linux-kernel - Re: [GIT PULL] SELinux patches for v5.8

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Date:   Wed, 3 Jun 2020 10:37:47 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Paul Moore <paul@...l-moore.com>, selinux@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] SELinux patches for v5.8

On Wed, Jun 3, 2020 at 10:20 AM Casey Schaufler <casey@...aufler-ca.com> wrote:
>
> We could have inode->i_security be the blob, rather than a pointer to it.
> That will have its own performance issues.

It wouldn't actually really fix anything, because the inode is so big
and sparsely accessed that it doesn't even really help the cache
density issue. Yeah, it gets rid of the pointer access, but that's
pretty much it. The fact that we randomize the order means that we
can't even really try to aim for any cache density.

And it would actually not be possible with the current layered
security model anyway, since those blob sizes are dynamic at runtime.

If we had _only_ SELinux, we could perhaps have hidden the
sid/sclass/task_sid directly in the inode (it would be only slightly
larger than the pointer is, anyway), but even that ship sailed long
long ago due to the whole "no security person can ever agree with
another one on fundamentals".

So don't try to blame the rest of the system design. This is on the
security people. We've been able to handle other layers fairly well
because they generally agree on fundamentals (although it can take
decades before they then end up merging their code - things like the
filesystem people standardizing on iomap and other core concepts). And
as mentioned, when there is agreed-upon security rules (ie "struct
cred") we've been able to spend the effort to architect it so that it
doesn't add unnecessary overheads.

             Linus