linux-kernel - Re: [PATCH v3 1/5] x86/sgx: Fix a resource leak in sgx

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Date:   Wed, 10 Mar 2021 07:49:29 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>, linux-sgx@...r.kernel.org,
        stable@...r.kernel.org, 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>,
        Serge Ayoun <serge.ayoun@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] x86/sgx: Fix a resource leak in sgx_init()

On Wed, Mar 10, 2021, Jarkko Sakkinen wrote:
> On Wed, Mar 03, 2021 at 08:56:52AM -0800, Dave Hansen wrote:
> > On 3/3/21 7:03 AM, Jarkko Sakkinen wrote:
> > > If sgx_page_cache_init() fails in the middle, a trivial return
> > > statement causes unused memory and virtual address space reserved for
> > > the EPC section, not freed. Fix this by using the same rollback, as
> > > when sgx_page_reclaimer_init() fails.
> > ...
> > > @@ -708,8 +708,10 @@ static int __init sgx_init(void)
> > >  	if (!cpu_feature_enabled(X86_FEATURE_SGX))
> > >  		return -ENODEV;
> > >  
> > > -	if (!sgx_page_cache_init())
> > > -		return -ENOMEM;
> > > +	if (!sgx_page_cache_init()) {
> > > +		ret = -ENOMEM;
> > > +		goto err_page_cache;
> > > +	}
> > 
> > 
> > Currently, the only way sgx_page_cache_init() can fail is in the case
> > that there are no sections:
> > 
> >         if (!sgx_nr_epc_sections) {
> >                 pr_err("There are zero EPC sections.\n");
> >                 return false;
> >         }
> > 
> > That only happened if all sgx_setup_epc_section() calls failed.
> > sgx_setup_epc_section() never both allocates memory with vmalloc for
> > section->pages *and* fails.  If sgx_setup_epc_section() has a successful
> > memremap() but a failed vmalloc(), it cleans up with memunmap().
> > 
> > In other words, I see how this _looks_ like a memory leak from
> > sgx_init(), but I don't see an actual leak in practice.
> > 
> > Am I missing something?
> 
> In sgx_setup_epc_section():
> 
> 
> 	section->pages = vmalloc(nr_pages * sizeof(struct sgx_epc_page));
> 	if (!section->pages) {
> 		memunmap(section->virt_addr);
> 		return false;
> 	}
> 
> I.e. this rollback does not happen without this fix applied:
> 
> 	for (i = 0; i < sgx_nr_epc_sections; i++) {
> 		vfree(sgx_epc_sections[i].pages);
> 		memunmap(sgx_epc_sections[i].virt_addr);
> 	}

Dave is pointing out that sgx_page_cache_init() fails if and only if _all_
sections fail sgx_setup_epc_section(), and if all sections fail then
sgx_nr_epc_sections is '0' and the above is a nop.

That behavior is by design, as we didn't want to kill SGX if a single section
failed to initialize for whatever reason.