[PATCH v2] arm64/simd: Avoid pointless clearing of FP/SIMD buffer

[PATCH v2] arm64/simd: Avoid pointless clearing of FP/SIMD buffer

[Ard Biesheuvel]

The buffer provided to kernel_neon_begin() is only used if the task is
scheduled out while the FP/SIMD is in use by the kernel, or when such a
section is interrupted by a softirq that also uses the FP/SIMD.

IOW, this happens rarely, and even if it happened often, there is still
no reason for this buffer to be cleared beforehand, which happens
unconditionally, due to the use of a compound literal expression.

So define that buffer variable explicitly, and mark it as
__uninitialized so that it will not get cleared, even when
-ftrivial-auto-var-init is in effect.

This requires some preprocessor gymnastics, due to the fact that the
variable must be defined throughout the entire guarded scope, and the
expression

  ({ struct user_fpsimd_state __uninitialized st; &st; })

is problematic in that regard, even though the compilers seem to
permit it. So instead, repeat for 'for ()' trick that is also used in
the implementation of the guarded scope helpers.

Cc: Will Deacon <will@kernel.org>,
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Cc: Kees Cook <keescook@chromium.org>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/include/asm/simd.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index 0941f6f58a14..69ecbd69ca8c 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -48,6 +48,13 @@ DEFINE_LOCK_GUARD_1(ksimd,
 		    kernel_neon_begin(_T->lock),
 		    kernel_neon_end(_T->lock))
 
-#define scoped_ksimd()	scoped_guard(ksimd, &(struct user_fpsimd_state){})
+#define __scoped_ksimd(_label)					\
+	for (struct user_fpsimd_state __uninitialized __st;	\
+	     true; ({ goto _label; }))				\
+		if (0) {					\
+_label:			break;					\
+		} else scoped_guard(ksimd, &__st)
+
+#define scoped_ksimd()	__scoped_ksimd(__UNIQUE_ID(label))
 
 #endif

base-commit: 3f9f0252130e7dd60d41be0802bf58f6471c691d
-- 
2.47.3

Re: [PATCH v2] arm64/simd: Avoid pointless clearing of FP/SIMD buffer

[Eric Biggers]

On Tue, Dec 09, 2025 at 06:48:49AM +0100, Ard Biesheuvel wrote:
> The buffer provided to kernel_neon_begin() is only used if the task is
> scheduled out while the FP/SIMD is in use by the kernel, or when such a
> section is interrupted by a softirq that also uses the FP/SIMD.
> 
> IOW, this happens rarely, and even if it happened often, there is still
> no reason for this buffer to be cleared beforehand, which happens
> unconditionally, due to the use of a compound literal expression.
> 
> So define that buffer variable explicitly, and mark it as
> __uninitialized so that it will not get cleared, even when
> -ftrivial-auto-var-init is in effect.
> 
> This requires some preprocessor gymnastics, due to the fact that the
> variable must be defined throughout the entire guarded scope, and the
> expression
> 
>   ({ struct user_fpsimd_state __uninitialized st; &st; })
> 
> is problematic in that regard, even though the compilers seem to
> permit it. So instead, repeat for 'for ()' trick that is also used in
> the implementation of the guarded scope helpers.
> 
> Cc: Will Deacon <will@kernel.org>,
> Cc: Catalin Marinas <catalin.marinas@arm.com>,
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Eric Biggers <ebiggers@kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/include/asm/simd.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index 0941f6f58a14..69ecbd69ca8c 100644
> --- a/arch/arm64/include/asm/simd.h
> +++ b/arch/arm64/include/asm/simd.h
> @@ -48,6 +48,13 @@ DEFINE_LOCK_GUARD_1(ksimd,
>  		    kernel_neon_begin(_T->lock),
>  		    kernel_neon_end(_T->lock))
>  
> -#define scoped_ksimd()	scoped_guard(ksimd, &(struct user_fpsimd_state){})
> +#define __scoped_ksimd(_label)					\
> +	for (struct user_fpsimd_state __uninitialized __st;	\
> +	     true; ({ goto _label; }))				\
> +		if (0) {					\
> +_label:			break;					\
> +		} else scoped_guard(ksimd, &__st)
> +
> +#define scoped_ksimd()	__scoped_ksimd(__UNIQUE_ID(label))
>  
>  #endif
> 

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-fixes

I added:

    Fixes: 4fa617cc6851 ("arm64/fpsimd: Allocate kernel mode FP/SIMD buffers on the stack")

- Eric