linux-kernel - Re: [PATCH 5/7] x86/uaccess: Warn on uaccess faults other than #PF

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Date:	Wed, 25 May 2016 11:49:07 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 5/7] x86/uaccess: Warn on uaccess faults other than #PF

On Tue, May 24, 2016 at 03:48:42PM -0700, Andy Lutomirski wrote:
> If a uaccess instruction fails due to an8 error other than #PF,
> warn.  If the fault is #GP, it most likely indicates access to a
> non-canonical address, which means that an access_ok check is
> missing, and that's bad.  If the fault is something else (#UD?),
> then something is very wrong and we should diagnose it rather
> than ignoring it.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/mm/extable.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 658292fdee5e..c1933471fce7 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -29,6 +29,19 @@ EXPORT_SYMBOL(ex_handler_default);
>  static bool uaccess_fault_okay(int trapnr, unsigned long error_code,
>  			       unsigned long extra)
>  {
> +	/*
> +	 * For uaccess, only page faults should be fixed up.  I can't see
> +	 * any exploit mitigation value in OOPSing on other types of faults,
> +	 * so just warn and continue if that happens.  This means that
> +	 * uaccess faults to non-canonical addresses will warn.  That's okay
> +	 * -- this will only happen if an access_ok is missing, and we want to
> +	 * detect that error if it happens.
> +	 */
> +	if (WARN_ONCE(trapnr != X86_TRAP_PF,
> +		      "unexpected uaccess trap %d (may indicate a missing access_ok on a non-canonical address)\n",
> +		      trapnr))

Perhaps dump also regs->ip and make the warn message more helpful...

> +		return true;  /* no good reason to OOPS. */

You love those side comments, don'tcha? :-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.