On Mon, Oct 28, 2024 at 06:07:49PM +0200, Alexander Shishkin wrote:
> From: Sohil Mehta <sohil.mehta@xxxxxxxxx>
>
> Linear Address Space Separation (LASS) is a security feature that
> intends to prevent malicious virtual address space accesses across
> user/kernel mode.
>
> Such mode based access protection already exists today with paging and
> features such as SMEP and SMAP. However, to enforce these protections,
> the processor must traverse the paging structures in memory. Malicious
> software can use timing information resulting from this traversal to
> determine details about the paging structures, and these details may
> also be used to determine the layout of the kernel memory.
>
> The LASS mechanism provides the same mode-based protections as paging
> but without traversing the paging structures. Because the protections
> enforced by LASS are applied before paging, software will not be able to
> derive paging-based timing information from the various caching
> structures such as the TLBs, mid-level caches, page walker, data caches,
> etc.
>
> LASS enforcement relies on the typical kernel implementation to divide
> the 64-bit virtual address space into two halves:
> Addr[63]=0 -> User address space
> Addr[63]=1 -> Kernel address space
>
> Any data access or code execution across address spaces typically
> results in a #GP fault.
SDM mentions #SS for LASS violations on stack instructions. Do we care to
provide a sensible error message on #SS as we do for #GP?
> The LASS enforcement for kernel data access is dependent on CR4.SMAP
> being set. The enforcement can be disabled by toggling the RFLAGS.AC bit
> similar to SMAP.
>
> Define the CPU feature bits to enumerate this feature and include
> feature dependencies to reflect the same.
>
> Co-developed-by: Yian Chen <yian.chen@xxxxxxxxx>
> Signed-off-by: Yian Chen <yian.chen@xxxxxxxxx>
> Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/cpufeatures.h | 1 +
> arch/x86/include/asm/disabled-features.h | 4 +++-
> arch/x86/include/asm/smap.h | 18 ++++++++++++++++++
> arch/x86/include/uapi/asm/processor-flags.h | 2 ++
> arch/x86/kernel/cpu/cpuid-deps.c | 1 +
> tools/arch/x86/include/asm/cpufeatures.h | 1 +
> 6 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index ea33439a5d00..acb3ccea2bd7 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -319,6 +319,7 @@
> /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
> #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* "avx_vnni" AVX VNNI instructions */
> #define X86_FEATURE_AVX512_BF16 (12*32+ 5) /* "avx512_bf16" AVX512 BFLOAT16 instructions */
> +#define X86_FEATURE_LASS (12*32+ 6) /* "lass" Linear Address Space Separation */
> #define X86_FEATURE_CMPCCXADD (12*32+ 7) /* CMPccXADD instructions */
> #define X86_FEATURE_ARCH_PERFMON_EXT (12*32+ 8) /* Intel Architectural PerfMon Extension */
> #define X86_FEATURE_FZRM (12*32+10) /* Fast zero-length REP MOVSB */
> diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
> index c492bdc97b05..76c7d362af94 100644
> --- a/arch/x86/include/asm/disabled-features.h
> +++ b/arch/x86/include/asm/disabled-features.h
> @@ -22,12 +22,14 @@
> # define DISABLE_CYRIX_ARR (1<<(X86_FEATURE_CYRIX_ARR & 31))
> # define DISABLE_CENTAUR_MCR (1<<(X86_FEATURE_CENTAUR_MCR & 31))
> # define DISABLE_PCID 0
> +# define DISABLE_LASS 0
> #else
> # define DISABLE_VME 0
> # define DISABLE_K6_MTRR 0
> # define DISABLE_CYRIX_ARR 0
> # define DISABLE_CENTAUR_MCR 0
> # define DISABLE_PCID (1<<(X86_FEATURE_PCID & 31))
> +# define DISABLE_LASS (1<<(X86_FEATURE_LASS & 31))
> #endif /* CONFIG_X86_64 */
>
> #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> @@ -146,7 +148,7 @@
> #define DISABLED_MASK11 (DISABLE_RETPOLINE|DISABLE_RETHUNK|DISABLE_UNRET| \
> DISABLE_CALL_DEPTH_TRACKING|DISABLE_USER_SHSTK)
> #define DISABLED_MASK12 (DISABLE_FRED|DISABLE_LAM)
> -#define DISABLED_MASK13 0
> +#define DISABLED_MASK13 (DISABLE_LASS)
> #define DISABLED_MASK14 0
> #define DISABLED_MASK15 0
> #define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP| \
> diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
> index bab490379c65..8cb6f004800b 100644
> --- a/arch/x86/include/asm/smap.h
> +++ b/arch/x86/include/asm/smap.h
> @@ -27,6 +27,12 @@
>
> #else /* __ASSEMBLY__ */
>
> +/*
> + * The CLAC/STAC instructions toggle enforcement of X86_FEATURE_SMAP.
> + * Add dedicated lass_*() variants for cases that are necessitated by
> + * LASS (X86_FEATURE_LASS) enforcement, which helps readability and
> + * avoids AC flag flipping on CPUs that don't support LASS.
> + */
Maybe add a new line here? The comment is for the group of helpers, not
for clac() specifically.
> static __always_inline void clac(void)
> {
> /* Note: a barrier is implicit in alternative() */
> @@ -39,6 +45,18 @@ static __always_inline void stac(void)
> alternative("", __ASM_STAC, X86_FEATURE_SMAP);
> }
>
> +static __always_inline void lass_clac(void)
> +{
> + /* Note: a barrier is implicit in alternative() */
> + alternative("", __ASM_CLAC, X86_FEATURE_LASS);
> +}
> +
> +static __always_inline void lass_stac(void)
> +{
> + /* Note: a barrier is implicit in alternative() */
> + alternative("", __ASM_STAC, X86_FEATURE_LASS);
> +}
> +
> static __always_inline unsigned long smap_save(void)
> {
> unsigned long flags;
> diff --git a/arch/x86/include/uapi/asm/processor-flags.h b/arch/x86/include/uapi/asm/processor-flags.h
> index f1a4adc78272..81d0c8bf1137 100644
> --- a/arch/x86/include/uapi/asm/processor-flags.h
> +++ b/arch/x86/include/uapi/asm/processor-flags.h
> @@ -136,6 +136,8 @@
> #define X86_CR4_PKE _BITUL(X86_CR4_PKE_BIT)
> #define X86_CR4_CET_BIT 23 /* enable Control-flow Enforcement Technology */
> #define X86_CR4_CET _BITUL(X86_CR4_CET_BIT)
> +#define X86_CR4_LASS_BIT 27 /* enable Linear Address Space Separation support */
> +#define X86_CR4_LASS _BITUL(X86_CR4_LASS_BIT)
> #define X86_CR4_LAM_SUP_BIT 28 /* LAM for supervisor pointers */
> #define X86_CR4_LAM_SUP _BITUL(X86_CR4_LAM_SUP_BIT)
>
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index 8bd84114c2d9..3f73c4b03348 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -83,6 +83,7 @@ static const struct cpuid_dep cpuid_deps[] = {
> { X86_FEATURE_AMX_TILE, X86_FEATURE_XFD },
> { X86_FEATURE_SHSTK, X86_FEATURE_XSAVES },
> { X86_FEATURE_FRED, X86_FEATURE_LKGS },
> + { X86_FEATURE_LASS, X86_FEATURE_SMAP },
> {}
> };
>
> diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
> index 23698d0f4bb4..538930159f9f 100644
> --- a/tools/arch/x86/include/asm/cpufeatures.h
> +++ b/tools/arch/x86/include/asm/cpufeatures.h
> @@ -319,6 +319,7 @@
> /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
> #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* "avx_vnni" AVX VNNI instructions */
> #define X86_FEATURE_AVX512_BF16 (12*32+ 5) /* "avx512_bf16" AVX512 BFLOAT16 instructions */
> +#define X86_FEATURE_LASS (12*32+ 6) /* "lass" Linear Address Space Separation */
> #define X86_FEATURE_CMPCCXADD (12*32+ 7) /* CMPccXADD instructions */
> #define X86_FEATURE_ARCH_PERFMON_EXT (12*32+ 8) /* Intel Architectural PerfMon Extension */
> #define X86_FEATURE_FZRM (12*32+10) /* Fast zero-length REP MOVSB */
> --
> 2.45.2
>
--
Kiryl Shutsemau / Kirill A. Shutemov
