linux-hardening - Re: [RFC PATCH v2 1/1] exec: seal system mappings

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Message-ID: <CABi2SkXKWGuvttUWHH+CL-MnKDvF=o6i2nGz55r+-6WvSJex5Q@mail.gmail.com>
Date: Wed, 16 Oct 2024 15:06:06 -0700
From: Jeff Xu <jeffxu@...omium.org>
To: Kees Cook <kees@...nel.org>
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Subject: Re: [RFC PATCH v2 1/1] exec: seal system mappings

On Wed, Oct 16, 2024 at 2:26 PM Kees Cook <kees@...nel.org> wrote:
>
> (I don't think this needs "RFC" any more)
>
> On Mon, Oct 14, 2024 at 09:50:20PM +0000, jeffxu@...omium.org wrote:
> > From: Jeff Xu <jeffxu@...omium.org>
> >
> > Seal vdso, vvar, sigpage, uprobes and vsyscall.
> >
> > Those mappings are readonly or executable only, sealing can protect
> > them from ever changing during the life time of the process. For
> > complete descriptions of memory sealing, please see mseal.rst [1].
> >
> > System mappings such as vdso, vvar, and sigpage (for arm) are
> > generated by the kernel during program initialization. These mappings
> > are designated as non-writable, and sealing them will prevent them
> > from ever becoming writeable.
> >
> > Unlike the aforementioned mappings, the uprobe mapping is not
> > established during program startup. However, its lifetime is the same
> > as the process's lifetime [2], thus sealable.
> >
> > The vdso, vvar, sigpage, and uprobe mappings all invoke the
> > _install_special_mapping() function. As no other mappings utilize this
> > function, it is logical to incorporate sealing logic within
> > _install_special_mapping(). This approach avoids the necessity of
> > modifying code across various architecture-specific implementations.
> >
> > The vsyscall mapping, which has its own initialization function, is
> > sealed in the XONLY case, it seems to be the most common and secure
> > case of using vsyscall.
> >
> > It is important to note that the CHECKPOINT_RESTORE feature (CRIU) may
> > alter the mapping of vdso, vvar, and sigpage during restore
> > operations. Consequently, this feature cannot be universally enabled
> > across all systems. To address this, a kernel configuration option has
> > been introduced to enable or disable this functionality. Note, uprobe
> > is always sealed and not controlled by this kernel configuration.
> >
> > [1] Documentation/userspace-api/mseal.rst
> > [2] https://lore.kernel.org/all/CABi2SkU9BRUnqf70-nksuMCQ+yyiWjo3fM4XkRkL-NrCZxYAyg@mail.gmail.com/
> >
> > Signed-off-by: Jeff Xu <jeffxu@...omium.org>
> > ---
> >  .../admin-guide/kernel-parameters.txt         | 10 ++++
> >  arch/x86/entry/vsyscall/vsyscall_64.c         |  9 +++-
> >  fs/exec.c                                     | 53 +++++++++++++++++++
> >  include/linux/fs.h                            |  1 +
> >  kernel/events/uprobes.c                       |  2 +-
> >  mm/mmap.c                                     |  1 +
> >  security/Kconfig                              | 26 +++++++++
> >  7 files changed, 99 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index e7bfe1bde49e..02e5eb23d76f 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1538,6 +1538,16 @@
> >                       Permit 'security.evm' to be updated regardless of
> >                       current integrity status.
> >
> > +     exec.seal_system_mappings = [KNL]
> > +                     Format: { never | always }
> > +                     Seal system mappings: vdso, vvar, sigpage, uprobes,
> > +                     vsyscall.
> > +                     This overwrites KCONFIG CONFIG_SEAL_SYSTEM_MAPPINGS_*
> > +                     - 'never':  never seal system mappings.
> > +                     - 'always': always seal system mappings.
> > +                     If not specified or invalid, default is the KCONFIG value.
> > +                     This option has no effect if CONFIG_64BIT=n
> > +
>
> Any reason for "always"/"never" instead of the more traditional y/n
> enabled/disabled, etc?
>
Yes. I like to leave room for  future extension, in case someone ever
needs a prctl for pre-process opt-in, e.g.
Format:{never|prctl|always}

> Otherwise, this all makes sense to me.
>
> --
> Kees Cook