linux-kernel - Re: [PATCH v34 10/24] mm: Add vm

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Date:   Wed, 8 Jul 2020 19:10:27 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jethro Beekman <jethro@...tanix.com>,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.com, haitao.huang@...el.com,
        josh@...htriplett.org, kai.huang@...el.com, kai.svahn@...el.com,
        kmoy@...gle.com, ludloff@...gle.com, luto@...nel.org,
        nhorman@...hat.com, npmccallum@...hat.com, puiterwijk@...hat.com,
        rientjes@...gle.com, tglx@...utronix.de, yaozhangx@...gle.com
Subject: Re: [PATCH v34 10/24] mm: Add vm_ops->mprotect()

On Wed, Jul 08, 2020 at 03:37:08PM +0100, Matthew Wilcox wrote:
> On Wed, Jul 08, 2020 at 05:33:20PM +0300, Jarkko Sakkinen wrote:
> > I get the point but I don't think that your proposal could work given
> > that mprotect-callback takes neither 'prev' nor 'newflags' as its
> > parameters. The current callback has no means to call mprotect_fixup()
> > properly.
> > 
> > It would have to be extended
> > 
> > 	int (*mprotect)(struct vm_area_struct *vma,
> > 			struct vm_area_struct **pprev, unsigned long start,
> > 			unsigned long end, unsigned long prot,
> > 			unsigned long newflags);
> > 
> > Is this what you want?
> 
> https://lore.kernel.org/linux-mm/20200625173050.GF7703@casper.infradead.org/

Ugh, it's there as it should be. I'm sorry - I just misread the code.

I think that should work, and we do not have to do extra conversion
inside the callback.

There is still one thing that I'm wondering. 'page->vm_max_prot_bits'
contains some an union of subset of {VM_READ, VM_WRITE, VM_EXEC}, and
'newflags' can contain other bits set too.

The old implementation of sgx_vma_mprotect() is like this:

static int sgx_vma_mprotect(struct vm_area_struct *vma, unsigned long start,
			    unsigned long end, unsigned long prot)
{
	return sgx_encl_may_map(vma->vm_private_data, start, end,
				calc_vm_prot_bits(prot, 0));
}

The new one should be probably the implemented along the lines of

static int sgx_vma_mprotect(struct vm_area_struct *vma,
			    struct vm_area_struct **pprev, unsigned long start,
			    unsigned long end, unsigned long newflags)
{
	unsigned long masked_newflags = newflags &
					(VM_READ | VM_WRITE | VM_EXEC);
	int ret;

	ret = sgx_encl_may_map(vma->vm_private_data, start, end,
				   masked_newflags);
	if (ret)
		return ret;

	return mprotect_fixup(vma, pprev, start, end, newflags);
}

Alternatively the filtering can be inside sgx_encl_may_map(). Perhaps
that is a better place for it. This was just easier function to
represent the idea.

/Jarkko