linux-kernel - Re: [PATCH v6 1/7] mseal, system mappings: kernel config and header change

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Message-ID: <202502241123.D398A24@keescook>
Date: Mon, 24 Feb 2025 11:25:00 -0800
From: Kees Cook <kees@...nel.org>
To: Jeff Xu <jeffxu@...omium.org>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Dave Hansen <dave.hansen@...el.com>, akpm@...ux-foundation.org,
	jannh@...gle.com, torvalds@...ux-foundation.org, vbabka@...e.cz,
	lorenzo.stoakes@...cle.com, adhemerval.zanella@...aro.org,
	oleg@...hat.com, avagin@...il.com, benjamin@...solutions.net,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	linux-mm@...ck.org, jorgelo@...omium.org, sroettger@...gle.com,
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	mike.rapoport@...il.com
Subject: Re: [PATCH v6 1/7] mseal, system mappings: kernel config and header
 change

On Mon, Feb 24, 2025 at 11:10:22AM -0800, Jeff Xu wrote:
> On Mon, Feb 24, 2025 at 11:03 AM Liam R. Howlett
> <Liam.Howlett@...cle.com> wrote:
> >
> > * Jeff Xu <jeffxu@...omium.org> [250224 13:44]:
> > > On Mon, Feb 24, 2025 at 10:21 AM Dave Hansen <dave.hansen@...el.com> wrote:
> > > >
> > > > On 2/24/25 09:45, jeffxu@...omium.org wrote:
> > > > > +/*
> > > > > + * mseal of userspace process's system mappings.
> > > > > + */
> > > > > +#ifdef CONFIG_MSEAL_SYSTEM_MAPPINGS
> > > > > +#define MSEAL_SYSTEM_MAPPINGS_VM_FLAG        VM_SEALED
> > > > > +#else
> > > > > +#define MSEAL_SYSTEM_MAPPINGS_VM_FLAG        VM_NONE
> > > > > +#endif
> > > >
> > > > This ends up looking pretty wonky in practice:
> > > >
> > > > > +     vm_flags = VM_READ|VM_MAYREAD|VM_IO|VM_DONTDUMP|VM_PFNMAP;
> > > > > +     vm_flags |= MSEAL_SYSTEM_MAPPINGS_VM_FLAG;
> > > >
> > > > because MSEAL_SYSTEM_MAPPINGS_VM_FLAG is so much different from the
> > > > other ones.
> > > >
> > > > Would it really hurt to have
> > > >
> > > >  #ifdef CONFIG_64BIT
> > > >  /* VM is sealed, in vm_flags */
> > > >  #define VM_SEALED       _BITUL(63)
> > > > +#else
> > > > +#define VM_SEALED       VM_NONE
> > > >  #endif
> > > >
> > > > ?
> > > >
> > > VM_SEALED isn't defined in 32-bit systems, and mseal.c isn't part of
> > > the build. This is intentional. Any 32-bit code trying to use the
> > > sealing function or the VM_SEALED flag will immediately fail
> > > compilation. This makes it easier to identify incorrect usage.
> > >
> >
> > The reason that two #defines are needed is because you can have mseal
> > enabled while not sealing system mappings, so for this to be clean we
> > need two defines.
> >
> > However MSEAL_SYSTEM_MAPPINGS_VM_FLAG, is _way_ too long, in my opinion.
> > Keeping with "VM_SEALED" I'd suggest "VM_SYSTEM_SEALED".
> >
> How about MSEAL_SYSTME_MAPPINGS as Kees suggested ?
> 
> The VM_SYSTEM_SEALED doesn't have the MSEAL key or the MAPPING, so it
> might take longer for the new reader to understand what it is.

I think to address Dave's concern, it should start with "VM_". We use
"SEAL" already with VM_SEALED, so the "M" in "MSEAL" may be redundant,
and to clarify the system mapping, how avout VM_SEAL_SYSMAP  ? That
capture's, hopefully, Dave, Liam, and your thoughts about the naming?

-- 
Kees Cook