linux-kernel - Re: [RFC PATCH 0/5] madvise MADV

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Message-Id: <1764B08C-CC1E-4636-944A-DB95B81C7A8E@amacapital.net>
Date:   Tue, 28 Jul 2020 07:23:03 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Anthony Yznaga <anthony.yznaga@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org, mhocko@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        arnd@...db.de, ebiederm@...ssion.com, keescook@...omium.org,
        gerg@...ux-m68k.org, ktkhai@...tuozzo.com,
        christian.brauner@...ntu.com, peterz@...radead.org,
        esyr@...hat.com, jgg@...pe.ca, christian@...lner.me,
        areber@...hat.com, cyphar@...har.com, steven.sistare@...cle.com
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC



> On Jul 27, 2020, at 10:02 AM, Anthony Yznaga <anthony.yznaga@...cle.com> wrote:
> 
> This patchset adds support for preserving an anonymous memory range across
> exec(3) using a new madvise MADV_DOEXEC argument.  The primary benefit for
> sharing memory in this manner, as opposed to re-attaching to a named shared
> memory segment, is to ensure it is mapped at the same virtual address in
> the new process as it was in the old one.  An intended use for this is to
> preserve guest memory for guests using vfio while qemu exec's an updated
> version of itself.  By ensuring the memory is preserved at a fixed address,
> vfio mappings and their associated kernel data structures can remain valid.
> In addition, for the qemu use case, qemu instances that back guest RAM with
> anonymous memory can be updated.

This will be an amazing attack surface. Perhaps use of this flag should require no_new_privs?  Arguably it should also require a special flag to execve() to honor it.  Otherwise library helpers that do vfork()+exec() or posix_spawn() could be quite surprised.