linux-kernel - Re: hv_hypercall

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Message-ID: <20200616143940.GA15676@lst.de>
Date:   Tue, 16 Jun 2020 16:39:40 +0200
From:   Christoph Hellwig <hch@....de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christoph Hellwig <hch@....de>, Dexuan Cui <decui@...rosoft.com>,
        vkuznets <vkuznets@...hat.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Michael Kelley <mikelley@...rosoft.com>,
        Ju-Hyoung Lee <juhlee@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: hv_hypercall_pg page permissios

On Tue, Jun 16, 2020 at 01:24:15PM +0200, Peter Zijlstra wrote:
> > +void *module_alloc_prot(unsigned long size, pgprot_t prot)
> >  {
> > +	unsigned int flags = (pgprot_val(prot) & _PAGE_NX) ?
> > +			0 : VM_FLUSH_RESET_PERMS;
> >  	void *p;
> >  
> >  	if (PAGE_ALIGN(size) > MODULES_LEN)
> > @@ -75,7 +77,7 @@ void *module_alloc(unsigned long size)
> >  	p = __vmalloc_node_range(size, MODULE_ALIGN,
> >  				    MODULES_VADDR + get_module_load_offset(),
> >  				    MODULES_END, GFP_KERNEL,
> > -				    PAGE_KERNEL, 0, NUMA_NO_NODE,
> > +				    prot, flags, NUMA_NO_NODE,
> >  				    __builtin_return_address(0));
> >  	if (p && (kasan_module_alloc(p, size) < 0)) {
> >  		vfree(p);
> 
> Hurmm.. Yes it would. It just doesn't feel right though. Can't we
> unconditionally set the flag? At worst it makes free a little bit more
> expensive.
> 
> The thing is, I don't think _NX is the only prot that needs restoring.
> Any prot other than the default (RW IIRC) needs restoring.

If that actually is the case I think we should just check for
a non-default permission in __vmalloc_node_range itself and handle it
there, which seems like a nice solution.