Re: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-enhanced functions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.03.2022 11:00, Jiamei Xie wrote:
>> -----Original Message-----
>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
>> Jiamei Xie
>> Sent: 2022年3月17日 17:17
>>
>>> -----Original Message-----
>>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
>>> Ross Lagerwall
>>> Sent: 2022年3月10日 1:12
>>> To: Bjoern Doebel <doebel@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>>> Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
>>> <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
>>> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Konrad Rzeszutek Wilk
>>> <konrad.wilk@xxxxxxxxxx>
>>> Subject: Re: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
>>> enhanced functions
>>>
>>>> From: Bjoern Doebel <doebel@xxxxxxxxx>
>>>> Sent: Wednesday, March 9, 2022 2:53 PM
>>>> To: xen-devel@xxxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>>>> Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
>>> <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
>>> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Bjoern Doebel
>>> <doebel@xxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
>>> Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>>>> Subject: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
>>> enhanced functions
>>>>
>>>> Xen enabled CET for supporting architectures. The control flow aspect of
>>>> CET expects functions that can be called indirectly (i.e., via function
>>>> pointers) to start with an ENDBR64 instruction. Otherwise a control flow
>>>> exception is raised.
>>>>
>>>> This expectation breaks livepatching flows because we patch functions by
>>>> overwriting their first 5 bytes with a JMP + <offset>, thus breaking the
>>>> ENDBR64. We fix this by checking the start of a patched function for
>>>> being ENDBR64. In the positive case we move the livepatch JMP to start
>>>> behind the ENDBR64 instruction.
>>>>
>>>> To avoid having to guess the ENDBR64 offset again on patch reversal
>>>> (which might race with other mechanisms adding/removing ENDBR
>>>> dynamically), use the livepatch metadata to store the computed offset
>>>> along with the saved bytes of the overwritten function.
>>>>
>>>> Signed-off-by: Bjoern Doebel <doebel@xxxxxxxxx>
>>>> Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>>> CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>>>
>>> Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>>
>> Tested-by: Jiamei xie <jiamei.xie@xxxxxxx>
>>
>> Cheers,
>> Jiamei
> Sorry I forgot to add the scope I tested in last email. I tested it on 
> armv8a. It worked fine and  didn't break arm.
> Tested-by: Jiamei xie <jiamei.xie@xxxxxxx>

Yet in any event there's meanwhile been a v6, so I'm unsure of taking the
tag over there.

Jan