2022-03-28 20:48:37

by Guoqing Jiang

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

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:[email protected]
> Fixes: 70bcecdb1534 ("amd-cluster: Improve md_reload_sb to be less error prone")

amd-cluster? ????

> Signed-off-by: Xiaomeng Tong<[email protected]>
> ---
> 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

2022-03-28 22:57:32

by Xiaomeng Tong

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

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

Thank you for the suggestion, i will take it in PATCH v2.

> > - 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.

No. At least from the pr_warn log, the list can be empty or no element found in it.
If this cases happen, the 'rdev' will be an invalid pointer that point to a invalid
struct containning the HEAD '&((mddev)->disks)'. So this fix is necessary.

--
Xiaomeng Tong