linux-hardening - Re: [PATCH 37/82] aio: Refactor intentional wrap-around test

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Message-ID: <20240123180330.2br6kzg2m3lbyrva@quack3>
Date: Tue, 23 Jan 2024 19:03:30 +0100
From: Jan Kara <jack@...e.cz>
To: Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org, Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-aio@...ck.org, linux-fsdevel@...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 37/82] aio: Refactor intentional wrap-around test

On Mon 22-01-24 16:27:12, 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 wrap-around addition test to use add_would_overflow().
> 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: Benjamin LaHaise <bcrl@...ck.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Jan Kara <jack@...e.cz>
> Cc: linux-aio@...ck.org
> Cc: linux-fsdevel@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/aio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index bb2ff48991f3..edd19be3f4b1 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -796,7 +796,7 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
>  	/* limit the number of system wide aios */
>  	spin_lock(&aio_nr_lock);
>  	if (aio_nr + ctx->max_reqs > aio_max_nr ||
> -	    aio_nr + ctx->max_reqs < aio_nr) {
> +	    add_would_overflow(aio_nr, ctx->max_reqs)) {
>  		spin_unlock(&aio_nr_lock);
>  		err = -EAGAIN;
>  		goto err_ctx;
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR