linux-kernel - Re: [PATCH v30 12/20] x86/sgx: Add a page reclaimer

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Message-ID: <20200522195705.GA121599@linux.intel.com>
Date:   Fri, 22 May 2020 22:57:05 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, nhorman@...hat.com, npmccallum@...hat.com,
        haitao.huang@...el.com, andriy.shevchenko@...ux.intel.com,
        tglx@...utronix.de, kai.svahn@...el.com, bp@...en8.de,
        josh@...htriplett.org, luto@...nel.org, kai.huang@...el.com,
        rientjes@...gle.com, cedric.xing@...el.com, puiterwijk@...hat.com,
        linux-mm@...ck.org, Jethro Beekman <jethro@...tanix.com>,
        Jordan Hand <jorhand@...ux.microsoft.com>,
        Chunyang Hui <sanqian.hcy@...fin.com>,
        Seth Moore <sethmo@...gle.com>
Subject: Re: [PATCH v30 12/20] x86/sgx: Add a page reclaimer

On Thu, May 21, 2020 at 11:58:02PM -0700, Sean Christopherson wrote:
> > +		kref_put(&encl_page->encl->refcount, sgx_encl_release);
> > +
> > +		spin_lock(&sgx_active_page_list_lock);
> > +		list_add_tail(&epc_page->list, &sgx_active_page_list);
> > +		spin_unlock(&sgx_active_page_list_lock);
> 
> Ugh, this is wrong.  If the above kref_put() drops the last reference and
> releases the enclave, adding the page to the active page list will result
> in a use-after-free as the enclave will have been freed.  It also leaks the
> EPC page because sgx_encl_destroy() skips pages that are in the process of
> being reclaimed (as detected by list_empty()).
> 
> The "original" code did the put() after list_add_tail(), but was moved in
> v15 to fix a bug where the put() could drop a reference to the wrong enclave
> if the page was freed and reallocated by a different CPU between
> list_add_tail() and put().  But, that particular bug only occurred because
> the code at the time was:
> 
> 	sgx_encl_page_put(epc_page);
> 
> I.e. the backpointer in epc_page was consumed after dropping the spin lock.
> So long as epc_page->owner (well, epc_page in general) isn't dereferenced,
> I'm 99% certain this can be fixed simply by doing kref_put() after moving
> the page back to the active page list.

Yes. It is certainly a regression to not call it after
sgx_active_page_list. That was a good catch, thanks.

v31:
* Unset SGX_ENCL_IOCTL in the error path of checking encl->flags in order
  to prevent leaving it set and thus block any further ioctl calls.
* Added missing cleanup_srcu_struct() call to sgx_encl_release().
* Take encl->lock in sgx_encl_add_page() in order to prevent races with
  the page reclaimer.
* Fix a use-after-free bug from page reclaimer. Call kref_put() for the
  encl->refcount only after putting enclave page back to the active page
  list because it could be the last ref to the enclave.

I'm ready to send a new version of the patch set once there is a
conclusion with the sigstruct vendor field.

/Jarkko