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Date:   Tue, 22 Sep 2020 09:43:02 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     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()
Message-ID: <20200922164301.GB30874@linux.intel.com>
References: <20200915112842.897265-1-jarkko.sakkinen@linux.intel.com>
 <20200915112842.897265-11-jarkko.sakkinen@linux.intel.com>
 <CALCETrX9T1ZUug=M5ba9g4H5B7kV=yL5RzuTaeAEdy3uAieN_A@mail.gmail.com>
 <20200918235337.GA21189@sjchrist-ice>
 <20200921124946.GF6038@linux.intel.com>
 <20200921165758.GA24156@linux.intel.com>
 <20200921210736.GB58176@linux.intel.com>
 <20200921211849.GA25403@linux.intel.com>
 <20200922052957.GA97272@linux.intel.com>
 <20200922053515.GA97687@linux.intel.com>
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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".