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SubjectRe: [PATCH v6 2/8] debugfs: prevent access to removed files' private data
Nicolai Stange <> writes:

> Sasha Levin <> writes:
>> On 05/18/2016 12:05 PM, Greg Kroah-Hartman wrote:
>>> On Wed, May 18, 2016 at 11:18:16AM -0400, Sasha Levin wrote:
>>>> On 05/18/2016 11:01 AM, Nicolai Stange wrote:
>>>>> Thanks a million for reporting!
>>>>> 1.) Do you have lockdep enabled?
>>>> Yup, nothing there.
>>>>> 2.) Does this happen before or after userspace init has been spawned,
>>>>> i.e. does the lockup happen at debugfs file creation time or
>>>>> possibly at usage time?
>>>> So I looked closer, and it seems to happen after starting syzkaller, which
>>>> as far as I know tries to open many different debugfs files.
>>>> Is there debug code I can add it that'll help us figure out what's up?
>>> Trying to figure out _which_ debugfs file is causing this would be
>>> great, if at all possible. strace?
>> What seems to be failing is syzkaller's attempt to mmap the coverage
>> debugfs file. So this isn't actually a kernel deadlock but syzkaller
>> misbehaves when that scenario happens.
>> Either way, it only fails to mmap with that commit that I've pointed
>> out.
> That info is really helpful here: the proxy file_operations introduced by
> this commit doesn't have a ->mmap() defined, i.e. it is NULL from the
> VFS layer's point of view.
> The simple reason is that at the time I submitted this series, my
> Coccinelle script didn't find any debugfs user with a ->mmap()
> defined. Thus either that script was broken or things have changed in
> the meanwhile.

Thankfully, it's the latter :)
See the attached cocci script I used back then.

It now reports:

./drivers/staging/android/sync_debug.c:330:1-20: unsupported file_operations given to debugfs
./kernel/kcov.c:267:6-25: unsupported file_operations given to debugfs

The kcov's ->mmap() has been introduced by

5c9a8750a640 ("kernel: add kcov code coverage")

dated from March this year.

Since that kcov debugfs file is never removed, it needs no protecting
proxy and thus, a replacement of debugfs_create_file() by
debugfs_create_file_unsafe() will do the trick here.

I'll send patches addressing the above two issues.

>> th->cover_fd = open("/sys/kernel/debug/kcov", O_RDWR);
>> if (th->cover_fd == -1)
>> fail("open of /sys/kernel/debug/kcov failed");
>> if (ioctl(th->cover_fd, KCOV_INIT_TRACE, kCoverSize))
>> fail("cover enable write failed");
>> th->cover_data = (uintptr_t*)mmap(NULL, kCoverSize * sizeof(th->cover_data[0]), PROT_READ | PROT_WRITE, MAP_SHARED, th->cover_fd, 0);
>> if ((void*)th->cover_data == MAP_FAILED)
>> fail("cover mmap failed");
>> And it's the mmap() that fails with -ENODEV.

virtual report
virtual org

identifier fops;
expression e;
identifier m != {owner, open, release, llseek, read, write, poll, unlocked_ioctl};
struct file_operations fops = {
.m = e,

expression name, mode, parent, data;
identifier unsupp_fops.fops;
position p;
debugfs_create_file@p(name, mode, parent, data, &fops)

@script:python depends on report@
p << unsupp_debugfs_fops.p;
@@[0], "unsupported file_operations given to debugfs")

@script:python depends on org@
p << unsupp_debugfs_fops.p;
cocci.print_main("unsupported file_operations given to debugfs", p)
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