Re: Stupid VFS name lookup interface..

[Casey Schaufler]

On 5/26/2013 11:17 AM, Linus Torvalds wrote:
> On Sun, May 26, 2013 at 10:59 AM, Casey Schaufler
> <casey@xxxxxxxxxxxxxxxx> wrote:
>> The whole secid philosophy comes out of the need to keep security out
>> of other people's way. It has performance impact. Sure, SELinux
>> hashes lookups, but a blob pointer gets you right where you want to be.
>> When we are constrained in unnatural ways there are going to be
>> consequences. Performance is one. Code complexity is another.
> Quite frankly, I'd like to possibly introduce a cache of security
> decisions at least for the common filesystem operations (and the
> per-pathcomponent lookup is *the* most common one, but the per-stat
> one is pretty bad too), and put that cache at the VFS level, so that
> the security people can *not* screw it up, and so that we don't call
> down to the security layer at all 99% of the time.
>
> Once that happens, we don't care any more what security people do.
>
> It has been how we have fixed performance problems for filesystems
> every single time. It's simply not possible to have a generic
> interface to 50+ different filesystems and expect that kind of generic
> interface to be high-performance - but when we've been able to
> abstract it out as a cache in front of the filesystem operations, it
> is suddenly quite reasonable to spend a lot of effort making that
> cache go fast like a bat out of hell.
>
> It started with the dentry cache and the page cache, but we now do the
> POSIX ACL's that way too, because it just wasn't reasonable to call
> down to the filesystem to look up ACL's and have all the complex "we
> do this with RCU locks held" semantics.
>
> Is there something similar we could do for the security layer? We
> don't have 50+ different security models, but we do have several. If
> the different security modules could agree on some kind of generic
> "security ID" model so that we could cache things (see fs/namei.c and
> get_cached_acl_rcu() for example), it would be a great thing.
>
> Then selinux could get rid of it's hashed lookups entirely, because
> that whole "cache the security ID" would be handled by generic code.
>
> But that *would* require that there would be some abstract notion of
> security ID/context that we could use in generic code *WITHOUT* the
> need to call down the the security subsystem.
>
> The indirect calls are expensive, but they are expensive not because
> an indirect call itself is particularly expensive (although that's
> true on some architectures too), but because the whole notion of "I'm
> calling down to the lower-level non-generic code" means that we can't
> do inlining, we can't optimize locking, we can't do anything clever.
>
> My selinux patch kept the indirect call, but at least made it cheap.
> Could we do even better? And keep it generic?

Probably, but we'll need help from people who really understand VFS,
caching and RCU.

> Btw, if we can do something like that, then nested security modules
> likely get much easier to do too, because the nesting would all be
> behind the cache. Once it's behind the cache, it doesn't matter if
> we'd need to traverse lists etc. The hot case would be able to ignore
> it all.

Indeed. It may require significant revision to the existing LSMs.

>
>                   Linus
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