Re: suspicious __GFP_NOMEMALLOC in selinux

From: Michal Hocko
Date: Thu Aug 03 2017 - 04:12:03 EST

[CC Mel]

On Wed 02-08-17 17:45:56, Paul Moore wrote:
> 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]

Sorry about that. Will keep it in mind for next posts

> 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.

Thanks for the pointer. That makes much more sense now. Back in 2012 we
really didn't have a good way to distinguish non sleeping and atomic
with reserves allocations.

> 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.

What do you think about the following? I haven't tested it but it should
be rather straightforward.