Revision 8ec7791bae1327b1c279c5cd6e929c3b12daaf0a authored by Michael Ellerman on 06 May 2021, 04:49:58 UTC, committed by Michael Ellerman on 14 May 2021, 07:27:36 UTC
The STF (store-to-load forwarding) barrier mitigation can be enabled/disabled at runtime via a debugfs file (stf_barrier), which causes the kernel to patch itself to enable/disable the relevant mitigations. However depending on which mitigation we're using, it may not be safe to do that patching while other CPUs are active. For example the following crash: User access of kernel address (c00000003fff5af0) - exploit attempt? (uid: 0) segfault (11) at c00000003fff5af0 nip 7fff8ad12198 lr 7fff8ad121f8 code 1 code: 40820128 e93c00d0 e9290058 7c292840 40810058 38600000 4bfd9a81 e8410018 code: 2c030006 41810154 3860ffb6 e9210098 <e94d8ff0> 7d295279 39400000 40820a3c Shows that we returned to userspace without restoring the user r13 value, due to executing the partially patched STF exit code. Fix it by doing the patching under stop machine. The CPUs that aren't doing the patching will be spinning in the core of the stop machine logic. That is currently sufficient for our purposes, because none of the patching we do is to that code or anywhere in the vicinity. Fixes: a048a07d7f45 ("powerpc/64s: Add support for a store forwarding barrier at kernel entry/exit") Cc: stable@vger.kernel.org # v4.17+ Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20210506044959.1298123-1-mpe@ellerman.id.au
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xor.c
// SPDX-License-Identifier: GPL-2.0-or-later
/*
* xor.c : Multiple Devices driver for Linux
*
* Copyright (C) 1996, 1997, 1998, 1999, 2000,
* Ingo Molnar, Matti Aarnio, Jakub Jelinek, Richard Henderson.
*
* Dispatch optimized RAID-5 checksumming functions.
*/
#define BH_TRACE 0
#include <linux/module.h>
#include <linux/gfp.h>
#include <linux/raid/xor.h>
#include <linux/jiffies.h>
#include <linux/preempt.h>
#include <asm/xor.h>
#ifndef XOR_SELECT_TEMPLATE
#define XOR_SELECT_TEMPLATE(x) (x)
#endif
/* The xor routines to use. */
static struct xor_block_template *active_template;
void
xor_blocks(unsigned int src_count, unsigned int bytes, void *dest, void **srcs)
{
unsigned long *p1, *p2, *p3, *p4;
p1 = (unsigned long *) srcs[0];
if (src_count == 1) {
active_template->do_2(bytes, dest, p1);
return;
}
p2 = (unsigned long *) srcs[1];
if (src_count == 2) {
active_template->do_3(bytes, dest, p1, p2);
return;
}
p3 = (unsigned long *) srcs[2];
if (src_count == 3) {
active_template->do_4(bytes, dest, p1, p2, p3);
return;
}
p4 = (unsigned long *) srcs[3];
active_template->do_5(bytes, dest, p1, p2, p3, p4);
}
EXPORT_SYMBOL(xor_blocks);
/* Set of all registered templates. */
static struct xor_block_template *__initdata template_list;
#ifndef MODULE
static void __init do_xor_register(struct xor_block_template *tmpl)
{
tmpl->next = template_list;
template_list = tmpl;
}
static int __init register_xor_blocks(void)
{
active_template = XOR_SELECT_TEMPLATE(NULL);
if (!active_template) {
#define xor_speed do_xor_register
// register all the templates and pick the first as the default
XOR_TRY_TEMPLATES;
#undef xor_speed
active_template = template_list;
}
return 0;
}
#endif
#define BENCH_SIZE 4096
#define REPS 800U
static void __init
do_xor_speed(struct xor_block_template *tmpl, void *b1, void *b2)
{
int speed;
int i, j;
ktime_t min, start, diff;
tmpl->next = template_list;
template_list = tmpl;
preempt_disable();
min = (ktime_t)S64_MAX;
for (i = 0; i < 3; i++) {
start = ktime_get();
for (j = 0; j < REPS; j++) {
mb(); /* prevent loop optimization */
tmpl->do_2(BENCH_SIZE, b1, b2);
mb();
}
diff = ktime_sub(ktime_get(), start);
if (diff < min)
min = diff;
}
preempt_enable();
// bytes/ns == GB/s, multiply by 1000 to get MB/s [not MiB/s]
if (!min)
min = 1;
speed = (1000 * REPS * BENCH_SIZE) / (unsigned int)ktime_to_ns(min);
tmpl->speed = speed;
pr_info(" %-16s: %5d MB/sec\n", tmpl->name, speed);
}
static int __init
calibrate_xor_blocks(void)
{
void *b1, *b2;
struct xor_block_template *f, *fastest;
fastest = XOR_SELECT_TEMPLATE(NULL);
if (fastest) {
printk(KERN_INFO "xor: automatically using best "
"checksumming function %-10s\n",
fastest->name);
goto out;
}
b1 = (void *) __get_free_pages(GFP_KERNEL, 2);
if (!b1) {
printk(KERN_WARNING "xor: Yikes! No memory available.\n");
return -ENOMEM;
}
b2 = b1 + 2*PAGE_SIZE + BENCH_SIZE;
/*
* If this arch/cpu has a short-circuited selection, don't loop through
* all the possible functions, just test the best one
*/
#define xor_speed(templ) do_xor_speed((templ), b1, b2)
printk(KERN_INFO "xor: measuring software checksum speed\n");
template_list = NULL;
XOR_TRY_TEMPLATES;
fastest = template_list;
for (f = fastest; f; f = f->next)
if (f->speed > fastest->speed)
fastest = f;
pr_info("xor: using function: %s (%d MB/sec)\n",
fastest->name, fastest->speed);
#undef xor_speed
free_pages((unsigned long)b1, 2);
out:
active_template = fastest;
return 0;
}
static __exit void xor_exit(void) { }
MODULE_LICENSE("GPL");
#ifndef MODULE
/* when built-in xor.o must initialize before drivers/md/md.o */
core_initcall(register_xor_blocks);
#endif
module_init(calibrate_xor_blocks);
module_exit(xor_exit);
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