linux-kernel - Re: [PATCH] x86/sgx: Do not limit EAUG'd pages by pre-initialization policy

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Date:   Fri, 4 Mar 2022 03:32:28 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     linux-sgx@...r.kernel.org
Cc:     Reinette Chatre <reinette.chatre@...el.com>,
        Nathaniel McCallum <nathaniel@...fian.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/sgx: Do not limit EAUG'd pages by pre-initialization
 policy

On Fri, Mar 04, 2022 at 03:25:38AM +0200, Jarkko Sakkinen wrote:
> On Fri, Mar 04, 2022 at 03:16:35AM +0200, Jarkko Sakkinen wrote:
> > Pre-initialization policy is meant for EADD'd pages because they are
> > part of the enclave identity. It's a good practice to not let touch the
> > permissions after initialization, and does provide guarantees to e.g.
> > LSM's about the enclave.
> > 
> > For EAUG'd pages it should be sufficient to let mmap(), mprotect() and
> > SGX opcodes to control the permissions. Thus effectively disable
> > pre-initialization policy by setting vm_max_prot_bit and
> > vm_run_prot_bits to RWX.
> > 
> > Cc: Reinette Chatre <reinette.chatre@...el.com>
> > Cc: Nathaniel McCallum <nathaniel@...fian.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > ---
> >  arch/x86/kernel/cpu/sgx/encl.c | 8 +-------
> >  1 file changed, 1 insertion(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> > index 5fe7189eac9d..17feb6fa5578 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -200,13 +200,7 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
> >  	encl_page->desc = addr;
> >  	encl_page->encl = encl;
> >  
> > -	/*
> > -	 * Adding a regular page that is architecturally allowed to only
> > -	 * be created with RW permissions.
> > -	 * TBD: Interface with user space policy to support max permissions
> > -	 * of RWX.
> > -	 */
> > -	prot = PROT_READ | PROT_WRITE;
> > +	prot = PROT_READ | PROT_WRITE | PROT_EXEC;
> >  	encl_page->vm_run_prot_bits = calc_vm_prot_bits(prot, 0);
> >  	encl_page->vm_max_prot_bits = encl_page->vm_run_prot_bits;
> >  
> > -- 
> > 2.35.1
> > 
> 
> This does not break any existing ABI and at least makes the current
> patch set usable.

Also it would be a sane limitation to deny EMODPR and EMODT completely for
EADD'd pages.

Then, you can discard vm_run_prot_bits. It's not needed for anything
anymore.

This should make implementation considerably less obfuscated.

BR, Jarkko