linux-kernel - Re: [PATCH 2/2] x86/sgx: Maintain encl->refcount for each encl->mm

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Date:   Sun, 7 Feb 2021 23:32:06 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>, Borislav Petkov <bp@...en8.de>
Cc:     linux-sgx@...r.kernel.org,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/sgx: Maintain encl->refcount for each
 encl->mm_list entry

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