Re: [PATCH] arm64: Ensure bits ASID[15:8] are masked out when the kernel uses 8-bit ASIDs

[Marc Zyngier <maz@kernel.org>]

On Tue, 03 Dec 2024 15:19:41 +0000,
Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> Linux currently sets the TCR_EL1.AS bit unconditionally during CPU
> bring-up. On an 8-bit ASID CPU, this is RES0 and ignored, otherwise
> 16-bit ASIDs are enabled. However, if running in a VM and the hypervisor
> reports 8-bit ASIDs (ID_AA64MMFR0_EL1.ASIDBits == 0) on a 16-bit ASIDs
> CPU, Linux uses bits 8 to 63 as a generation number for tracking old
> process ASIDs. The bottom 8 bits of this generation end up being written
> to TTBR1_EL1 and also used for the ASID-based TLBI operations as the
> upper 8 bits of the ASID. Following an ASID roll-over event we can have
> threads of the same application with the same 8-bit ASID but different
> generation numbers running on separate CPUs. Both TLB caching and the
> TLBI operations will end up using different actual 16-bit ASIDs for the
> same process.
> 
> A similar scenario can happen in a big.LITTLE configuration if the boot
> CPU only uses 8-bit ASIDs while secondary CPUs have 16-bit ASIDs.
> 
> Ensure that the ASID generation is only tracked by bits 16 and up,
> leaving bits 15:8 as 0 if the kernel uses 8-bit ASIDs. Note that
> clearing TCR_EL1.AS is not sufficient since the architecture requires
> that the top 8 bits of the ASID passed to TLBI instructions are 0 rather
> than ignored in such configuration.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: <stable@vger.kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: James Morse <james.morse@arm.com>
> ---
>  arch/arm64/mm/context.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
> index 188197590fc9..b2ac06246327 100644
> --- a/arch/arm64/mm/context.c
> +++ b/arch/arm64/mm/context.c
> @@ -32,9 +32,9 @@ static unsigned long nr_pinned_asids;
>  static unsigned long *pinned_asid_map;
>  
>  #define ASID_MASK		(~GENMASK(asid_bits - 1, 0))
> -#define ASID_FIRST_VERSION	(1UL << asid_bits)
> +#define ASID_FIRST_VERSION	(1UL << 16)
>  
> -#define NUM_USER_ASIDS		ASID_FIRST_VERSION
> +#define NUM_USER_ASIDS		(1UL << asid_bits)
>  #define ctxid2asid(asid)	((asid) & ~ASID_MASK)
>  #define asid2ctxid(asid, genid)	((asid) | (genid))

Acked-by: Marc Zyngier <maz@kernel.org>

In the light of this, I think we also need to prevent VM migration
from machines that have 8bit ASIDs to those that have 16bit ASIDs,
because lying to the guest is potentially deadly, for the exact
reasons outlined in the commit message.

I'll post something tomorrow.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.