Re: [PATCH v3] livepatch: account for patch offset when applying NOP patch

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Apr 07, 2022 at 03:44:16PM +0000, Ross Lagerwall wrote:
> > From: Jan Beulich <jbeulich@xxxxxxxx>
> > Sent: Thursday, March 31, 2022 9:42 AM
> > To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Ross 
> > Lagerwall <ross.lagerwall@xxxxxxxxxx>; Konrad Wilk 
> > <konrad.wilk@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Wei 
> > Liu <wl@xxxxxxx>; Bjoern Doebel <doebel@xxxxxxxxx>
> > Subject: Re: [PATCH v3] livepatch: account for patch offset when applying 
> > NOP patch 
> >  
> > [CAUTION - EXTERNAL EMAIL] DO NOT reply, click links, or open attachments 
> > unless you have verified the sender and know the content is safe.
> > 
> > On 31.03.2022 10:21, Roger Pau Monné wrote:
> > > On Thu, Mar 31, 2022 at 08:49:46AM +0200, Jan Beulich wrote:
> > >> While not triggered by the trivial xen_nop in-tree patch on
> > >> staging/master, that patch exposes a problem on the stable trees, where
> > >> all functions have ENDBR inserted. When NOP-ing out a range, we need to
> > >> account for this. Handle this right in livepatch_insn_len().
> > >>
> > >> This requires livepatch_insn_len() to be called _after_ ->patch_offset
> > >> was set.
> > >>
> > >> Fixes: 6974c75180f1 ("xen/x86: Livepatch: support patching CET-enhanced 
> > >> functions")
> > >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > 
> > > Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > 
> > Thanks.
> > 
> > As a note to the livepatch maintainers: I'm going to put this in
> > without further waiting for suitable acks. Just in case I'll put
> > it on the 4.16 branch only for starters, to see that it actually
> > helps there (it's unusual to put something on the stable
> > branches before it having passed the push gate to master).
> 
> Thanks (was on PTO and away from email).
> 
> > 
> > > Albeit I don't think I understand how the in-place patching is done. I
> > > would expect the !func->new_addr branch of the if in
> > > arch_livepatch_apply to fill the insn buffer with the in-place
> > > replacement instructions, but I only see the buffer getting filled
> > > with nops. I'm likely missing something (not that this patch changes
> > > any of this).
> > 
> > Well, as per the v2 thread: There's no in-place patching except
> > to NOP out certain insns.
> 
> Correct. FWIW I'm not really aware of a valid use case for this
> 
> > 
> > > I'm also having trouble figuring out how we assert that the len value
> > > (which is derived from new_size if !new_addr) is not greater than
> > > LIVEPATCH_OPAQUE_SIZE, which is the limit of the insn buffer. Maybe
> > > that's already checked elsewhere.
> > 
> > That's what my 3rd post-commit-message remark was (partly) about.
> 
> old_size specifies the length of the existing function to be patched.
> 
> If new_addr is zero (NOP case), then new_size specifies the number of
> bytes to overwrite with NOP. That's why new_size is used as the memcpy
> length (minus patch offset).

Sorry, maybe a naive question, but why not use old_size directly to
overwrite with NOPs?

Is this because you could generate a patch that just removed code from
a function and then you would ideally just overwrite with NOPs the
section to be removed while leaving the rest of the function as-is (so
no jump added)?

I wonder whether we exercise this functionality at all, as I would
imagine is quite hard to come with such payload?

Thanks, Roger.