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

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Date:   Tue, 22 Sep 2020 08:35:15 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 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:
> > On Tue, Sep 22, 2020 at 12:07:36AM +0300, Jarkko Sakkinen wrote:
> > > On Mon, Sep 21, 2020 at 09:57:58AM -0700, Sean Christopherson wrote:
> > > > On Mon, Sep 21, 2020 at 03:49:46PM +0300, Jarkko Sakkinen wrote:
> > > > > On Fri, Sep 18, 2020 at 04:53:37PM -0700, Sean Christopherson wrote:
> > > > > > a noexec filesystem by loading code into an enclave, and to give the kernel the
> > > > > > option of adding enclave specific LSM policies in the future.
> > > > > > 
> > > > > > The source file (if one exists) for the enclave is long gone when the enclave
> > > > > > is actually mmap()'d and mprotect()'d.  To enforce noexec, the requested
> > > > > > permissions for a given page are snapshotted when the page is added to the
> > > > > > enclave, i.e. when the enclave is built.  Enclave pages that will be executable
> > > > > > must originate from an a MAYEXEC VMA, e.g. the source page can't come from a
> > > > > > noexec file system.
> > > > > 
> > > > > noexec check is done in __sgx_encl_add_page(), not in this callback.
> > > > > sgx_vma_mprotect() calls sgx_encl_may_map(), which iterates the
> > > > > addresses, checks that permissions are not surpassed and there are
> > > > > no holes.
> > > > 
> > > > Yes, that's what I said.
> > > 
> > > sgx_encl_add_page() will remove such page. The callback does not
> > > interact with this process as such pages never get to the enclave.
> > 
> > I think we're in violent agreement, mostly.
> > 
> > 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.

/Jarkko