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

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Date:   Thu, 7 Apr 2022 17:37:55 -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] md: fix an incorrect NULL check in md_reload_sb

On Mon, Mar 28, 2022 at 1:06 AM Xiaomeng Tong <xiam0nd.tong@...il.com> wrote:
>
> The bug is here:
>         if (!rdev || rdev->desc_nr != nr) {
>
> The list iterator value 'rdev' will *always* be set and non-NULL
> by rdev_for_each_rcu(), so it is incorrect to assume that the
> iterator value will be NULL if the list is empty or no element
> found (In fact, it will be a bogus pointer to an invalid struct
> object containing the HEAD). Otherwise it will bypass the 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
> point to the found element.
>
> Cc: stable@...r.kernel.org
> Fixes: 70bcecdb1534 ("amd-cluster: Improve md_reload_sb to be less error prone")

s/amd-cluster/md-cluster/

> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>
> ---
>
> changes from v1:
>  - rephrase the subject (Guoqing Jiang)
>
> v1:https://lore.kernel.org/lkml/20220327080111.12028-1-xiam0nd.tong@gmail.com/
>
> ---
>  drivers/md/md.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 7476fc204172..f156678c08bc 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -9794,16 +9794,18 @@ static int read_rdev(struct mddev *mddev, struct md_rdev *rdev)
>
>  void md_reload_sb(struct mddev *mddev, int nr)
>  {
> -       struct md_rdev *rdev;
> +       struct md_rdev *rdev = NULL, *iter;
>         int err;
>
>         /* Find the rdev */
> -       rdev_for_each_rcu(rdev, mddev) {
> -               if (rdev->desc_nr == nr)
> +       rdev_for_each_rcu(iter, mddev) {
> +               if (iter->desc_nr == nr) {
> +                       rdev = iter;
>                         break;
> +               }
>         }
>
> -       if (!rdev || rdev->desc_nr != nr) {
> +       if (!rdev) {
>                 pr_warn("%s: %d Could not find rdev with nr %d\n", __func__, __LINE__, nr);
>                 return;
>         }
> --
> 2.17.1
>