Re: [PATCH] fs/binfmt_elf.c: allocate initialized memory in fill_thread_core_info()

[Jann Horn]

+x86 folks

(rest of thread is on lore
<https://lore.kernel.org/lkml/20200419100848.63472-1-glider@xxxxxxxxxx/>,
with original bug report on github
<https://github.com/google/kmsan/issues/76>)

On Tue, Apr 21, 2020 at 2:54 PM Alexander Potapenko <glider@xxxxxxxxxx> wrote:
> On Tue, Apr 21, 2020 at 5:42 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, Apr 20, 2020 at 03:41:40PM -0700, Kees Cook wrote:
> > > On Mon, Apr 20, 2020 at 03:33:52PM -0700, Andrew Morton wrote:
> > > > On Sun, 19 Apr 2020 12:08:48 +0200 glider@xxxxxxxxxx wrote:
> > > >
> > > > > KMSAN reported uninitialized data being written to disk when dumping
> > > > > core. As a result, several kilobytes of kmalloc memory may be written to
> > > > > the core file and then read by a non-privileged user.
> > >
> > > Ewww. That's been there for 12 years. Did something change in
> > > regset_size() or regset->get()? Do you know what leaves the hole?
> >
> > Not lately and I would also like to hear the details; which regset it is?
> > Should be reasonably easy to find - just memset() the damn thing to something
> > recognizable, do whatever triggers that KMSAN report and look at that
> > resulting coredump.
> >
>
> Seems to be REGSET_XSTATE filled by xstateregs_get().
> Is there a ptrace interface also using that function?

It looks to me like the problem KMSAN found is that
copy_xstate_to_kernel() will not fill out memory for unused xstates? I
think this may have been introduced by commit 91c3dba7dbc1
("x86/fpu/xstate: Fix PTRACE frames for XSAVES", introduced in v4.8).

There seem to be no other functions that reach that path other than
coredumping; I think the correct fix would be to change
copy_xstate_to_kernel() to always fully initialize the output buffer.

The ptrace path uses copy_xstate_to_user(); there, instead of leaking
kernel memory to userspace, parts of the userspace buffer are simply
not written to at all.