linux-kernel - Re: [PATCH v2] blktrace: fix dereference after null check

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Date:   Thu, 5 Mar 2020 16:47:45 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     Cengiz Can <cengiz@...nel.wtf>
Cc:     Jens Axboe <axboe@...nel.dk>, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] blktrace: fix dereference after null check

On Wed, Mar 4, 2020 at 6:59 PM Cengiz Can <cengiz@...nel.wtf> wrote:
>
> There was a recent change in blktrace.c that added a RCU protection to
> `q->blk_trace` in order to fix a use-after-free issue during access.
>
> However the change missed an edge case that can lead to dereferencing of
> `bt` pointer even when it's NULL:
>
> Coverity static analyzer marked this as a FORWARD_NULL issue with CID
> 1460458.
>
> ```
> /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
> 1898            ret = 0;
> 1899            if (bt == NULL)
> 1900                    ret = blk_trace_setup_queue(q, bdev);
> 1901
> 1902            if (ret == 0) {
> 1903                    if (attr == &dev_attr_act_mask)
> >>>     CID 1460458:  Null pointer dereferences  (FORWARD_NULL)
> >>>     Dereferencing null pointer "bt".
> 1904                            bt->act_mask = value;
> 1905                    else if (attr == &dev_attr_pid)
> 1906                            bt->pid = value;
> 1907                    else if (attr == &dev_attr_start_lba)
> 1908                            bt->start_lba = value;
> 1909                    else if (attr == &dev_attr_end_lba)
> ```
>
> Added a reassignment with RCU annotation to fix the issue.
>
> Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
>
> Signed-off-by: Cengiz Can <cengiz@...nel.wtf>
> ---
>
>     Patch Changelog
>     * v2: Added RCU annotation to assignment
>
>  kernel/trace/blktrace.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 4560878f0bac..ca39dc3230cb 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -1896,8 +1896,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
>         }
>
>         ret = 0;
> -       if (bt == NULL)
> +       if (bt == NULL) {
>                 ret = blk_trace_setup_queue(q, bdev);
> +               bt = rcu_dereference_protected(q->blk_trace,
> +                               lockdep_is_held(&q->blk_trace_mutex));
> +       }
>
>         if (ret == 0) {
>                 if (attr == &dev_attr_act_mask)
> --
> 2.25.1
>

Reviewed-by: Ming Lei <ming.lei@...hat.com>

-- 
Ming Lei