lkdtm: Add REPORT_STACK for checking stack offsets

For validating the stack offset behavior, report the offset from a given
process's first seen stack address. Add s script to calculate the results
to the LKDTM kselftests.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210401232347.2791257-7-keescook@chromium.org

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 110f5a8..0e8254d 100644 (file)
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -134,6 +134,23 @@ noinline void lkdtm_CORRUPT_STACK_STRONG(void)
        __lkdtm_CORRUPT_STACK((void *)&data);
 }
 
+static pid_t stack_pid;
+static unsigned long stack_addr;
+
+void lkdtm_REPORT_STACK(void)
+{
+       volatile uintptr_t magic;
+       pid_t pid = task_pid_nr(current);
+
+       if (pid != stack_pid) {
+               pr_info("Starting stack offset tracking for pid %d\n", pid);
+               stack_pid = pid;
+               stack_addr = (uintptr_t)&magic;
+       }
+
+       pr_info("Stack offset: %d\n", (int)(stack_addr - (uintptr_t)&magic));
+}
+
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
 {
        static u8 data[5] __attribute__((aligned(4))) = {1, 2, 3, 4, 5};

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index b2aff4d..8024b6a 100644 (file)
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -110,6 +110,7 @@ static const struct crashtype crashtypes[] = {
        CRASHTYPE(EXHAUST_STACK),
        CRASHTYPE(CORRUPT_STACK),
        CRASHTYPE(CORRUPT_STACK_STRONG),
+       CRASHTYPE(REPORT_STACK),
        CRASHTYPE(CORRUPT_LIST_ADD),
        CRASHTYPE(CORRUPT_LIST_DEL),
        CRASHTYPE(STACK_GUARD_PAGE_LEADING),

diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index 5ae48c6..99f90d3 100644 (file)
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -17,6 +17,7 @@ void lkdtm_LOOP(void);
 void lkdtm_EXHAUST_STACK(void);
 void lkdtm_CORRUPT_STACK(void);
 void lkdtm_CORRUPT_STACK_STRONG(void);
+void lkdtm_REPORT_STACK(void);
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void);
 void lkdtm_SOFTLOCKUP(void);
 void lkdtm_HARDLOCKUP(void);

diff --git a/tools/testing/selftests/lkdtm/.gitignore b/tools/testing/selftests/lkdtm/.gitignore
index f262126..d4b0be8 100644 (file)
--- a/tools/testing/selftests/lkdtm/.gitignore
+++ b/tools/testing/selftests/lkdtm/.gitignore
@@ -1,2 +1,3 @@
 *.sh
 !run.sh
+!stack-entropy.sh

diff --git a/tools/testing/selftests/lkdtm/Makefile b/tools/testing/selftests/lkdtm/Makefile
index 1bcc9ee..c71109c 100644 (file)
--- a/tools/testing/selftests/lkdtm/Makefile
+++ b/tools/testing/selftests/lkdtm/Makefile
@@ -5,6 +5,7 @@ include ../lib.mk
 
 # NOTE: $(OUTPUT) won't get default value if used before lib.mk
 TEST_FILES := tests.txt
+TEST_PROGS := stack-entropy.sh
 TEST_GEN_PROGS = $(patsubst %,$(OUTPUT)/%.sh,$(shell awk '{print $$1}' tests.txt | sed -e 's/\#//'))
 all: $(TEST_GEN_PROGS)
 

diff --git a/tools/testing/selftests/lkdtm/stack-entropy.sh b/tools/testing/selftests/lkdtm/stack-entropy.sh
new file mode 100755 (executable)
index 0000000..b1b8a50
--- /dev/null
+++ b/tools/testing/selftests/lkdtm/stack-entropy.sh
@@ -0,0 +1,36 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+#
+# Measure kernel stack entropy by sampling via LKDTM's REPORT_STACK test.
+set -e
+samples="${1:-1000}"
+
+# Capture dmesg continuously since it may fill up depending on sample size.
+log=$(mktemp -t stack-entropy-XXXXXX)
+dmesg --follow >"$log" & pid=$!
+report=-1
+for i in $(seq 1 $samples); do
+        echo "REPORT_STACK" >/sys/kernel/debug/provoke-crash/DIRECT
+       if [ -t 1 ]; then
+               percent=$(( 100 * $i / $samples ))
+               if [ "$percent" -ne "$report" ]; then
+                       /bin/echo -en "$percent%\r"
+                       report="$percent"
+               fi
+       fi
+done
+kill "$pid"
+
+# Count unique offsets since last run.
+seen=$(tac "$log" | grep -m1 -B"$samples"0 'Starting stack offset' | \
+       grep 'Stack offset' | awk '{print $NF}' | sort | uniq -c | wc -l)
+bits=$(echo "obase=2; $seen" | bc | wc -L)
+echo "Bits of stack entropy: $bits"
+rm -f "$log"
+
+# We would expect any functional stack randomization to be at least 5 bits.
+if [ "$bits" -lt 5 ]; then
+       exit 1
+else
+       exit 0
+fi