linux-kernel - Re: [PATCH v3] md: fix an incorrect NULL check in does_sb_need

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Date:   Fri, 8 Apr 2022 07:23:48 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     song@...nel.org, guoqing.jiang@...ux.dev,
        linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v3] md: fix an incorrect NULL check in
 does_sb_need_changing

On 16:37 08/04, Xiaomeng Tong wrote:
> The bug is here:
> 	if (!rdev)
> 
> The list iterator value 'rdev' will *always* be set and non-NULL
> by rdev_for_each(), so it is incorrect to assume that the iterator
> value will be NULL if the list is empty or no element found.
> Otherwise it will bypass the NULL check and lead to invalid memory
> access passing the check.
> 
> To fix the bug, use a new variable 'iter' as the list iterator,
> while using the original variable 'rdev' as a dedicated pointer to
> point to the found element.
> 
> Cc: stable@...r.kernel.org
> Fixes: 2aa82191ac36 ("md-cluster: Perform a lazy update")
> Acked-by: Guoqing Jiang <guoqing.jiang@...ux.dev>
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>

Also safeguards from reading sb from a faulty device if all devices are
faulty.

Acked-by: Goldwyn Rodrigues <rgoldwyn@...e.com>

> v2:https://lore.kernel.org/lkml/20220328081127.26148-1-xiam0nd.tong@gmail.com/
> v1:https://lore.kernel.org/lkml/20220327080002.11923-1-xiam0nd.tong@gmail.com/
> 
> ---
>  drivers/md/md.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 4d38bd7dadd6..7476fc204172 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -2629,14 +2629,16 @@ static void sync_sbs(struct mddev *mddev, int nospares)
>  
>  static bool does_sb_need_changing(struct mddev *mddev)
>  {
> -	struct md_rdev *rdev;
> +	struct md_rdev *rdev = NULL, *iter;
>  	struct mdp_superblock_1 *sb;
>  	int role;
>  
>  	/* Find a good rdev */
> -	rdev_for_each(rdev, mddev)
> -		if ((rdev->raid_disk >= 0) && !test_bit(Faulty, &rdev->flags))
> +	rdev_for_each(iter, mddev)
> +		if ((iter->raid_disk >= 0) && !test_bit(Faulty, &iter->flags)) {
> +			rdev = iter;
>  			break;
> +		}
>  
>  	/* No good device found. */
>  	if (!rdev)
> -- 
> 2.17.1
> 

-- 
Goldwyn