linux-kernel - Re: [PATCH v2] Change OPT_STR max to be 1 less than the size of choices array

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Message-ID: <sxtugjw3gbifdvountioelfdpeqa7hnx4jxd3a76nh5uklf3du@vd7wof7b7nge>
Date: Wed, 6 Nov 2024 15:57:16 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Piotr Zalewski <pZ010001011111@...ton.me>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	skhan@...uxfoundation.org, syzbot+bee87a0c3291c06aa8c6@...kaller.appspotmail.com
Subject: Re: [PATCH v2] Change OPT_STR max to be 1 less than the size of
 choices array

On Wed, Nov 06, 2024 at 07:46:30PM +0000, Piotr Zalewski wrote:
> Change OPT_STR max value to be 1 less than the "ARRAY_SIZE" of "_choices"
> array. As a result, remove -1 from (opt->max-1) in bch2_opt_to_text.
> 
> The "_choices" array is a null-terminated array, so computing the maximum
> using "ARRAY_SIZE" without subtracting 1 yields an incorrect result. Since
> bch2_opt_validate don't subtract 1, as bch2_opt_to_text does, values
> bigger than the actual maximum would pass through option validation.
> 
> Reported-by: syzbot+bee87a0c3291c06aa8c6@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=bee87a0c3291c06aa8c6
> Fixes: 63c4b2545382 ("bcachefs: Better superblock opt validation")
> Suggested-by: Kent Overstreet <kent.overstreet@...ux.dev>
> Signed-off-by: Piotr Zalewski <pZ010001011111@...ton.me>

Thanks! Applied

> ---
> 
> Notes:
>     Changes in v2:
>         - Instead of subtracting 1 in bch2_opt_validate as in
>           bch2_opt_to_text add -1 in OPT_STR macro definition
>           and remove subtraction in bch2_opt_to_text.
> 
>     link to v1: https://lore.kernel.org/linux-bcachefs/20241031231823.688918-2-pZ010001011111@proton.me/
> 
>  fs/bcachefs/opts.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/bcachefs/opts.c b/fs/bcachefs/opts.c
> index 83f55cf99d46..49c59aec6954 100644
> --- a/fs/bcachefs/opts.c
> +++ b/fs/bcachefs/opts.c
> @@ -227,7 +227,7 @@ const struct bch_option bch2_opt_table[] = {
>  #define OPT_UINT(_min, _max)	.type = BCH_OPT_UINT,			\
>  				.min = _min, .max = _max
>  #define OPT_STR(_choices)	.type = BCH_OPT_STR,			\
> -				.min = 0, .max = ARRAY_SIZE(_choices),	\
> +				.min = 0, .max = ARRAY_SIZE(_choices) - 1, \
>  				.choices = _choices
>  #define OPT_STR_NOLIMIT(_choices)	.type = BCH_OPT_STR,		\
>  				.min = 0, .max = U64_MAX,		\
> @@ -429,7 +429,7 @@ void bch2_opt_to_text(struct printbuf *out,
>  			prt_printf(out, "%lli", v);
>  		break;
>  	case BCH_OPT_STR:
> -		if (v < opt->min || v >= opt->max - 1)
> +		if (v < opt->min || v >= opt->max)
>  			prt_printf(out, "(invalid option %lli)", v);
>  		else if (flags & OPT_SHOW_FULL_LIST)
>  			prt_string_option(out, opt->choices, v);
> -- 
> 2.47.0
> 
>