1 #!/bin/sh
2 # SPDX-License-Identifier: GPL-2.0
3 #
4 # Measure kernel stack entropy by sampling via LKDTM's REPORT_STACK test.
5 set -e
6 samples="${1:-1000}"
7 TRIGGER=/sys/kernel/debug/provoke-crash/DIRECT
8 KSELFTEST_SKIP_TEST=4
9
10 # Verify we have LKDTM available in the kernel.
11 if [ ! -r $TRIGGER ] ; then
12 /sbin/modprobe -q lkdtm || true
13 if [ ! -r $TRIGGER ] ; then
14 echo "Cannot find $TRIGGER (missing CONFIG_LKDTM?)"
15 else
16 echo "Cannot write $TRIGGER (need to run as root?)"
17 fi
18 # Skip this test
19 exit $KSELFTEST_SKIP_TEST
20 fi
21
22 # Capture dmesg continuously since it may fill up depending on sample size.
23 log=$(mktemp -t stack-entropy-XXXXXX)
24 dmesg --follow >"$log" & pid=$!
25 report=-1
26 for i in $(seq 1 $samples); do
27 echo "REPORT_STACK" > $TRIGGER
28 if [ -t 1 ]; then
29 percent=$(( 100 * $i / $samples ))
30 if [ "$percent" -ne "$report" ]; then
31 /bin/echo -en "$percent%\r"
32 report="$percent"
33 fi
34 fi
35 done
36 kill "$pid"
37
38 # Count unique offsets since last run.
39 seen=$(tac "$log" | grep -m1 -B"$samples"0 'Starting stack offset' | \
40 grep 'Stack offset' | awk '{print $NF}' | sort | uniq -c | wc -l)
41 bits=$(echo "obase=2; $seen" | bc | wc -L)
42 echo "Bits of stack entropy: $bits"
43 rm -f "$log"
44
45 # We would expect any functional stack randomization to be at least 5 bits.
46 if [ "$bits" -lt 5 ]; then
47 echo "Stack entropy is low! Booted without 'randomize_kstack_offset=y'?"
48 exit 1
49 else
50 exit 0
51 fi
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