linux-kernel - Re: Possible Memory Leak in KCOV Linux 4.15-rc1

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Date:   Sat, 20 Jan 2018 16:06:57 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Shankara Pailoor <sp3485@...umbia.edu>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: Possible Memory Leak in KCOV Linux 4.15-rc1

On Sat, Jan 20, 2018 at 4:01 PM, Shankara Pailoor <sp3485@...umbia.edu> wrote:
> Hi Dmitry,
>
> I will try and get something to you tomorrow. Just wondering, but what
> happens to the old struct kcov if a task opens /sys/kernel/debug/kcov
> twice? I am looking here
> https://elixir.free-electrons.com/linux/v4.15-rc8/source/kernel/kcov.c#L381
> and I don't see where the previous struct would get freed.

Good question. Perhaps we need something like:

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 7594c033d98a..c76498018500 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -358,7 +358,7 @@ static int kcov_ioctl_locked(struct kcov *kcov,
unsigned int cmd,
                 */
                if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
                        return -EINVAL;
-               if (kcov->t != NULL)
+               if (kcov->t != NULL || t->kcov != NULL)
                        return -EBUSY;
                if (arg == KCOV_TRACE_PC)
                        kcov->mode = KCOV_MODE_TRACE_PC;



> On Sat, Jan 20, 2018 at 4:38 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> On Fri, Jan 19, 2018 at 8:29 PM, Shankara Pailoor <sp3485@...umbia.edu> wrote:
>>> Hi Dmitry,
>>>
>>> I added support for kcov in strace and I have been tracing a fairly
>>> large program but after a little while, I notice that when I mmap a
>>> new cover buffer, the call fails with ENOMEM. After killing the
>>> program, I try and rerun and I notice that there is nearly no memory
>>> on the system. When I do a kmemleak scan I get the following reports:
>>>
>>> I believe the problem occurs when I try and setup the kcov buffer
>>> again after an exec. Instead of reusing the old file descriptor I open
>>> kcov again within that process. In that case, I don't know what
>>> happens to the old kcov struct.
>>>
>>> I don't see a maintainers list for kcov so I decided to email you
>>> directly. Let me know what more information I can provide.
>>
>>
>> Hi Shankara,
>>
>> Looks bad. Can you provide a reproducer?
>> We extensively use kcov with syzkaller, but have not observed such
>> leaks. Also I don't see anything obvious in the code.
>>
>> Thanks
>