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https://github.com/ipxe/ipxe
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06a9330004
The timer and entropy seed CSRs will, by design, return different values each time they are read. Add the missing volatile qualifiers on the inline assembly to prevent gcc from assuming that repeated invocations may be elided. Signed-off-by: Michael Brown <mcb30@ipxe.org>
112 lines
2.9 KiB
C
112 lines
2.9 KiB
C
/*
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* Copyright (C) 2024 Michael Brown <mbrown@fensystems.co.uk>.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation; either version 2 of the
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* License, or any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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* 02110-1301, USA.
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*
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* You can also choose to distribute this program under the terms of
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* the Unmodified Binary Distribution Licence (as given in the file
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* COPYING.UBDL), provided that you have satisfied its requirements.
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*/
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FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
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/** @file
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*
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* Entropy source extension (Zkr)
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*
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*/
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#include <errno.h>
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#include <ipxe/hart.h>
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#include <ipxe/entropy.h>
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#include <ipxe/drbg.h>
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struct entropy_source zkr_entropy __entropy_source ( ENTROPY_PREFERRED );
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/** Seed CSR operational state */
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#define ZKR_SEED_OPST_MASK 0xc0000000UL
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#define ZKR_SEED_OPST_ES16 0x80000000UL /**< 16 bits of entropy available */
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/** Number of times to retry reading from seed CSR */
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#define ZKR_SEED_MAX_RETRY 1024
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/** Colour for debug messages */
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#define colour &zkr_entropy
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/**
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* Enable entropy gathering
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*
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* @ret rc Return status code
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*/
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static int zkr_entropy_enable ( void ) {
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int rc;
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/* Check if Zkr extension is supported */
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if ( ( rc = hart_supported ( "_zkr" ) ) != 0 ) {
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DBGC ( colour, "ZKR not supported: %s\n", strerror ( rc ) );
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return rc;
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}
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/* RISC-V ISA mandates that 128 bits of full entropy shall be
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* obtained from 256 entropy bits read from the seed CSR.
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*
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* Each 16-bit sample therefore contains 8 bits of
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* min-entropy.
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*/
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entropy_init ( &zkr_entropy, MIN_ENTROPY ( 8.0 ) );
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return 0;
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}
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/**
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* Get noise sample
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*
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* @ret noise Noise sample
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* @ret rc Return status code
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*/
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static int zkr_get_noise ( noise_sample_t *noise ) {
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unsigned long seed;
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unsigned int i;
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/* Read entropy from seed CSR */
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for ( i = 0 ; i < ZKR_SEED_MAX_RETRY ; i++ ) {
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/* Read seed CSR */
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__asm__ __volatile__ ( "csrrw %0, seed, zero" :
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"=r" ( seed ) );
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/* Check operationsl state */
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if ( ( seed & ZKR_SEED_OPST_MASK ) == ZKR_SEED_OPST_ES16 ) {
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/* Return entropy from both halves of the
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* 16-bit entropy source value.
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*/
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*noise = ( seed ^ ( seed >> 8 ) );
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return 0;
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}
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}
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DBGC ( colour, "ZKR could not source entropy (seed %#08lx)\n", seed );
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return -EBUSY;
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}
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/** Hardware entropy source */
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struct entropy_source zkr_entropy __entropy_source ( ENTROPY_PREFERRED ) = {
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.name = "zkr",
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.enable = zkr_entropy_enable,
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.get_noise = zkr_get_noise,
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};
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