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

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Date:   Fri, 5 Feb 2021 11:36:57 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>, linux-sgx@...r.kernel.org
Cc:     Haitao Huang <haitao.huang@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        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 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.

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;

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).

View attachment "raw.patch" of type "text/x-patch" (2926 bytes)