Date:   Tue, 8 Dec 2020 22:13:41 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     David Howells <dhowells@...hat.com>,
        syzbot <syzbot+86dc6632faaca40133ab@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: memory leak in generic_parse_monolithic [+PATCH]

On 12/8/20 10:03 PM, Dmitry Vyukov wrote:
> On Wed, Dec 9, 2020 at 12:15 AM Randy Dunlap <rdunlap@...radead.org> wrote:
>>
>> On 12/8/20 2:54 PM, David Howells wrote:
>>> Randy Dunlap <rdunlap@...radead.org> wrote:
>>>
>>>>> Now the backtrace only shows what the state was when the string was allocated;
>>>>> it doesn't show what happened to it after that, so another possibility is that
>>>>> the filesystem being mounted nicked what vfs_parse_fs_param() had rightfully
>>>>> stolen, transferring fc->source somewhere else and then failed to release it -
>>>>> most likely on mount failure (ie. it's an error handling bug in the
>>>>> filesystem).
>>>>>
>>>>> Do we know what filesystem it was?
>>>>
>>>> Yes, it's call AFS (or kAFS).
>>>
>>> Hmmm...  afs parses the string in afs_parse_source() without modifying it,
>>> then moves the pointer to fc->source (parallelling vfs_parse_fs_param()) and
>>> doesn't touch it again.  fc->source should be cleaned up by do_new_mount()
>>> calling put_fs_context() at the end of the function.
>>>
>>> As far as I can tell with the attached print-insertion patch, it works, called
>>> by the following commands, some of which are correct and some which aren't:
>>>
>>> # mount -t afs none /xfstest.test/ -o dyn
>>> # umount /xfstest.test
>>> # mount -t afs "" /xfstest.test/ -o foo
>>> mount: /xfstest.test: bad option; for several filesystems (e.g. nfs, cifs) you might need a /sbin/mount.<type> helper program.
>>> # umount /xfstest.test
>>> umount: /xfstest.test: not mounted.
>>> # mount -t afs %xfstest.test20 /xfstest.test/ -o foo
>>> mount: /xfstest.test: bad option; for several filesystems (e.g. nfs, cifs) you might need a /sbin/mount.<type> helper program.
>>> # umount /xfstest.test
>>> umount: /xfstest.test: not mounted.
>>> # mount -t afs %xfstest.test20 /xfstest.test/
>>> # umount /xfstest.test
>>>
>>> Do you know if the mount was successful and what the mount parameters were?
>>
>> Here's the syzbot reproducer:
>> https://syzkaller.appspot.com/x/repro.c?x=129ca3d6500000
>>
>> The "interesting" mount params are:
>>         source=%^]$[+%](${:\017k[)-:,source=%^]$[+.](%{:\017\200[)-:,\000
>>
>> There is no other AFS activity: nothing mounted, no cells known (or
>> whatever that is), etc.
>>
>> I don't recall if the mount was successful and I can't test it just now.
>> My laptop is mucked up.
>>
>>
>> Be aware that this report could just be a false positive: it waits
>> for 5 seconds then looks for a memleak. AFAIK, it's possible that the "leaked"
>> memory is still in valid use and will be freed some day.
> 
> FWIW KMEMLEAK scans memory for pointers. If it claims a memory leak,
> it means the heap object is not referenced anywhere anymore. There are
> no live pointers to it to call kfree or anything else.
> Some false positives are theoretically possible, but so I don't
> remember any, all reported ones were true leaks:
> https://syzkaller.appspot.com/upstream/fixed?manager=ci-upstream-gce-leak
> 

OK, great, thanks for the info.

> 
> 
>>> David
>>> ---
>>> diff --git a/fs/afs/super.c b/fs/afs/super.c
>>> index 6c5900df6aa5..4c44ec0196c9 100644
>>> --- a/fs/afs/super.c
>>> +++ b/fs/afs/super.c
>>> @@ -299,7 +299,7 @@ static int afs_parse_source(struct fs_context *fc, struct fs_parameter *param)
>>>               ctx->cell = cell;
>>>       }
>>>
>>> -     _debug("CELL:%s [%p] VOLUME:%*.*s SUFFIX:%s TYPE:%d%s",
>>> +     kdebug("CELL:%s [%p] VOLUME:%*.*s SUFFIX:%s TYPE:%d%s",
>>>              ctx->cell->name, ctx->cell,
>>>              ctx->volnamesz, ctx->volnamesz, ctx->volname,
>>>              suffix ?: "-", ctx->type, ctx->force ? " FORCE" : "");
>>> @@ -318,6 +318,8 @@ static int afs_parse_param(struct fs_context *fc, struct fs_parameter *param)
>>>       struct afs_fs_context *ctx = fc->fs_private;
>>>       int opt;
>>>
>>> +     kenter("%s,%p '%s'", param->key, param->string, param->string);
>>> +
>>>       opt = fs_parse(fc, afs_fs_parameters, param, &result);
>>>       if (opt < 0)
>>>               return opt;
>>> diff --git a/fs/fs_context.c b/fs/fs_context.c
>>> index 2834d1afa6e8..f530a33876ce 100644
>>> --- a/fs/fs_context.c
>>> +++ b/fs/fs_context.c
>>> @@ -450,6 +450,8 @@ void put_fs_context(struct fs_context *fc)
>>>       put_user_ns(fc->user_ns);
>>>       put_cred(fc->cred);
>>>       put_fc_log(fc);
>>> +     if (strcmp(fc->fs_type->name, "afs") == 0)
>>> +             printk("PUT %p '%s'\n", fc->source, fc->source);
>>>       put_filesystem(fc->fs_type);
>>>       kfree(fc->source);
>>>       kfree(fc);
>>> @@ -671,6 +673,8 @@ void vfs_clean_context(struct fs_context *fc)
>>>       fc->s_fs_info = NULL;
>>>       fc->sb_flags = 0;
>>>       security_free_mnt_opts(&fc->security);
>>> +     if (strcmp(fc->fs_type->name, "afs") == 0)
>>> +             printk("CLEAN %p '%s'\n", fc->source, fc->source);
>>>       kfree(fc->source);
>>>       fc->source = NULL;
>>>
>>>
>>
>> I'll check more after my test machine is working again.
>>
>> thanks.
>> --
>> ~Randy
>>
>> --
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-- 
~Randy

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