linux-kernel - Re: [PATCH 2/3, V2] kernel: Move groups_sort to the caller of set

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Message-ID: <87efoatfsb.fsf@notabene.neil.brown.name>
Date:   Tue, 05 Dec 2017 07:11:00 +1100
From:   NeilBrown <neilb@...e.com>
To:     Thiago Rafael Becker <thiago.becker@...il.com>
Cc:     Thiago Rafael Becker <thiago.becker@...il.com>,
        bfields@...ldses.org, linux-nfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3, V2] kernel: Move groups_sort to the caller of set_groups.

On Mon, Dec 04 2017, Thiago Rafael Becker wrote:

> On Mon, 4 Dec 2017, NeilBrown wrote:
>
>> I think you need to add groups_sort() in a few more places.
>> Almost anywhere that calls groups_alloc() should be considered.
>> net/sunrpc/svcauth_unix.c, net/sunrpc/auth_gss/svcauth_gss.c,
>> fs/nfsd/auth.c definitely need it.
>
> So are any other functions that modify group_info. OK, I think I'll 
> implement the type detection below as it helps detecting where these 
> situations are located.
>
> This may take some time to make sane. I wonder if we shouldn't 
> accept the first change suggested to fix the corruption detected in 
> auth.unix.gid while I work on a new set of patches. 

As we don't seem to be pursuing this possibility is probably isn't very
important, but I'd like to point out that the original fix isn't a true
fix.
It just sorts a shared group_info early.  This does not stop corruption.
Every time a thread calls set_groups() on that group_info it will be
sorted again.
The sort algorithm used is the heap sort, and a heap sort always moves
elements in the array around - it does not leave a sorted array
untouched (unlike e.g. the quick sort which doesn't move anything in a
sorted array).
So it is still possible for two calls to groups_sort() to race.
We *need* to move groups_sort() out of set_groups().

Thanks,
NeilBrown


>    Also, that patch 
> doesn't change behavior of set_groups, and is easier to backport if 
> distros relying on older kernels need to do so and change behavior. The 
> first suggestion is undergoing tests, and so far we didn't detect any 
> new corruptions on auth.unix.gid.
>
>> Maybe it could be done with types.
>
> I changed the interfaces on groups_{alloc,sort} to check. There are some 
> extra changes needed in groups_from_user and others to make this viable, 
> but I like it and I'll try to make it happen.
>
>> Thanks,
>> NeilBrown
>>
>
> Thanks,
> trbecker

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