1Tainted kernels 2--------------- 3 4The kernel will mark itself as 'tainted' when something occurs that might be 5relevant later when investigating problems. Don't worry too much about this, 6most of the time it's not a problem to run a tainted kernel; the information is 7mainly of interest once someone wants to investigate some problem, as its real 8cause might be the event that got the kernel tainted. That's why bug reports 9from tainted kernels will often be ignored by developers, hence try to reproduce 10problems with an untainted kernel. 11 12Note the kernel will remain tainted even after you undo what caused the taint 13(i.e. unload a proprietary kernel module), to indicate the kernel remains not 14trustworthy. That's also why the kernel will print the tainted state when it 15notices an internal problem (a 'kernel bug'), a recoverable error 16('kernel oops') or a non-recoverable error ('kernel panic') and writes debug 17information about this to the logs ``dmesg`` outputs. It's also possible to 18check the tainted state at runtime through a file in ``/proc/``. 19 20 21Tainted flag in bugs, oops or panics messages 22~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 23 24You find the tainted state near the top in a line starting with 'CPU:'; if or 25why the kernel was tainted is shown after the Process ID ('PID:') and a shortened 26name of the command ('Comm:') that triggered the event:: 27 28 BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 29 Oops: 0002 [#1] SMP PTI 30 CPU: 0 PID: 4424 Comm: insmod Tainted: P W O 4.20.0-0.rc6.fc30 #1 31 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011 32 RIP: 0010:my_oops_init+0x13/0x1000 [kpanic] 33 [...] 34 35You'll find a 'Not tainted: ' there if the kernel was not tainted at the 36time of the event; if it was, then it will print 'Tainted: ' and characters 37either letters or blanks. In the example above it looks like this:: 38 39 Tainted: P W O 40 41The meaning of those characters is explained in the table below. In this case 42the kernel got tainted earlier because a proprietary Module (``P``) was loaded, 43a warning occurred (``W``), and an externally-built module was loaded (``O``). 44To decode other letters use the table below. 45 46 47Decoding tainted state at runtime 48~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 49 50At runtime, you can query the tainted state by reading 51``cat /proc/sys/kernel/tainted``. If that returns ``0``, the kernel is not 52tainted; any other number indicates the reasons why it is. The easiest way to 53decode that number is the script ``tools/debugging/kernel-chktaint``, which your 54distribution might ship as part of a package called ``linux-tools`` or 55``kernel-tools``; if it doesn't, you can download the script from 56`git.kernel.org <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/tools/debugging/kernel-chktaint>`_ 57and execute it with ``sh kernel-chktaint``, which would print something like 58this on the machine that had the statements in the logs that were quoted earlier:: 59 60 Kernel is Tainted for following reasons: 61 * Proprietary module was loaded (#0) 62 * Kernel issued warning (#9) 63 * Externally-built ('out-of-tree') module was loaded (#12) 64 See Documentation/admin-guide/tainted-kernels.rst in the Linux kernel or 65 https://www.kernel.org/doc/html/latest/admin-guide/tainted-kernels.html for 66 a more details explanation of the various taint flags. 67 Raw taint value as int/string: 4609/'P W O ' 68 69You can try to decode the number yourself. That's easy if there was only one 70reason that got your kernel tainted, as in this case you can find the number 71with the table below. If there were multiple reasons you need to decode the 72number, as it is a bitfield, where each bit indicates the absence or presence of 73a particular type of taint. It's best to leave that to the aforementioned 74script, but if you need something quick you can use this shell command to check 75which bits are set:: 76 77 $ for i in $(seq 18); do echo $(($i-1)) $(($(cat /proc/sys/kernel/tainted)>>($i-1)&1));done 78 79Table for decoding tainted state 80~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 81 82=== === ====== ======================================================== 83Bit Log Number Reason that got the kernel tainted 84=== === ====== ======================================================== 85 0 G/P 1 proprietary module was loaded 86 1 _/F 2 module was force loaded 87 2 _/S 4 kernel running on an out of specification system 88 3 _/R 8 module was force unloaded 89 4 _/M 16 processor reported a Machine Check Exception (MCE) 90 5 _/B 32 bad page referenced or some unexpected page flags 91 6 _/U 64 taint requested by userspace application 92 7 _/D 128 kernel died recently, i.e. there was an OOPS or BUG 93 8 _/A 256 ACPI table overridden by user 94 9 _/W 512 kernel issued warning 95 10 _/C 1024 staging driver was loaded 96 11 _/I 2048 workaround for bug in platform firmware applied 97 12 _/O 4096 externally-built ("out-of-tree") module was loaded 98 13 _/E 8192 unsigned module was loaded 99 14 _/L 16384 soft lockup occurred 100 15 _/K 32768 kernel has been live patched 101 16 _/X 65536 auxiliary taint, defined for and used by distros 102 17 _/T 131072 kernel was built with the struct randomization plugin 103 18 _/N 262144 an in-kernel test has been run 104=== === ====== ======================================================== 105 106Note: The character ``_`` is representing a blank in this table to make reading 107easier. 108 109More detailed explanation for tainting 110~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 111 112 0) ``G`` if all modules loaded have a GPL or compatible license, ``P`` if 113 any proprietary module has been loaded. Modules without a 114 MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by 115 insmod as GPL compatible are assumed to be proprietary. 116 117 1) ``F`` if any module was force loaded by ``insmod -f``, ``' '`` if all 118 modules were loaded normally. 119 120 2) ``S`` if the kernel is running on a processor or system that is out of 121 specification: hardware has been put into an unsupported configuration, 122 therefore proper execution cannot be guaranteed. 123 Kernel will be tainted if, for example: 124 125 - on x86: PAE is forced through forcepae on intel CPUs (such as Pentium M) 126 which do not report PAE but may have a functional implementation, an SMP 127 kernel is running on non officially capable SMP Athlon CPUs, MSRs are 128 being poked at from userspace. 129 - on arm: kernel running on certain CPUs (such as Keystone 2) without 130 having certain kernel features enabled. 131 - on arm64: there are mismatched hardware features between CPUs, the 132 bootloader has booted CPUs in different modes. 133 - certain drivers are being used on non supported architectures (such as 134 scsi/snic on something else than x86_64, scsi/ips on non 135 x86/x86_64/itanium, have broken firmware settings for the 136 irqchip/irq-gic on arm64 ...). 137 - x86/x86_64: Microcode late loading is dangerous and will result in 138 tainting the kernel. It requires that all CPUs rendezvous to make sure 139 the update happens when the system is as quiescent as possible. However, 140 a higher priority MCE/SMI/NMI can move control flow away from that 141 rendezvous and interrupt the update, which can be detrimental to the 142 machine. 143 144 3) ``R`` if a module was force unloaded by ``rmmod -f``, ``' '`` if all 145 modules were unloaded normally. 146 147 4) ``M`` if any processor has reported a Machine Check Exception, 148 ``' '`` if no Machine Check Exceptions have occurred. 149 150 5) ``B`` If a page-release function has found a bad page reference or some 151 unexpected page flags. This indicates a hardware problem or a kernel bug; 152 there should be other information in the log indicating why this tainting 153 occurred. 154 155 6) ``U`` if a user or user application specifically requested that the 156 Tainted flag be set, ``' '`` otherwise. 157 158 7) ``D`` if the kernel has died recently, i.e. there was an OOPS or BUG. 159 160 8) ``A`` if an ACPI table has been overridden. 161 162 9) ``W`` if a warning has previously been issued by the kernel. 163 (Though some warnings may set more specific taint flags.) 164 165 10) ``C`` if a staging driver has been loaded. 166 167 11) ``I`` if the kernel is working around a severe bug in the platform 168 firmware (BIOS or similar). 169 170 12) ``O`` if an externally-built ("out-of-tree") module has been loaded. 171 172 13) ``E`` if an unsigned module has been loaded in a kernel supporting 173 module signature. 174 175 14) ``L`` if a soft lockup has previously occurred on the system. 176 177 15) ``K`` if the kernel has been live patched. 178 179 16) ``X`` Auxiliary taint, defined for and used by Linux distributors. 180 181 17) ``T`` Kernel was build with the randstruct plugin, which can intentionally 182 produce extremely unusual kernel structure layouts (even performance 183 pathological ones), which is important to know when debugging. Set at 184 build time. 185 186 18) ``N`` if an in-kernel test, such as a KUnit test, has been run. 187