Messages in this thread | | | From | Jann Horn <> | Date | Tue, 21 Apr 2020 17:09:44 +0200 | Subject | Re: [PATCH] fs/binfmt_elf.c: allocate initialized memory in fill_thread_core_info() |
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+x86 folks
(rest of thread is on lore <https://lore.kernel.org/lkml/20200419100848.63472-1-glider@google.com/>, 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@google.com> wrote: > On Tue, Apr 21, 2020 at 5:42 AM Al Viro <viro@zeniv.linux.org.uk> 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@google.com 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.
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