linux-kernel - Re: [PATCH v38 10/24] mm: Add vm

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Message-ID: <op.0rgp5h0hwjvjmi@mqcpg7oapc828.gar.corp.intel.com>
Date:   Thu, 24 Sep 2020 14:11:37 -0500
From:   "Haitao Huang" <haitao.huang@...ux.intel.com>
To:     "Sean Christopherson" <sean.j.christopherson@...el.com>,
        "Jarkko Sakkinen" <jarkko.sakkinen@...ux.intel.com>
Cc:     "Andy Lutomirski" <luto@...nel.org>, "X86 ML" <x86@...nel.org>,
        linux-sgx@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Matthew Wilcox" <willy@...radead.org>,
        "Jethro Beekman" <jethro@...tanix.com>,
        "Darren Kenny" <darren.kenny@...cle.com>,
        "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
        asapek@...gle.com, "Borislav Petkov" <bp@...en8.de>,
        "Xing, Cedric" <cedric.xing@...el.com>, chenalexchen@...gle.com,
        "Conrad Parker" <conradparker@...gle.com>, cyhanish@...gle.com,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        "Josh Triplett" <josh@...htriplett.org>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Svahn, Kai" <kai.svahn@...el.com>,
        "Keith Moyer" <kmoy@...gle.com>,
        "Christian Ludloff" <ludloff@...gle.com>,
        "Neil Horman" <nhorman@...hat.com>,
        "Nathaniel McCallum" <npmccallum@...hat.com>,
        "Patrick Uiterwijk" <puiterwijk@...hat.com>,
        "David Rientjes" <rientjes@...gle.com>,
        "Thomas Gleixner" <tglx@...utronix.de>, yaozhangx@...gle.com
Subject: Re: [PATCH v38 10/24] mm: Add vm_ops->mprotect()

On Wed, 23 Sep 2020 08:50:56 -0500, Jarkko Sakkinen  
<jarkko.sakkinen@...ux.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?