[PATCH] 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 by
default when using a compiler that supports -ftrivial-auto-var-init.

So mark the buffer as __uninitialized. Given that this is a variable
attribute not a type attribute, this requires that the expression is
tweaked a bit.

Cc: Will Deacon <will at kernel.org>,
Cc: Catalin Marinas <catalin.marinas at arm.com>,
Cc: Kees Cook <keescook at chromium.org>
Cc: Eric Biggers <ebiggers at kernel.org>
Cc: Justin Stitt <justinstitt at google.com>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/simd.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

The issue here is that returning a pointer to an automatic variable as
it goes out of scope is slightly dodgy, especially in the context of
__attribute__((cleanup())), on which the scoped guard API relies
heavily. However, in this case it should be safe, given that this
expression is the input to the guarded variable type's constructor.

It is definitely not pretty, though, so hopefully here is a better way
to attach this.

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index 0941f6f58a14..825b7fe94003 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -48,6 +48,7 @@ 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()	\
+	scoped_guard(ksimd, ({ struct user_fpsimd_state __uninitialized s; &s; }))
 
 #endif
-- 
2.47.3