linux-hardening - Re: [PATCH 19/82] fs: Refactor intentional wrap-around calculation

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Message-ID: <20240123180128.d2hgvlbjq66rkfdc@quack3>
Date: Tue, 23 Jan 2024 19:01:28 +0100
From: Jan Kara <jack@...e.cz>
To: Kees Cook <keescook@...omium.org>
Cc: linux-hardening@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	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 19/82] fs: Refactor intentional wrap-around calculation

On Mon 22-01-24 16:26:54, 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: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Jan Kara <jack@...e.cz>
> 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/read_write.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/read_write.c b/fs/read_write.c
> index d4c036e82b6c..e24b94a8937d 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1417,6 +1417,7 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  	struct inode *inode_out = file_inode(file_out);
>  	uint64_t count = *req_count;
>  	loff_t size_in;
> +	loff_t sum_in, sum_out;
>  	int ret;
>  
>  	ret = generic_file_rw_checks(file_in, file_out);
> @@ -1451,7 +1452,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  		return -ETXTBSY;
>  
>  	/* Ensure offsets don't wrap. */
> -	if (pos_in + count < pos_in || pos_out + count < pos_out)
> +	if (check_add_overflow(pos_in, count, &sum_in) ||
> +	    check_add_overflow(pos_out, count, &sum_out))
>  		return -EOVERFLOW;
>  
>  	/* Shorten the copy to EOF */
> @@ -1467,8 +1469,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  
>  	/* Don't allow overlapped copying within the same file. */
>  	if (inode_in == inode_out &&
> -	    pos_out + count > pos_in &&
> -	    pos_out < pos_in + count)
> +	    sum_out > pos_in &&
> +	    pos_out < sum_in)
>  		return -EINVAL;
>  
>  	*req_count = count;
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR