linux-kernel - Re: [PATCH v2] md: fix an incorrect NULL check in sync

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Date:   Thu, 7 Apr 2022 17:36:48 -0700
From:   Song Liu <song@...nel.org>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     rgoldwyn@...e.com, Guoqing Jiang <guoqing.jiang@...ux.dev>,
        linux-raid <linux-raid@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] md: fix an incorrect NULL check in sync_sbs

On Mon, Mar 28, 2022 at 1:11 AM Xiaomeng Tong <xiam0nd.tong@...il.com> 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 'pdev' as a dedicated pointer to

s/pdev/rdev/

> point to the found element.
>
> Cc: stable@...r.kernel.org
> Fixes: 2aa82191ac36c ("md-cluster: Perform a lazy update")

"Fixes" should use a hash of 12 characters (13 given here). Did
checkpatch.pl complain about it?

> Acked-by: Guoqing Jiang <guoqing.jiang@...ux.dev>
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>
> ---
>
> changes since v1:
>  - rephrase the subject (Guoqing Jiang)
>  - add Acked-by: for Guoqing Jiang
> 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
>