Re: [PATCH] x86/build: Unilaterally disable -fcf-protection

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/05/2020 03:35, Jason Andryuk wrote:
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> On Tue, May 12, 2020 at 3:11 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> See comment for details.  Works around a GCC-9 bug which breaks the build on
>> Ubuntu.
>>
>> Reported-by: Jason Andryuk <jandryuk@xxxxxxxxx>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Tested-by: Jason Andryuk <jandryuk@xxxxxxxxx>
> Reviewed-by: Jason Andryuk <jandryuk@xxxxxxxxx>
>
>> diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk
>> index 2a51553edb..93e30e4bea 100644
>> --- a/xen/arch/x86/arch.mk
>> +++ b/xen/arch/x86/arch.mk
>> @@ -67,6 +67,15 @@ CFLAGS-$(CONFIG_INDIRECT_THUNK) += 
>> -mindirect-branch=thunk-extern
>>  CFLAGS-$(CONFIG_INDIRECT_THUNK) += -mindirect-branch-register
>>  CFLAGS-$(CONFIG_INDIRECT_THUNK) += -fno-jump-tables
>>
>> +# Xen doesn't support CET-IBT yet.  At a minimum, logic is required to
>> +# enable it for supervisor use, but the Livepatch functionality needs
>> +# to learn not to overwrite ENDBR64 instructions.
> Is the problem that existing functions start with ENDBR64, but the
> livepatch overwrites with a "real" instruction?

We livepatch by creating a new complete copy of the function, and
putting `jmp new` at the head of the old one.

This means we don't need to patch every callsite and track every
function pointer to the old function, and we can fully revert by
replacing the 5 bytes which became `jmp new`.

With CET-IBT in the mix, livepatch will have to learn to spot an ENDBR64
instruction and leave it intact, patching instead the next 5 bytes, so
an old function pointer still lands on the ENDBR64 instruction.

~Andrew