[PATCH] Documentation/srso: Document a method for checking safe RET operates properly
From: Borislav Petkov
Date: Wed Jul 31 2024 - 12:05:45 EST
From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
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: Borislav Petkov (AMD) <bp@xxxxxxxxx>
---
Documentation/admin-guide/hw-vuln/srso.rst | 69 +++++++++++++++++++++
tools/testing/selftests/x86/Makefile | 2 +-
tools/testing/selftests/x86/srso.c | 70 ++++++++++++++++++++++
3 files changed, 140 insertions(+), 1 deletion(-)
create mode 100644 tools/testing/selftests/x86/srso.c
diff --git a/Documentation/admin-guide/hw-vuln/srso.rst b/Documentation/admin-guide/hw-vuln/srso.rst
index 4bd3ce3ba171..2ad1c05b8c88 100644
--- a/Documentation/admin-guide/hw-vuln/srso.rst
+++ b/Documentation/admin-guide/hw-vuln/srso.rst
@@ -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
diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 5c8757a25998..d51249f14e2f 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -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
diff --git a/tools/testing/selftests/x86/srso.c b/tools/testing/selftests/x86/srso.c
new file mode 100644
index 000000000000..1dcc49faf5a2
--- /dev/null
+++ b/tools/testing/selftests/x86/srso.c
@@ -0,0 +1,70 @@
+#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;
+}
+
--
2.43.0