linux-kernel - Re: [PATCH] coredump: Also dump first pages of non-executable ELF libraries

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Message-ID: <CAG48ez1RzO8KrbAv_9kwRCyjS7E3K+adJzdqV6uLqKPoyQmPnw@mail.gmail.com>
Date:   Fri, 4 Feb 2022 00:59:59 +0100
From:   Jann Horn <jannh@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Bill Messmer <wmessmer@...rosoft.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>, stable@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] coredump: Also dump first pages of non-executable ELF libraries

On Wed, Feb 2, 2022 at 6:43 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Wed, Feb 2, 2022 at 4:19 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> >
> > Jann Horn <jannh@...gle.com> writes:
> >
> > > When I rewrote the VMA dumping logic for coredumps, I changed it to
> > > recognize ELF library mappings based on the file being executable instead
> > > of the mapping having an ELF header. But turns out, distros ship many ELF
> > > libraries as non-executable, so the heuristic goes wrong...
> > >
> > > Restore the old behavior where FILTER(ELF_HEADERS) dumps the first page of
> > > any offset-0 readable mapping that starts with the ELF magic.
> > >
> > > This fix is technically layer-breaking a bit, because it checks for
> > > something ELF-specific in fs/coredump.c; but since we probably want to
> > > share this between standard ELF and FDPIC ELF anyway, I guess it's fine?
> > > And this also keeps the change small for backporting.
> >
> > In light of the conflict with my other changes, and in light of the pain
> > of calling get_user.
> >
> > Is there any reason why the doesn't unconditionally dump all headers?
> > Something like the diff below?
> >
> > I looked in the history and the code was filtering for ELF headers
> > there already.  I am just thinking this feels like a good idea
> > regardless of the file format to help verify the file on-disk
> > is the file we think was mapped.
>
> Yeah, I guess that's reasonable. The main difference will probably be
> that the starting pages of some font files, locale files and python
> libraries will also be logged.

Are you planning to turn that into a proper patch and take it through
your tree, or something like that? If so, we should tell akpm to take
this one back out...