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SubjectRe: [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
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().


> 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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