linux-kernel - Re: [RFC PATCH v1 0/8] Introduce mseal() syscall

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Message-ID: <CAHk-=wj87GMTH=5901ob=SjQqegAm2JYBE7E4J7skJzE64U-wQ@mail.gmail.com>
Date:   Tue, 17 Oct 2023 10:22:16 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jeff Xu <jeffxu@...gle.com>
Cc:     jeffxu@...omium.org, akpm@...ux-foundation.org,
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Subject: Re: [RFC PATCH v1 0/8] Introduce mseal() syscall

On Tue, 17 Oct 2023 at 02:08, Jeff Xu <jeffxu@...gle.com> wrote:
>
> It is probably worth noting that I choose to check one and only
> one sealing type per syscall. i.e. munmap(2) checks
> MM_SEAL_MUNMAP only.

Yeah, this is wrong.

It's wrong exactly because other system calls will unmap things too.

Using mmap() to over-map something will unmap the old one.

Same goes for mremap() to move over an existing mapping.

So the whole "do things by the name of the system call" is not workable.

All that matters is what the system calls *do*, not what their name is.

And mmap() will fundamentally munmap() as part of the action.

This is why I absolutely hated the old "ON_BEHALF_OF_xyz" flag, and
why I still absolutely hate the "randomly pass different sealing flags
fto do_munmap()".

You should *not* be passing any flags at all to do_munmap(). Because
*regardless* of who calls it, and regardless of which system call
started the action, do_munmap() unmaps a virtual memory area.

See what I'm saying?

                 Linus