linux-hardening - Re: [PATCH v8 2/4] mseal: add mseal syscall

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Date: Thu, 1 Feb 2024 19:30:29 -0800
From: Jeff Xu <jeffxu@...omium.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: akpm@...ux-foundation.org, keescook@...omium.org, jannh@...gle.com, 
	sroettger@...gle.com, willy@...radead.org, gregkh@...uxfoundation.org, 
	torvalds@...ux-foundation.org, usama.anjum@...labora.com, 
	rdunlap@...radead.org, jeffxu@...gle.com, jorgelo@...omium.org, 
	groeck@...omium.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, pedro.falcato@...il.com, 
	dave.hansen@...el.com, linux-hardening@...r.kernel.org, deraadt@...nbsd.org
Subject: Re: [PATCH v8 2/4] mseal: add mseal syscall

On Thu, Feb 1, 2024 at 3:11 PM Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Wed, Jan 31, 2024 at 05:50:24PM +0000, jeffxu@...omium.org wrote:
> > [PATCH v8 2/4] mseal: add mseal syscall
> [...]
> > +/*
> > + * The PROT_SEAL defines memory sealing in the prot argument of mmap().
> > + */
> > +#define PROT_SEAL    0x04000000      /* _BITUL(26) */
> > +
> >  /* 0x01 - 0x03 are defined in linux/mman.h */
> >  #define MAP_TYPE     0x0f            /* Mask for type of mapping */
> >  #define MAP_FIXED    0x10            /* Interpret addr exactly */
> > @@ -33,6 +38,9 @@
> >  #define MAP_UNINITIALIZED 0x4000000  /* For anonymous mmap, memory could be
> >                                        * uninitialized */
> >
> > +/* map is sealable */
> > +#define MAP_SEALABLE 0x8000000       /* _BITUL(27) */
>
> IMO this patch is misleading, as it claims to just be adding a new syscall, but
> it actually adds three new UAPIs, only one of which is the new syscall.  The
> other two new UAPIs are new flags to the mmap syscall.
>
The description does include all three. I could update the patch title.

> Based on recent discussions, it seems the usefulness of the new mmap flags has
> not yet been established.  Note also that there are only a limited number of
> mmap flags remaining, so we should be careful about allocating them.
>
> Therefore, why not start by just adding the mseal syscall, without the new mmap
> flags alongside it?
>
> I'll also note that the existing PROT_* flags seem to be conventionally used for
> the CPU page protections, as opposed to kernel-specific properties of the VMA
> object.  As such, PROT_SEAL feels a bit out of place anyway.  If it's added at
> all it perhaps should be a MAP_* flag, not PROT_*.  I'm not sure this aspect has
> been properly discussed yet, seeing as the patchset is presented as just adding
> sys_mseal().  Some reviewers may not have noticed or considered the new flags.
>
MAP_ flags is more used for type of mapping, such as MAP_FIXED_NOREPLACE.

The PROT_SEAL might make more sense because sealing the protection bit
is the main functionality of the sealing at this moment.

Thanks
-Jeff




> - Eric