linux-kernel - Re: [PATCH 76/82] udf: Refactor intentional wrap-around test

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Date: Tue, 23 Jan 2024 18:14:36 +0100
From: Jan Kara <jack@...e.cz>
To: Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org, Jan Kara <jack@...e.com>,
	"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 76/82] udf: Refactor intentional wrap-around test

On Mon 22-01-24 16:27:51, 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: Jan Kara <jack@...e.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>

Looks good. Feel free to add:

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

								Honza

> ---
>  fs/udf/balloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/udf/balloc.c b/fs/udf/balloc.c
> index ab3ffc355949..5c88300c3de7 100644
> --- a/fs/udf/balloc.c
> +++ b/fs/udf/balloc.c
> @@ -139,7 +139,7 @@ static void udf_bitmap_free_blocks(struct super_block *sb,
>  
>  	mutex_lock(&sbi->s_alloc_mutex);
>  	partmap = &sbi->s_partmaps[bloc->partitionReferenceNum];
> -	if (bloc->logicalBlockNum + count < count ||
> +	if (add_would_overflow(count, bloc->logicalBlockNum) ||
>  	    (bloc->logicalBlockNum + count) > partmap->s_partition_len) {
>  		udf_debug("%u < %d || %u + %u > %u\n",
>  			  bloc->logicalBlockNum, 0,
> @@ -390,7 +390,7 @@ static void udf_table_free_blocks(struct super_block *sb,
>  
>  	mutex_lock(&sbi->s_alloc_mutex);
>  	partmap = &sbi->s_partmaps[bloc->partitionReferenceNum];
> -	if (bloc->logicalBlockNum + count < count ||
> +	if (add_would_overflow(count, bloc->logicalBlockNum) ||
>  	    (bloc->logicalBlockNum + count) > partmap->s_partition_len) {
>  		udf_debug("%u < %d || %u + %u > %u\n",
>  			  bloc->logicalBlockNum, 0,
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR