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

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Message-ID: <e6fe9883-2f51-a249-c5d2-ce11f6b449da@kernel.dk>
Date:   Thu, 5 Mar 2020 13:43:04 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Cengiz Can <cengiz@...nel.wtf>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] blktrace: fix dereference after null check

On 3/4/20 3:58 AM, Cengiz Can 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.

Applied, thanks.

-- 
Jens Axboe