tools/testing/selftests/lkdtm/stack-entropy.sh

   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