linux-kernel - Re: [PATCH v2] lib/crypto: Add SHA3-224, SHA3-256, SHA3-384, SHA-512, SHAKE128, SHAKE256

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Message-ID: <20250923174541.GA2695109@google.com>
Date: Tue, 23 Sep 2025 17:45:41 +0000
From: Eric Biggers <ebiggers@...nel.org>
To: David Howells <dhowells@...hat.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Harald Freudenberger <freude@...ux.ibm.com>,
	Holger Dengler <dengler@...ux.ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Stephan Mueller <smueller@...onox.de>, Simo Sorce <simo@...hat.com>,
	linux-crypto@...r.kernel.org, linux-s390@...r.kernel.org,
	keyrings@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] lib/crypto: Add SHA3-224, SHA3-256, SHA3-384,
 SHA-512, SHAKE128, SHAKE256

On Tue, Sep 23, 2025 at 06:36:21PM +0100, David Howells wrote:
> Eric Biggers <ebiggers@...nel.org> wrote:
> 
> > > > and that the functions can be called in any context.
> > > 
> > > "Context" as in?
> > 
> > See the "Function context" section of
> > Documentation/doc-guide/kernel-doc.rst
> 
> Btw, in include/crypto/sha1.h:
> 
> /**
>  * hmac_sha1_update() - Update an HMAC-SHA1 context with message data
>  * @ctx: the HMAC context to update; must have been initialized
>  * @data: the message data
>  * @data_len: the data length in bytes
>  *
>  * This can be called any number of times.
>  *
>  * Context: Any context.
>  */
> static inline void hmac_sha1_update(struct hmac_sha1_ctx *ctx,
> 				    const u8 *data, size_t data_len)
> {
> 	sha1_update(&ctx->sha_ctx, data, data_len);
> }
> 
> for example, your specification of "Context: Any context." is probably not
> correct if FPU/Vector registers are used by optimised assembly as part of the
> function.  See:
> 
> void kernel_fpu_begin_mask(unsigned int kfpu_mask)
> {
> 	if (!irqs_disabled())
> 		fpregs_lock();
> 
> 	WARN_ON_FPU(!irq_fpu_usable());
> 
> 	/* Toggle kernel_fpu_allowed to false: */
> 	WARN_ON_FPU(!this_cpu_read(kernel_fpu_allowed));
> 	this_cpu_write(kernel_fpu_allowed, false);
> 
> 	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
> 	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
> 		set_thread_flag(TIF_NEED_FPU_LOAD);
> 		save_fpregs_to_fpstate(x86_task_fpu(current));
> 	}
> 	__cpu_invalidate_fpregs_state();
> 
> 	/* Put sane initial values into the control registers. */
> 	if (likely(kfpu_mask & KFPU_MXCSR) && boot_cpu_has(X86_FEATURE_XMM))
> 		ldmxcsr(MXCSR_DEFAULT);
> 
> 	if (unlikely(kfpu_mask & KFPU_387) && boot_cpu_has(X86_FEATURE_FPU))
> 		asm volatile ("fninit");
> }
> 
> If you try and access the function in IRQ mode, for example, you'll get a
> warning, and if IRQs are not disabled, it will disable BH/preemption.
> 
> You also can't use it from inside something else that uses FPU registers.
> 
> I suggest something like:
> 
>  * Context: Arch-dependent: May use the FPU/Vector unit registers.

Kernel-mode FPU is used only when irq_fpu_usable().

The tests verify that the functions do work in IRQ context.

- Eric