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fix: sgx: on locked down kernels, fallback to CPUID bit for detection

on locked down kernels (Fedora / Red Hat feature that prevents writing
to MSRs from userspace, even if root), we can't write to FLUSH_CMD MSR
to verify that it's present. So fallback to checking the existence of
the L1D flush CPUID feature bit to infer that the microcode has been
updated in a recent enough version that also mitigates SGX (fixes for
both issues have been included in the same microcode updates for all
Intel CPUs)
pull/327/head
Stéphane Lesimple 3 years ago
parent
commit
18df38fae6
  1. 38
      spectre-meltdown-checker.sh

38
spectre-meltdown-checker.sh

@ -2181,6 +2181,7 @@ write_msr() @@ -2181,6 +2181,7 @@ write_msr()
if [ -n "$(eval echo \$$_mockvarname)" ]; then
_debug "write_msr: MOCKING enabled for msr $_msr func returns $(eval echo \$$_mockvarname)"
mocked=1
[ "$(eval echo \$$_mockvarname)" = 202 ] && msr_locked_down=1
return "$(eval echo \$$_mockvarname)"
fi
@ -2200,18 +2201,6 @@ write_msr() @@ -2200,18 +2201,6 @@ write_msr()
_debug "write_msr: using perl"
ret=1
perl -e "open(M,'>','/dev/cpu/$_cpu/msr') and seek(M,$_msr_dec,0) and exit(syswrite(M,pack('H16',0)))"; [ $? -eq 8 ] && ret=0
if [ "$ret" = 1 ]; then
# Fedora (and probably Red Hat) have a "kernel lock down" feature that prevents us to write to MSRs
# when this mode is enabled and EFI secure boot is enabled (see issue #303)
# https://src.fedoraproject.org/rpms/kernel/blob/master/f/efi-lockdown.patch
# when this happens, any write will fail and dmesg will have a msg printed "msr: Direct access to MSR"
# we don't use dmesg_grep() because we don't care if dmesg is truncated here, as the message has just been printed
if dmesg | grep -qF "msr: Direct access to MSR"; then
_debug "write_msr: locked down kernel detected"
mockme=$(printf "%b\n%b" "$mockme" "SMC_MOCK_WRMSR_${_msr}_RET=202")
return 202 # lockdown error
fi
fi
# fallback to dd if it supports seek_bytes
elif dd if=/dev/null of=/dev/null bs=8 count=1 seek="$_msr_dec" oflag=seek_bytes 2>/dev/null; then
_debug "write_msr: using dd"
@ -2221,6 +2210,19 @@ write_msr() @@ -2221,6 +2210,19 @@ write_msr()
mockme=$(printf "%b\n%b" "$mockme" "SMC_MOCK_WRMSR_${_msr}_RET=201")
return 201 # missing tool error
fi
if [ "$ret" = 1 ]; then
# Fedora (and probably Red Hat) have a "kernel lock down" feature that prevents us to write to MSRs
# when this mode is enabled and EFI secure boot is enabled (see issue #303)
# https://src.fedoraproject.org/rpms/kernel/blob/master/f/efi-lockdown.patch
# when this happens, any write will fail and dmesg will have a msg printed "msr: Direct access to MSR"
# we don't use dmesg_grep() because we don't care if dmesg is truncated here, as the message has just been printed
if dmesg | grep -qF "msr: Direct access to MSR"; then
_debug "write_msr: locked down kernel detected"
mockme=$(printf "%b\n%b" "$mockme" "SMC_MOCK_WRMSR_${_msr}_RET=202")
msr_locked_down=1
return 202 # lockdown error
fi
fi
fi
# normalize ret
[ "$ret" != 0 ] && ret=1
@ -2643,6 +2645,7 @@ check_cpu() @@ -2643,6 +2645,7 @@ check_cpu()
read_cpuid 0x7 $EDX 28 1 1; ret=$?
if [ $ret -eq 0 ]; then
pstatus green YES "L1D flush feature bit"
cpuid_l1df=1
elif [ $ret -eq 1 ]; then
pstatus yellow NO
elif [ $ret -eq 2 ]; then
@ -4205,10 +4208,15 @@ check_CVE_2018_3615() @@ -4205,10 +4208,15 @@ check_CVE_2018_3615()
_info "\033[1;34m$cve aka '$(cve2name "$cve")'\033[0m"
_info_nol "* CPU microcode mitigates the vulnerability: "
if [ "$cpu_flush_cmd" = 1 ] && [ "$cpuid_sgx" = 1 ]; then
if ( [ "$cpu_flush_cmd" = 1 ] || ( [ "$msr_locked_down" = 1 ] && [ "$cpuid_l1df" = 1 ] ) ) && [ "$cpuid_sgx" = 1 ]; then
# no easy way to detect a fixed SGX but we know that
# microcodes that have the FLUSH_CMD MSR also have the
# fixed SGX (for CPUs that support it)
# fixed SGX (for CPUs that support it), because Intel
# delivered fixed microcodes for both issues at the same time
#
# if the system we're running on is locked down (no way to write MSRs),
# make the assumption that if the L1D flush CPUID bit is set, probably
# that FLUSH_CMD MSR is here too
pstatus green YES
elif [ "$cpuid_sgx" = 1 ]; then
pstatus red NO
@ -4219,7 +4227,7 @@ check_CVE_2018_3615() @@ -4219,7 +4227,7 @@ check_CVE_2018_3615()
if ! is_cpu_vulnerable "$cve"; then
# override status & msg in case CPU is not vulnerable after all
pvulnstatus $cve OK "your CPU vendor reported your CPU model as not vulnerable"
elif [ "$cpu_flush_cmd" = 1 ]; then
elif [ "$cpu_flush_cmd" = 1 ] || ( [ "$msr_locked_down" = 1 ] && [ "$cpuid_l1df" = 1 ] ) ; then
pvulnstatus $cve OK "your CPU microcode mitigates the vulnerability"
else
pvulnstatus $cve VULN "your CPU supports SGX and the microcode is not up to date"

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