Re: [kvm-unit-tests PATCH v4 16/18] x86: cet: Enable NOTRACK handling for IBT tests

[Mathias Krause <minipli@grsecurity.net>]

On 14.11.25 21:50, Sean Christopherson wrote:
> gcc's jump table handling makes use of 'notrack' indirect jumps, causing
> spurious #CP(3) exceptions.
> 

Missing a "From: Mathias Krause <minipli@grsecurity.net>", maybe?

> Enable 'notrack' handling for the IBT tests instead of disabling jump
> tables as we may want to make use of 'notrack' ourselves in future
> tests.  This will allow using report() in IBT tests, as gcc likes to
> generate a small jump table for exception_mnemonic():
> 
>  000000000040707c <exception_mnemonic>:
>   40707c:       endbr64
>   407080:       cmp    $0x1e,%edi
>   407083:       ja     407117 <exception_mnemonic+0x9b>
>   407089:       mov    %edi,%edi
>   40708b:       notrack jmp *0x4107e0(,%rdi,8)
>     ::
>   4070b1:       mov    $0x411c7c,%eax	# <-- #CP(3) here
> 
> Link: https://lore.kernel.org/all/fc886a22-49f3-4627-8ba6-933099e7640d@grsecurity.net
> Signed-off-by: Mathias Krause <minipli@grsecurity.net>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  x86/cet.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/x86/cet.c b/x86/cet.c
> index 26cd1c9b..74d3f701 100644
> --- a/x86/cet.c
> +++ b/x86/cet.c
> @@ -82,8 +82,9 @@ static uint64_t cet_ibt_func(void)
>  #define CP_ERR_SETSSBSY	0x0005
>  #define CP_ERR_ENCL		BIT(15)
>  
> -#define ENABLE_SHSTK_BIT 0x1
> -#define ENABLE_IBT_BIT   0x4
> +#define CET_ENABLE_SHSTK			BIT(0)
> +#define CET_ENABLE_IBT				BIT(2)
> +#define CET_ENABLE_NOTRACK			BIT(4)
>  
>  static void test_shstk(void)
>  {
> @@ -112,7 +113,7 @@ static void test_shstk(void)
>  	install_pte(current_page_table(), 1, shstk_virt, pte, 0);
>  
>  	/* Enable shadow-stack protection */
> -	wrmsr(MSR_IA32_U_CET, ENABLE_SHSTK_BIT);
> +	wrmsr(MSR_IA32_U_CET, CET_ENABLE_SHSTK);
>  
>  	/* Store shadow-stack pointer. */
>  	wrmsr(MSR_IA32_PL3_SSP, (u64)(shstk_virt + 0x1000));
> @@ -140,8 +141,8 @@ static void test_ibt(void)
>  		return;
>  	}
>  
> -	/* Enable indirect-branch tracking */
> -	wrmsr(MSR_IA32_U_CET, ENABLE_IBT_BIT);
> +	/* Enable indirect-branch tracking (notrack handling for jump tables) */
> +	wrmsr(MSR_IA32_U_CET, CET_ENABLE_IBT | CET_ENABLE_NOTRACK);
>  
>  	run_in_user(cet_ibt_func, CP_VECTOR, 0, 0, 0, 0, &rvc);
>  	report(rvc && exception_error_code() == CP_ERR_ENDBR,

Otherwise, LGTM!

Thanks,
Mathias