Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()

From: "Haitao Huang" <haitao.huang@linux.intel.com>
To: "Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Jarkko Sakkinen" <jarkko.sakkinen@linux.intel.com>
Cc: "Andy Lutomirski" <luto@kernel.org>, "X86 ML" <x86@kernel.org>,
	linux-sgx@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Jethro Beekman" <jethro@fortanix.com>,
	"Darren Kenny" <darren.kenny@oracle.com>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	asapek@google.com, "Borislav Petkov" <bp@alien8.de>,
	"Xing, Cedric" <cedric.xing@intel.com>,
	chenalexchen@google.com,
	"Conrad Parker" <conradparker@google.com>,
	cyhanish@google.com, "Dave Hansen" <dave.hansen@intel.com>,
	"Huang, Haitao" <haitao.huang@intel.com>,
	"Josh Triplett" <josh@joshtriplett.org>,
	"Huang, Kai" <kai.huang@intel.com>,
	"Svahn, Kai" <kai.svahn@intel.com>,
	"Keith Moyer" <kmoy@google.com>,
	"Christian Ludloff" <ludloff@google.com>,
	"Neil Horman" <nhorman@redhat.com>,
	"Nathaniel McCallum" <npmccallum@redhat.com>,
	"Patrick Uiterwijk" <puiterwijk@redhat.com>,
	"David Rientjes" <rientjes@google.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	yaozhangx@google.com
Subject: Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()
Date: Thu, 24 Sep 2020 14:11:37 -0500	[thread overview]
Message-ID: <op.0rgp5h0hwjvjmi@mqcpg7oapc828.gar.corp.intel.com> (raw)
In-Reply-To: <20200923135056.GD5160@linux.intel.com>

On Wed, 23 Sep 2020 08:50:56 -0500, Jarkko Sakkinen  
<jarkko.sakkinen@linux.intel.com> wrote:

> On Tue, Sep 22, 2020 at 09:43:02AM -0700, Sean Christopherson wrote:
>> On Tue, Sep 22, 2020 at 08:35:15AM +0300, Jarkko Sakkinen wrote:
>> > On Tue, Sep 22, 2020 at 08:30:06AM +0300, Jarkko Sakkinen wrote:
>> > > On Mon, Sep 21, 2020 at 02:18:49PM -0700, Sean Christopherson wrote:
>> > > > Userspace can add the page without EXEC permissions in the EPCM,  
>> and thus
>> > > > avoid the noexec/VM_MAYEXEC check.  The enclave can then do  
>> EMODPE to gain
>> > > > EXEC permissions in the EPMC.  Without the ->mprotect() hook, we  
>> wouldn't
>> > > > be able to detect/prevent such shenanigans.
>> > >
>> > > Right, the VM_MAYEXEC in the code is nested under VM_EXEC check.
>> > >
>> > > I'm only wondering why not block noexec completely with any  
>> permissions,
>> > > i.e. why not just have unconditional VM_MAYEXEC check?
>> >
>> > I.e. why not this:
>> >
>> > static int __sgx_encl_add_page(struct sgx_encl *encl,
>> > 			       struct sgx_encl_page *encl_page,
>> > 			       struct sgx_epc_page *epc_page,
>> > 			       struct sgx_secinfo *secinfo, unsigned long src)
>> > {
>> > 	struct sgx_pageinfo pginfo;
>> > 	struct vm_area_struct *vma;
>> > 	struct page *src_page;
>> > 	int ret;
>> >
>> > 	vma = find_vma(current->mm, src);
>> > 	if (!vma)
>> > 		return -EFAULT;
>> >
>> > 	if (!(vma->vm_flags & VM_MAYEXEC))
>> > 		return -EACCES;
>> >
>> > I'm not seeing the reason for "partial support" for noexec partitions.
>> >
>> > If there is a good reason, fine, let's just then document it.
>>
>> There are scenarios I can contrive, e.g. loading an enclave from a  
>> noexec
>> filesystem without having to copy the entire enclave to anon memory, or
>> loading a data payload from a noexec FS.
>>
>> They're definitely contrived scenarios, but given that we also want the
>> ->mprotect() hook/behavior for potential LSM interaction, supporting  
>> said
>> contrived scenarios costs is "free".
>
> For me this has caused months of confusion and misunderstanding of this
> feature. I only recently realized that "oh, right, we invented this".
>
> They are contrived scenarios enough that they should be considered when
> the workloads hit.
>
> Either we fully support noexec or not at all. Any "partial" thing is a
> two edged sword: it can bring some robustness with the price of
> complexity and possible unknown uknown scenarios where they might become
> API issue.
>
> I rather think later on how to extend API in some way to enable such
> contrivid scenarios rather than worrying about how this could be abused.
>
> The whole SGX is complex beast already so lets not add any extra when
> there is no a hard requirement to do so.
>
> I'll categorically deny noexec in the next patch set version.
>
> /Jarkko

There are use cases supported currently in which enclave binary is  
received via IPC/RPC and held in buffers before EADD. Denying noexec  
altogether would break those, right?