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Re: [PATCH] x86/spec-ctrl: Disable retpolines with CET-IBT
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 1 Mar 2022 08:59:47 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 01 Mar 2022 08:00:03 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 28.02.2022 23:51, Andrew Cooper wrote:
> CET-IBT depend on executing indirect branches for protections to apply.
> Extend the clobber for CET-SS to all of CET.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> I can't decide if this wants a fixes tag or not. If I'd remembered during the
> CET series, it would have been its own patch.
On this basis I'd say no Fixes: tag.
> --- a/xen/arch/x86/spec_ctrl.c
> +++ b/xen/arch/x86/spec_ctrl.c
> @@ -944,10 +944,11 @@ void __init init_speculation_mitigations(void)
> boot_cpu_has(X86_FEATURE_IBRS));
>
> /*
> - * First, disable the use of retpolines if Xen is using shadow stacks, as
> - * they are incompatible.
> + * First, disable the use of retpolines if Xen is using CET. Retpolines
> + * are a ROP gadget so incompatbile with Shadow Stacks, while IBT depends
> + * on executing indirect branches for the safety properties to apply.
> */
> - if ( cpu_has_xen_shstk &&
> + if ( (read_cr4() & X86_CR4_CET) &&
> (opt_thunk == THUNK_DEFAULT || opt_thunk == THUNK_RETPOLINE) )
> thunk = THUNK_JMP;
Just for my own understanding: Why unconditionally THUNK_JMP and not possibly
THUNK_LFENCE?
Jan
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