Re: [RFC PATCH v1 0/1] seal system mappings

[Liam R. Howlett]

* Jeff Xu <jeffxu@xxxxxxxxxxxx> [241008 11:01]:
> Hi Liam,
> 
> On Mon, Oct 7, 2024 at 7:19 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
> >
> > * jeffxu@xxxxxxxxxxxx <jeffxu@xxxxxxxxxxxx> [241004 12:32]:
> > > From: Jeff Xu <jeffxu@xxxxxxxxxx>
> > >
> > > 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.
> > >
> > > 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.
> >
> > But it also means they cannot be unmapped, right?
> >
> > I'm not saying it's a thing people should, but recent conversations
> > with the ppc people seem to indicate that people do 'things' to the vdso
> > such as removing it.
> >
> > Won't this change mean they cannot do that, at least if mseal is enabled
> > on ppc64?  In which case we would have a different special mapping for
> > powerpc, or any other platform that wants to be able to unmap the vdso
> > (or vvar or whatever else?)
> >
> > In fact, I came across people removing the vdso to catch callers to
> > those functions which they didn't want to allow.  In this case enabling
> > the security of mseal would not allow them to stop applications from
> > vdso calls.  Again, I'm not saying this is a good (or bad) idea but it
> > happening.
> >
> > >
> > > Unlike the aforementioned mappings, the uprobe mapping is not
> > > established during program startup. However, its lifetime is the same
> > > as the process's lifetime [1], 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. I tested
> > > CONFIG_SEAL_SYSTEM_MAPPINGS_ALWAYS with ChromeOS, which doesn’t use
> > > CHECKPOINT_RESTORE, to verify the sealing works.
> >
> > I am hesitant to say that CRIU is the only user of moving the vdso, as
> > the ppc people wanted the ability for the fallback methods to still
> > function when the vdso was unmapped.
> >
> > I am not sure we can change the user expected behaviour based on a
> > configuration option; users may be able to mmap/munmap but may not be
> > able to boot their own kernel, but maybe it's okay?
> >
> The text doesn't say CRIU is the **only** feature that is not
> compatible with this.

Fair enough.

I read it that way since you pointed out breaking criu is the reason for
not enabling this by default, although it's probably the biggest reason
against doing this.

> 
> The default config is "CONFIG_SEAL_SYSTEM_MAPPINGS_NEVER", and
> distribution needs to opt-in for this feature, such as ChromeOS or
> Android or other safe-by-default systems that doesn't allow to unmap
> or remap vdso in production build.

Okay, but you never stated that they can't be unmapped or remapped in
this change; just that they will never become writeable.  It is worth
adding that detail in the description since it isn't entirely obvious
unless you know the workings of mseal.

> 
> Thanks
> -Jeff
> 
> 
> > >
> > > [1] https://lore.kernel.org/all/CABi2SkU9BRUnqf70-nksuMCQ+yyiWjo3fM4XkRkL-NrCZxYAyg@xxxxxxxxxxxxxx/
> > >
> > > Jeff Xu (1):
> > >   exec: seal system mappings
> > >
> > >  .../admin-guide/kernel-parameters.txt         |  9 ++++
> > >  arch/x86/entry/vsyscall/vsyscall_64.c         |  9 +++-
> > >  fs/exec.c                                     | 53 +++++++++++++++++++
> > >  include/linux/fs.h                            |  1 +
> > >  mm/mmap.c                                     |  1 +
> > >  security/Kconfig                              | 26 +++++++++
> > >  6 files changed, 97 insertions(+), 2 deletions(-)
> > >
> > > --
> > > 2.47.0.rc0.187.ge670bccf7e-goog
> > >