bit
hardware since boot_cpu_data.x86_virt_bits=48 even when running
in 32-bit mode.

Correct that and properly handle all 3 possible cases:

 1. 32-bit binary on 64-bit kernel
 2. 64-bit binary on 64-bit kernel
 3. 32-bit binary on 32-bit kernel

This manifested in having bounds tables not properly unmapped.
It "leaked" memory but had no functional impact otherwise.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: <stable@vger.kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
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: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20151111181934.FA7FAC34@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 6127c5e3a19ba29ad95b4a7703780f35f5365d71..1202d5ca2fb582d1a71cad1c85f65b61b907c441 100644 (file)
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -723,11 +723,23 @@ static unsigned long mpx_get_bt_entry_offset_bytes(struct mm_struct *mm,
  */
 static inline unsigned long bd_entry_virt_space(struct mm_struct *mm)
 {
-       unsigned long long virt_space = (1ULL << boot_cpu_data.x86_virt_bits);
-       if (is_64bit_mm(mm))
-               return virt_space / MPX_BD_NR_ENTRIES_64;
-       else
-               return virt_space / MPX_BD_NR_ENTRIES_32;
+       unsigned long long virt_space;
+       unsigned long long GB = (1ULL << 30);
+
+       /*
+        * This covers 32-bit emulation as well as 32-bit kernels
+        * running on 64-bit harware.
+        */
+       if (!is_64bit_mm(mm))
+               return (4ULL * GB) / MPX_BD_NR_ENTRIES_32;
+
+       /*
+        * 'x86_virt_bits' returns what the hardware is capable
+        * of, and returns the full >32-bit adddress space when
+        * running 32-bit kernels on 64-bit hardware.
+        */
+       virt_space = (1ULL << boot_cpu_data.x86_virt_bits);
+       return virt_space / MPX_BD_NR_ENTRIES_64;
 }
 
 /*