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

Andy Lutomirski
2015-12-15T18:41:14Z

On Tue, Dec 15, 2015 at 9:45 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> 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));

Shouldn't we switch those two lines? Arguably trying to set rwx
permissions on !PAE is a bug even if it makes no difference, whereas
setting rw- and getting rwx because we don't have NX support on the
platform or kernel build doesn't indicate a bug.

Anyway, I still think that we should apply my patches for 4.5 because
I think they're cleanups, but apparently I guessed wrong as to what
was causing Pavel's issue. But your patch would help diagnose it.

--Andy