Commit 40153505 authored by Borislav Petkov (AMD)'s avatar Borislav Petkov (AMD)
Browse files

Documentation/srso: Document a method for checking safe RET operates properly 



Add a method to quickly verify whether safe RET operates properly on
a given system using perf tool.

Also, add a selftest which does the same thing.

Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240731160531.28640-1-bp@kernel.org
parent 225f2bd0

Documentation/admin-guide/hw-vuln/srso.rst

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@@ -158,3 +158,72 @@ poisoned BTB entry and using that safe one for all function returns. 
In older Zen1 and Zen2, this is accomplished using a reinterpretation

technique similar to Retbleed one: srso_untrain_ret() and

srso_safe_ret().



Checking the safe RET mitigation actually works

-----------------------------------------------



In case one wants to validate whether the SRSO safe RET mitigation works

on a kernel, one could use two performance counters



* PMC_0xc8 - Count of RET/RET lw retired

* PMC_0xc9 - Count of RET/RET lw retired mispredicted



and compare the number of RETs retired properly vs those retired

mispredicted, in kernel mode. Another way of specifying those events

is::



        # perf list ex_ret_near_ret



        List of pre-defined events (to be used in -e or -M):



        core:

          ex_ret_near_ret

               [Retired Near Returns]

          ex_ret_near_ret_mispred

               [Retired Near Returns Mispredicted]



Either the command using the event mnemonics::



        # perf stat -e ex_ret_near_ret:k -e ex_ret_near_ret_mispred:k sleep 10s



or using the raw PMC numbers::



        # perf stat -e cpu/event=0xc8,umask=0/k -e cpu/event=0xc9,umask=0/k sleep 10s



should give the same amount. I.e., every RET retired should be

mispredicted::



        [root@brent: ~/kernel/linux/tools/perf> ./perf stat -e cpu/event=0xc8,umask=0/k -e cpu/event=0xc9,umask=0/k sleep 10s



         Performance counter stats for 'sleep 10s':



                   137,167      cpu/event=0xc8,umask=0/k

                   137,173      cpu/event=0xc9,umask=0/k



              10.004110303 seconds time elapsed



               0.000000000 seconds user

               0.004462000 seconds sys



vs the case when the mitigation is disabled (spec_rstack_overflow=off)

or not functioning properly, showing usually a lot smaller number of

mispredicted retired RETs vs the overall count of retired RETs during

a workload::



       [root@brent: ~/kernel/linux/tools/perf> ./perf stat -e cpu/event=0xc8,umask=0/k -e cpu/event=0xc9,umask=0/k sleep 10s



        Performance counter stats for 'sleep 10s':



                  201,627      cpu/event=0xc8,umask=0/k

                    4,074      cpu/event=0xc9,umask=0/k



             10.003267252 seconds time elapsed



              0.002729000 seconds user

              0.000000000 seconds sys



Also, there is a selftest which performs the above, go to

tools/testing/selftests/x86/ and do::



        make srso

        ./srso

tools/testing/selftests/x86/Makefile

+1 −1
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@@ -77,7 +77,7 @@ all_32: $(BINARIES_32) 


all_64: $(BINARIES_64)



EXTRA_CLEAN := $(BINARIES_32) $(BINARIES_64)

EXTRA_CLEAN := $(BINARIES_32) $(BINARIES_64) srso



$(BINARIES_32): $(OUTPUT)/%_32: %.c helpers.h

	$(CC) -m32 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $< $(EXTRA_FILES) -lrt -ldl -lm

tools/testing/selftests/x86/srso.c

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// SPDX-License-Identifier: GPL-2.0

#include <linux/perf_event.h>

#include <cpuid.h>

#include <errno.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/ioctl.h>

#include <sys/syscall.h>

#include <unistd.h>



int main(void)

{

	struct perf_event_attr ret_attr, mret_attr;

	long long count_rets, count_rets_mispred;

	int rrets_fd, mrrets_fd;

	unsigned int cpuid1_eax, b, c, d;



	__cpuid(1, cpuid1_eax, b, c, d);



	if (cpuid1_eax < 0x00800f00 ||

	    cpuid1_eax > 0x00afffff) {

		fprintf(stderr, "This needs to run on a Zen[1-4] machine (CPUID(1).EAX: 0x%x). Exiting...\n", cpuid1_eax);

		exit(EXIT_FAILURE);

	}



	memset(&ret_attr, 0, sizeof(struct perf_event_attr));

	memset(&mret_attr, 0, sizeof(struct perf_event_attr));



	ret_attr.type = mret_attr.type = PERF_TYPE_RAW;

	ret_attr.size = mret_attr.size = sizeof(struct perf_event_attr);

	ret_attr.config = 0xc8;

	mret_attr.config = 0xc9;

	ret_attr.disabled = mret_attr.disabled = 1;

	ret_attr.exclude_user = mret_attr.exclude_user = 1;

	ret_attr.exclude_hv = mret_attr.exclude_hv = 1;



	rrets_fd = syscall(SYS_perf_event_open, &ret_attr, 0, -1, -1, 0);

	if (rrets_fd == -1) {

		perror("opening retired RETs fd");

		exit(EXIT_FAILURE);

	}



	mrrets_fd = syscall(SYS_perf_event_open, &mret_attr, 0, -1, -1, 0);

	if (mrrets_fd == -1) {

		perror("opening retired mispredicted RETs fd");

		exit(EXIT_FAILURE);

	}



	ioctl(rrets_fd, PERF_EVENT_IOC_RESET, 0);

	ioctl(mrrets_fd, PERF_EVENT_IOC_RESET, 0);



	ioctl(rrets_fd, PERF_EVENT_IOC_ENABLE, 0);

	ioctl(mrrets_fd, PERF_EVENT_IOC_ENABLE, 0);



	printf("Sleeping for 10 seconds\n");

	sleep(10);



	ioctl(rrets_fd, PERF_EVENT_IOC_DISABLE, 0);

	ioctl(mrrets_fd, PERF_EVENT_IOC_DISABLE, 0);



	read(rrets_fd, &count_rets, sizeof(long long));

	read(mrrets_fd, &count_rets_mispred, sizeof(long long));



	printf("RETs: (%lld retired <-> %lld mispredicted)\n",

		count_rets, count_rets_mispred);

	printf("SRSO Safe-RET mitigation works correctly if both counts are almost equal.\n");



	return 0;

}