Re: unify pagetable accessors patch causes double fault II

Jeremy Fitzhardinge
2008-01-14T22:46:52Z

Andi Kleen wrote:
>> OK, I see the problem. The problem is that the _PAGE_X defines are
>> defined with _AC(UL, 1 << _PAGE_BIT_X), which has unsigned long type.
>> This means that ~_PAGE_X also has unsigned long type, and so when cast
>> to 64-bit in pte_mkX, it ends up &ing the pte with 0x00000000ffffffxxx,
>> with predictable results.
>>
>
> Actually I fixed some of that -- see the pgtable-nx patch on firstfloor -- but
> it still doesn't work. Or maybe my patch was not complete.
>

Yeah, that looks like the right sort of thing, but I wonder if there's
other places doing an open-coded "pte_val(pte) & ~_PAGE_FOO". My patch
changes the definition of _PAGE_FOO so it should be OK everywhere.

>> The original code just used signed constants for the _PAGE_X
>> definitions, which will sign-extend when cast to 64-bit, and so have the
>> upper bits set when masking. (Well, actually, the old code just
>> operated on pte_low, so the problem didn't arise; however, pgtable_64.h
>> also uses integers for its _PAGE_X, which has the same sign-extended
>> 32->64 casting property).
>>
>> I'll put together a fixup patch now.
>>
>
> I'm leaving now but can test later.
>

Can you try this out? It applies after "x86: move all asm/pgtable
constants into one place".

Subject: x86/pgtable: fix constant sign extension problem

When the _PAGE_FOO constants are defined as (1ul << _PAGE_BIT_FOO), they
become unsigned longs. In 32-bit PAE mode, these end up being
implicitly cast to 64-bit types when used to manipulate a pte, and
because they're unsigned the top 32-bits are 0, destroying the upper
bits of the pte.

When _PAGE_FOO constants are given a signed integer type, the cast to
64-bits will sign-extend so that the upper bits are all ones,
preserving the upper pte bits in manipulations.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>

---
include/asm-x86/pgtable.h | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)

===================================================================
--- a/include/asm-x86/pgtable.h
+++ b/include/asm-x86/pgtable.h
@@ -19,18 +19,18 @@
#define _PAGE_BIT_UNUSED3 11
#define _PAGE_BIT_NX 63 /* No execute: only valid after cpuid check */

-#define _PAGE_PRESENT (_AC(1, UL)<<_PAGE_BIT_PRESENT)
-#define _PAGE_RW (_AC(1, UL)<<_PAGE_BIT_RW)
-#define _PAGE_USER (_AC(1, UL)<<_PAGE_BIT_USER)
-#define _PAGE_PWT (_AC(1, UL)<<_PAGE_BIT_PWT)
-#define _PAGE_PCD (_AC(1, UL)<<_PAGE_BIT_PCD)
-#define _PAGE_ACCESSED (_AC(1, UL)<<_PAGE_BIT_ACCESSED)
-#define _PAGE_DIRTY (_AC(1, UL)<<_PAGE_BIT_DIRTY)
-#define _PAGE_PSE (_AC(1, UL)<<_PAGE_BIT_PSE) /* 2MB page */
-#define _PAGE_GLOBAL (_AC(1, UL)<<_PAGE_BIT_GLOBAL) /* Global TLB entry */
-#define _PAGE_UNUSED1 (_AC(1, UL)<<_PAGE_BIT_UNUSED1)
-#define _PAGE_UNUSED2 (_AC(1, UL)<<_PAGE_BIT_UNUSED2)
-#define _PAGE_UNUSED3 (_AC(1, UL)<<_PAGE_BIT_UNUSED3)
+#define _PAGE_PRESENT (_AC(1, L)<<_PAGE_BIT_PRESENT)
+#define _PAGE_RW (_AC(1, L)<<_PAGE_BIT_RW)
+#define _PAGE_USER (_AC(1, L)<<_PAGE_BIT_USER)
+#define _PAGE_PWT (_AC(1, L)<<_PAGE_BIT_PWT)
+#define _PAGE_PCD (_AC(1, L)<<_PAGE_BIT_PCD)
+#define _PAGE_ACCESSED (_AC(1, L)<<_PAGE_BIT_ACCESSED)
+#define _PAGE_DIRTY (_AC(1, L)<<_PAGE_BIT_DIRTY)
+#define _PAGE_PSE (_AC(1, L)<<_PAGE_BIT_PSE) /* 2MB page */
+#define _PAGE_GLOBAL (_AC(1, L)<<_PAGE_BIT_GLOBAL) /* Global TLB entry */
+#define _PAGE_UNUSED1 (_AC(1, L)<<_PAGE_BIT_UNUSED1)
+#define _PAGE_UNUSED2 (_AC(1, L)<<_PAGE_BIT_UNUSED2)
+#define _PAGE_UNUSED3 (_AC(1, L)<<_PAGE_BIT_UNUSED3)

#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
#define _PAGE_NX (_AC(1, ULL) << _PAGE_BIT_NX)