Re: [PATCH 2/2] x86/sgx: Maintain encl->refcount for each encl->mm_list entry

Jarkko Sakkinen
2021-02-07T21:33:18Z

On Sun, Feb 07, 2021 at 11:29:49PM +0200, Jarkko Sakkinen wrote:
> On Fri, Feb 05, 2021 at 11:36:57AM -0800, Dave Hansen wrote:
> > On 2/5/21 10:28 AM, Jarkko Sakkinen wrote:
> > > This has been shown in tests:
> > >
> > > [ +0.000008] WARNING: CPU: 3 PID: 7620 at kernel/rcu/srcutree.c:374 cleanup_srcu_struct+0xed/0x100
> > >
> > > There are two functions that drain encl->mm_list:
> > >
> > > - sgx_release() (i.e. VFS release) removes the remaining mm_list entries.
> > > - sgx_mmu_notifier_release() removes mm_list entry for the registered
> > > process, if it still exists.
> >
> > Jarkko, I like your approach. This actually has the potential to be a
> > lot more understandable than the fix we settled on before.
>
> Yeah, it's more like by-the-book use of refcount, each processs gets
> a reference. This way things should be always serialized correctly.
>
> > But I think the explanation needs some tweaking, and I think I can take
> > it a step further to make it even more straightforward. The issue here
> > isn't *really* mm_list, it's this:
> >
> > encl_mm->encl = encl;
>
> Agreed.
>
> This was also in center of thinking when I did this new patch.
>
> > That literally establishes a encl_mm to encl reference and needs a
> > reference count. That reference remains until 'encl_mm' is freed. I
> > don't think mm_list needs to even be taken into account.
> >
> > The most straightforward way to fix this is to take a refcount at
> > "encl_mm->encl = encl" and release it at kfree(encl_mm). That makes a
> > *lot* of logical sense to me, and it's also trivial to audit.
> >
> > Totally untested patch attached (adapted directly from yours).
>
> I tested this version, and it also seems to work. Boris, can you
> pick this refined version from Dave's attachment or do you prefer
> that I do a re-send?

Nevermind. I'll send a proper patch (just noticed that the attachment
did have short summary).

/Jarkko