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

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Message-ID: <CA+55aFyp91uoxPV=NTNpWOLWkfV9QB=0WKZBtF2cdBNC8Q6LSA@mail.gmail.com>
Date:	Tue, 15 Dec 2015 09:45:40 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Peter Anvin <hpa@...or.com>, Mike Galbraith <efault@....de>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 4.4-rc5: ugly warn on: 5 W+X pages found

On Tue, Dec 15, 2015 at 1:40 AM, Pavel Machek <pavel@....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
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