Re: [PATCH] x86/unwind/orc: Handle kretprobes_trampoline

[Kees Cook <keescook@chromium.org>]

On Wed, Oct 13, 2021 at 06:41:01PM -0700, Josh Poimboeuf wrote:
> On Mon, Oct 11, 2021 at 02:03:26PM -0700, Kees Cook wrote:
> > On Thu, Sep 02, 2021 at 07:13:26PM -0700, Kees Cook wrote:
> > > From: Marios Pomonis <pomonis@google.com>
> > > 
> > > Fix a bug in the ORC unwinder when kretprobes has replaced a return
> > > address with the address of `kretprobes_trampoline'. ORC mistakenly
> > > assumes that the address in the stack is a return address and decrements
> > > it by 1 in order to find the proper depth of the next frame.
> > > 
> > > This issue was discovered while testing the FG-KASLR series[0][1] and
> > > running the live patching test[2] that was originally failing[3].
> > > 
> > > [0] https://lore.kernel.org/kernel-hardening/20200923173905.11219-1-kristen@linux.intel.com/
> > > [1] https://github.com/KSPP/linux/issues/132
> > > [2] https://github.com/lpechacek/qa_test_klp
> > > [3] https://lore.kernel.org/lkml/alpine.LSU.2.21.2009251450260.13615@pobox.suse.cz/
> > > 
> > > Fixes: ee9f8fce9964 ("x86/unwind: Add the ORC unwinder")
> > > Signed-off-by: Marios Pomonis <pomonis@google.com>
> > > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > > Cc: Alexander Lobakin <alexandr.lobakin@intel.com>
> > > Cc: Kristen C Accardi <kristen.c.accardi@intel.com>
> > > Cc: Sami Tolvanen <samitolvanen@google.com>
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > 
> > Ping again; Josh can you take this please?
> 
> I'm confused how this still fixes anything after Masami's patch set,
> which is now in linux-next.
> 
> After those patches, for a CALL-type ORC entry, the unwinder sets
> state->ip to the address returned by unwind_recover_ret_addr().  In the
> case of a kretprobe, that means that state->ip will no longer point to
> kretprobes_trampoline() -- making the above patch description incorrect.
> 
> Instead, state->ip will then contain the original call return address
> which was replaced by kretpobes.  So it looks to the unwinder like a
> normal call return address, and 'state->signal' should remain false.
> 
> Am I missing something?

I'll let Marios answer in more detail, but my understanding is that
Masami's patch set didn't solve the FGKASLR-vs-kretprobes issue[1].
I don't understand _why_ yet, though.

-Kees

[1] https://lore.kernel.org/all/CAKXAmdgS3SL_qyjzjY32_DXe3WVTN+O=wYwJ9vkUXKhjmt87fA@mail.gmail.com/

-- 
Kees Cook