Re: suspicious __GFP_NOMEMALLOC in selinux

From: Paul Moore
Date: Wed Aug 02 2017 - 17:46:04 EST

On Wed, Aug 2, 2017 at 6:50 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> Hi,
> while doing something completely unrelated to selinux I've noticed a
> really strange __GFP_NOMEMALLOC usage pattern in selinux, especially
> GFP_ATOMIC | __GFP_NOMEMALLOC doesn't make much sense to me. GFP_ATOMIC
> on its own allows to access memory reserves while the later flag tells
> we cannot use memory reserves at all. The primary usecase for
> __GFP_NOMEMALLOC is to override a global PF_MEMALLOC should there be a
> need.
> It all leads to fa1aa143ac4a ("selinux: extended permissions for
> ioctls") which doesn't explain this aspect so let me ask. Why is the
> flag used at all? Moreover shouldn't GFP_ATOMIC be actually GFP_NOWAIT.
> What makes this path important to access memory reserves?

[NOTE: added the SELinux list to the CC line, please include that list
when asking SELinux questions]

The GFP_ATOMIC|__GFP_NOMEMALLOC use in SELinux appears to be limited
to security/selinux/avc.c, and digging a bit, I'm guessing commit
fa1aa143ac4a copied the combination from 6290c2c43973 ("selinux: tag
avc cache alloc as non-critical") and the avc_alloc_node() function.

I can't say that I'm an expert at the vm subsystem and the variety of
different GFP_* flags, but your suggestion of moving to GFP_NOWAIT in
security/selinux/avc.c seems reasonable and in keeping with the idea
behind commit 6290c2c43973.

paul moore
security @ redhat