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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.03.2022 12:43, Jan Beulich wrote:
> On 30.03.2022 12:19, Roger Pau Monné wrote:
>> On Wed, Mar 30, 2022 at 10:03:11AM +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().
>>>
>>> Fixes: 6974c75180f1 ("xen/x86: Livepatch: support patching CET-enhanced 
>>> functions")
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> ---
>>> Only build tested, as I don't have a live patching environment available.
>>>
>>> For Arm this assumes that the patch_offset field starts out as zero; I
>>> think we can make such an assumption, yet otoh on x86 explicit
>>> initialization was added by the cited commit.
>>>
>>> --- a/xen/include/xen/livepatch.h
>>> +++ b/xen/include/xen/livepatch.h
>>> @@ -90,7 +90,7 @@ static inline
>>>  unsigned int livepatch_insn_len(const struct livepatch_func *func)
>>>  {
>>>      if ( !func->new_addr )
>>> -        return func->new_size;
>>> +        return func->new_size - func->patch_offset;
>>>  
>>>      return ARCH_PATCH_INSN_SIZE;
>>>  }
>>
>> Don't you also need to move the call to livepatch_insn_len() in
>> arch_livepatch_apply() after func->patch_offset has been adjusted to
>> account for ENDBR presence?
> 
> Oh, yes, I definitely need to.

Actually - not quite. I need to call the function a 2nd time. And
this then also points out that is_endbr64() and is_endbr64_poison()
may overrun the range pointed to by old_ptr (which I'll take care
of at the same time).

Jan