linux-kernel - Re: [PATCH AUTOSEL 5.8 100/132] binfmt_elf: take the mmap lock around find_extend

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Date:   Tue, 27 Oct 2020 12:40:10 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michel Lespinasse <walken@...gle.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH AUTOSEL 5.8 100/132] binfmt_elf: take the mmap lock around find_extend_vma()

On Tue, Oct 27, 2020 at 12:54 AM Sasha Levin <sashal@...nel.org> wrote:
> [ Upstream commit b2767d97f5ff758250cf28684aaa48bbfd34145f ]
>
> create_elf_tables() runs after setup_new_exec(), so other tasks can
> already access our new mm and do things like process_madvise() on it.  (At
> the time I'm writing this commit, process_madvise() is not in mainline
> yet, but has been in akpm's tree for some time.)
>
> While I believe that there are currently no APIs that would actually allow
> another process to mess up our VMA tree (process_madvise() is limited to
> MADV_COLD and MADV_PAGEOUT, and uring and userfaultfd cannot reach an mm
> under which no syscalls have been executed yet), this seems like an
> accident waiting to happen.
>
> Let's make sure that we always take the mmap lock around GUP paths as long
> as another process might be able to see the mm.

While this commit makes the kernel less prone to future accidents, and
it is arguably fixing locking misbehavior, I don't think it belongs
into stable trees? As far as I know, it is not fixing any bugs that
can actually materialize in current or past kernels.