Re: [PATCH] md: md2: fix an incorrect NULL check on list iterator

[Guoqing Jiang <guoqing.jiang@linux.dev>]

Hi Xiaomeng,

I'd suggest to rephrase the subject to "md: fix an incorrect NULL check 
in md_reload_sb".

On 3/27/22 4:01 PM, Xiaomeng Tong 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 use the original variable 'pdev' as a dedicated pointer to
> point to the found element.
>
> Cc:stable@vger.kernel.org
> Fixes: 70bcecdb1534 ("amd-cluster: Improve md_reload_sb to be less error prone")

amd-cluster? 😉

> Signed-off-by: Xiaomeng Tong<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;
>   	}

I guess we only need to check desc_nr since rdev should always be valid 
, and IMO the fix tag
is not necessary.

@@ -9800,7 +9800,7 @@ void md_reload_sb(struct mddev *mddev, int nr)
                         break;
         }

-       if (!rdev || rdev->desc_nr != nr) {
+       if (rdev->desc_nr != nr) {

Thanks,
Guoqing