Re: 4.4-rc5: ugly warn on: 5 W+X pages found

Linus Torvalds
2015-12-15T17:45:43Z

On Tue, Dec 15, 2015 at 1:40 AM, Pavel Machek <pavel@ucw.cz> wrote:
>
> I tried applying:
>
> [PATCH 1/2] x86_32/mm: Set NX in __supported_pte_mask before enabling
> paging
>
> but I still get
>
> [ 2.691897] x86/mm: Found insecure W+X mapping at address ffe69000/0xffe69000

This may be an insane suggestion, but how about we try to detect when
that entry gets set, rather than after the fact.

Something really brute-force like

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 6ec0c8b2e9df..538c9bb239b9 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -337,6 +337,13 @@ static inline pmd_t pmd_clear_soft_dirty(pmd_t pmd)

#endif /* CONFIG_HAVE_ARCH_SOFT_DIRTY */

+static inline int kernel_write_execute_prot(pgprotval_t protval)
+{
+ return !(protval & _PAGE_USER) &&
+ !(protval & _PAGE_NX) &&
+ (protval & _PAGE_RW);
+}
+
/*
* Mask out unsupported bits in a present pgprot. Non-present pgprots
* can use those bits for other purposes, so leave them be.
@@ -345,8 +352,10 @@ static inline pgprotval_t massage_pgprot(pgprot_t pgprot)
{
pgprotval_t protval = pgprot_val(pgprot);

- if (protval & _PAGE_PRESENT)
+ if (protval & _PAGE_PRESENT) {
protval &= __supported_pte_mask;
+ WARN_ON_ONCE(kernel_write_execute_prot(protval));
+ }

return protval;
}

or similar?

The above is entirely untested. Maybe it doesn't compile. Or boot. Or work.

Linus