linux-hardening - Re: [PATCH 22/82] x86/sgx: Refactor intentional wrap-around calculation

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Message-Id: <CYLYZT8Q5YPS.BF3JHG87K4EJ@seitikki>
Date: Tue, 23 Jan 2024 09:15:16 +0000
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Kees Cook" <keescook@...omium.org>, <linux-hardening@...r.kernel.org>
Cc: "Dave Hansen" <dave.hansen@...ux.intel.com>, "Thomas Gleixner"
 <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov"
 <bp@...en8.de>, <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
 <linux-sgx@...r.kernel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "Bill Wendling" <morbo@...gle.com>, "Justin Stitt"
 <justinstitt@...gle.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 22/82] x86/sgx: Refactor intentional wrap-around
 calculation

On Tue Jan 23, 2024 at 12:26 AM UTC, Kees Cook wrote:
> In an effort to separate intentional arithmetic wrap-around from
> unexpected wrap-around, we need to refactor places that depend on this
> kind of math. One of the most common code patterns of this is:
>
> 	VAR + value < VAR
>
> Notably, this is considered "undefined behavior" for signed and pointer
> types, which the kernel works around by using the -fno-strict-overflow
> option in the build[1] (which used to just be -fwrapv). Regardless, we
> want to get the kernel source to the position where we can meaningfully
> instrument arithmetic wrap-around conditions and catch them when they
> are unexpected, regardless of whether they are signed[2], unsigned[3],
> or pointer[4] types.
>
> Refactor open-coded unsigned wrap-around addition test to use
> check_add_overflow(), retaining the result for later usage (which removes
> the redundant open-coded addition). This paves the way to enabling the
> wrap-around sanitizers in the future.
>
> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> Link: https://github.com/KSPP/linux/issues/26 [2]
> Link: https://github.com/KSPP/linux/issues/27 [3]
> Link: https://github.com/KSPP/linux/issues/344 [4]
> Cc: Jarkko Sakkinen <jarkko@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: x86@...nel.org
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: linux-sgx@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/x86/kernel/cpu/sgx/ioctl.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> index b65ab214bdf5..4b8f6c9f8ef5 100644
> --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> @@ -350,16 +350,18 @@ static int sgx_validate_offset_length(struct sgx_encl *encl,
>  				      unsigned long offset,
>  				      unsigned long length)
>  {
> +	unsigned long sum;
> +
>  	if (!IS_ALIGNED(offset, PAGE_SIZE))
>  		return -EINVAL;
>  
>  	if (!length || !IS_ALIGNED(length, PAGE_SIZE))
>  		return -EINVAL;
>  
> -	if (offset + length < offset)
> +	if (check_add_overflow(offset, length, &sum))
>  		return -EINVAL;
>  
> -	if (offset + length - PAGE_SIZE >= encl->size)
> +	if (sum - PAGE_SIZE >= encl->size)
>  		return -EINVAL;
>  
>  	return 0;

Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>

BR, Jarkko