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https://github.com/speed47/spectre-meltdown-checker.git
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feat: add --allow-msr-write, no longer write by default (#385), detect when writing is denied
This commit is contained in:
Stéphane Lesimple
parent
ee266d43b7
commit
2a5b965b98
@ -91,6 +91,7 @@ show_usage()
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--hw-only only check for CPU information, don't check for any variant
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--no-hw skip CPU information and checks, if you're inspecting a kernel not to be run on this host
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--vmm [auto,yes,no] override the detection of the presence of a hypervisor, default: auto
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--allow-msr-write allow probing for write-only MSRs, this might produce kernel logs or be blocked by your system
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--cpu [#,all] interact with CPUID and MSR of CPU core number #, or all (default: CPU core 0)
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--update-fwdb update our local copy of the CPU microcodes versions database (using the awesome
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MCExtractor project and the Intel firmwares GitHub repository)
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@ -157,6 +158,7 @@ opt_arch_prefix=''
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opt_hw_only=0
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opt_no_hw=0
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opt_vmm=-1
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opt_allow_msr_write=0
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opt_cpu=0
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opt_explain=0
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opt_paranoid=0
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@ -1047,6 +1049,9 @@ while [ -n "${1:-}" ]; do
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elif [ "$1" = "--no-hw" ]; then
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opt_no_hw=1
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shift
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elif [ "$1" = "--allow-msr-write" ]; then
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opt_allow_msr_write=1
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shift
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elif [ "$1" = "--cpu" ]; then
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opt_cpu=$2
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if [ "$opt_cpu" != all ]; then
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@ -2497,6 +2502,7 @@ write_msr_one_core()
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return $WRITE_MSR_RET_ERR
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fi
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_write_denied=0
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if [ "$os" != Linux ]; then
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cpucontrol -m "$_msr=0" "/dev/cpuctl$_core" >/dev/null 2>&1; ret=$?
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else
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@ -2509,15 +2515,23 @@ write_msr_one_core()
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elif command -v wrmsr >/dev/null 2>&1 && [ "${SMC_NO_WRMSR:-}" != 1 ]; then
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_debug "write_msr: using wrmsr"
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wrmsr $_msr_dec 0 2>/dev/null; ret=$?
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# ret=4: msr doesn't exist, ret=127: msr.allow_writes=off
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[ "$ret" = 127 ] && _write_denied=1
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# or fallback to dd if it supports seek_bytes, we prefer it over perl because we can tell the difference between EPERM and EIO
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elif dd if=/dev/null of=/dev/null bs=8 count=1 seek="$_msr_dec" oflag=seek_bytes 2>/dev/null && [ "${SMC_NO_DD:-}" != 1 ]; then
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_debug "write_msr: using dd"
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dd if=/dev/zero of=/dev/cpu/"$_core"/msr bs=8 count=1 seek="$_msr_dec" oflag=seek_bytes 2>/dev/null; ret=$?
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# if it failed, inspect stderrto look for EPERM
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if [ "$ret" != 0 ]; then
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if dd if=/dev/zero of=/dev/cpu/"$_core"/msr bs=8 count=1 seek="$_msr_dec" oflag=seek_bytes 2>&1 | grep -qF 'Operation not permitted'; then
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_write_denied=1
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fi
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fi
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# or if we have perl, use it, any 5.x version will work
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elif command -v perl >/dev/null 2>&1 && [ "${SMC_NO_PERL:-}" != 1 ]; then
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_debug "write_msr: using perl"
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ret=1
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perl -e "open(M,'>','/dev/cpu/$_core/msr') and seek(M,$_msr_dec,0) and exit(syswrite(M,pack('H16',0)))"; [ $? -eq 8 ] && ret=0
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# fallback to dd if it supports seek_bytes
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elif dd if=/dev/null of=/dev/null bs=8 count=1 seek="$_msr_dec" oflag=seek_bytes 2>/dev/null; then
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_debug "write_msr: using dd"
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dd if=/dev/zero of=/dev/cpu/"$_core"/msr bs=8 count=1 seek="$_msr_dec" oflag=seek_bytes 2>/dev/null; ret=$?
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else
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_debug "write_msr: got no wrmsr, perl or recent enough dd!"
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mockme=$(printf "%b\n%b" "$mockme" "SMC_MOCK_WRMSR_${_msr}_RET=$WRITE_MSR_RET_ERR")
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@ -2531,7 +2545,15 @@ write_msr_one_core()
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# when this happens, any write will fail and dmesg will have a msg printed "msr: Direct access to MSR"
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# * A version of this patch also made it to vanilla in 5.4+, in that case the message is: 'raw MSR access is restricted'
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# * we don't use dmesg_grep() because we don't care if dmesg is truncated here, as the message has just been printed
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if dmesg | grep -qF "msr: Direct access to MSR"; then
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# yet more recent versions of the msr module can be set to msr.allow_writes=off, in which case no dmesg message is printed,
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# but the write fails
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if [ "$_write_denied" = 1 ]; then
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_debug "write_msr: writing to msr has been denied"
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mockme=$(printf "%b\n%b" "$mockme" "SMC_MOCK_WRMSR_${_msr}_RET=$WRITE_MSR_RET_LOCKDOWN")
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msr_locked_down=1
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write_msr_msg="your kernel is configured to deny writes to MSRs from user space"
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return $WRITE_MSR_RET_LOCKDOWN
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elif dmesg | grep -qF "msr: Direct access to MSR"; then
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_debug "write_msr: locked down kernel detected (Red Hat / Fedora)"
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mockme=$(printf "%b\n%b" "$mockme" "SMC_MOCK_WRMSR_${_msr}_RET=$WRITE_MSR_RET_LOCKDOWN")
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msr_locked_down=1
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@ -2544,6 +2566,7 @@ write_msr_one_core()
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write_msr_msg="your kernel is locked down, please reboot with lockdown=none in the kernel cmdline and retry"
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return $WRITE_MSR_RET_LOCKDOWN
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fi
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unset _write_denied
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fi
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fi
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@ -2754,15 +2777,18 @@ check_cpu()
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# IBPB
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_info " * Indirect Branch Prediction Barrier (IBPB)"
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_info_nol " * PRED_CMD MSR is available: "
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# the new MSR 'PRED_CTRL' is at offset 0x49, write-only
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write_msr 0x49; ret=$?
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if [ $ret = $WRITE_MSR_RET_OK ]; then
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pstatus green YES
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elif [ $ret = $WRITE_MSR_RET_KO ]; then
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pstatus yellow NO
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else
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pstatus yellow UNKNOWN "$write_msr_msg"
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if [ "$opt_allow_msr_write" = 1 ]; then
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_info_nol " * PRED_CMD MSR is available: "
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# the new MSR 'PRED_CTRL' is at offset 0x49, write-only
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write_msr 0x49; ret=$?
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if [ $ret = $WRITE_MSR_RET_OK ]; then
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pstatus green YES
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elif [ $ret = $WRITE_MSR_RET_KO ]; then
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pstatus yellow NO
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else
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pstatus yellow UNKNOWN "$write_msr_msg"
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fi
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fi
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_info_nol " * CPU indicates IBPB capability: "
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@ -2912,18 +2938,21 @@ check_cpu()
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fi
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_info " * L1 data cache invalidation"
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_info_nol " * FLUSH_CMD MSR is available: "
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# the new MSR 'FLUSH_CMD' is at offset 0x10b, write-only
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write_msr 0x10b; ret=$?
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if [ $ret = $WRITE_MSR_RET_OK ]; then
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pstatus green YES
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cpu_flush_cmd=1
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elif [ $ret = $WRITE_MSR_RET_KO ]; then
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pstatus yellow NO
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cpu_flush_cmd=0
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else
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pstatus yellow UNKNOWN "$write_msr_msg"
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cpu_flush_cmd=-1
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if [ "$opt_allow_msr_write" = 1 ]; then
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_info_nol " * FLUSH_CMD MSR is available: "
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# the new MSR 'FLUSH_CMD' is at offset 0x10b, write-only
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write_msr 0x10b; ret=$?
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if [ $ret = $WRITE_MSR_RET_OK ]; then
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pstatus green YES
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cpu_flush_cmd=1
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elif [ $ret = $WRITE_MSR_RET_KO ]; then
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pstatus yellow NO
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cpu_flush_cmd=0
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else
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pstatus yellow UNKNOWN "$write_msr_msg"
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cpu_flush_cmd=-1
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fi
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fi
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# CPUID of L1D
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@ -2940,6 +2969,12 @@ check_cpu()
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cpuid_l1df=-1
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fi
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# if we weren't allowed to probe the write-only MSR but the CPUID
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# bit says that it shoul be there, make the assumption that it is
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if [ "$opt_allow_msr_write" != 1 ]; then
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cpu_flush_cmd=$cpuid_l1df
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fi
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if is_intel; then
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_info " * Microarchitectural Data Sampling"
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_info_nol " * VERW instruction is available: "
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