linux-hardening - Re: [PATCH v10 0/5] Introduce mseal

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Date: Fri, 19 Apr 2024 18:23:06 -0700
From: Jeff Xu <jeffxu@...omium.org>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	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, corbet@....net, 
	merimus@...gle.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, dave.hansen@...el.com, 
	linux-hardening@...r.kernel.org, deraadt@...nbsd.org
Subject: Re: [PATCH v10 0/5] Introduce mseal

On Fri, Apr 19, 2024 at 10:59 AM Pedro Falcato <pedro.falcato@...il.com> wrote:
>
> On Fri, Apr 19, 2024 at 2:22 AM Jeff Xu <jeffxu@...omium.org> wrote:
> > The overhead is likely to grow linearly with the number of VMA, since
> > it takes time to retrieve VMA's metadata.
> >
> > Let's use one data sample to look at impact:
> >
> > Test: munmap 1000 memory range, each memory range has 1 VMA
> >
> > syscall__       vmas    t       t_mseal delta_ns        per_vma %
> > munmap__        1       909     944     35      35      104%
> >
> > For those 1000 munmap calls, sealing adds 35000 ns in total, or 35 ns per call.
>
> Have you tried to spray around some likely() and unlikely()s? Does
> that make a difference? I'm thinking that sealing VMAs will be very
> rare, and mprotect/munmapping them is probably a programming error
> anyway, so the extra branches in the mprotect/munmap/madvice (etc)
> should be a nice target for some branch annotation.
>
Most cost will be in locating the node in the maple tree that stores
the VMAs, branch annotation is not possible there.

We could put unlikely() in the can_modify_mm check,  I suspect it
won't make a measurable difference in the real-world. On the other
hand, this also causes no harm, and existing mm code uses
unlikely/likely in a lot of places, so why not.

I will send a follow-up patch in the next few days.

Thanks
-Jeff
> --
> Pedro