Re: [PATCH v6 2/2] xen/x86: Livepatch: support patching CET-enhanced functions

[Julien Grall <julien@xxxxxxx>]

Hi Bjoern,

On 10/03/2022 07:35, Bjoern Doebel wrote:
> 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>
> Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> ----
> Changes since r1:
> * (v2) use sizeof_field() to avoid unused variable warning
> * (v3) make metadata variable const in arch_livepatch_revert
> * (v4) rebase on top and make use of Andrew Cooper's was_endbr64() patch
> * (v5) use padding byte to store offset rather than reducing opaque area
> * (v6) style fixes
For the record, this patch depends on 
https://lore.kernel.org/xen-devel/20220308140126.8815-1-andrew.cooper3@xxxxxxxxxx/.
Your patch will be merged once the prereq has been merged.

Cheers,

--
Julien Grall