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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 20); 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 19 _/J 524288 userspace used a mutating debug operation in fwctl
105=== === ====== ========================================================
106
107Note: The character ``_`` is representing a blank in this table to make reading
108easier.
109
110More detailed explanation for tainting
111~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
112
113 0) ``G`` if all modules loaded have a GPL or compatible license, ``P`` if
114 any proprietary module has been loaded. Modules without a
115 MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
116 insmod as GPL compatible are assumed to be proprietary.
117
118 1) ``F`` if any module was force loaded by ``insmod -f``, ``' '`` if all
119 modules were loaded normally.
120
121 2) ``S`` if the kernel is running on a processor or system that is out of
122 specification: hardware has been put into an unsupported configuration,
123 therefore proper execution cannot be guaranteed.
124 Kernel will be tainted if, for example:
125
126 - on x86: PAE is forced through forcepae on intel CPUs (such as Pentium M)
127 which do not report PAE but may have a functional implementation, an SMP
128 kernel is running on non officially capable SMP Athlon CPUs, MSRs are
129 being poked at from userspace.
130 - on arm: kernel running on certain CPUs (such as Keystone 2) without
131 having certain kernel features enabled.
132 - on arm64: there are mismatched hardware features between CPUs, the
133 bootloader has booted CPUs in different modes.
134 - certain drivers are being used on non supported architectures (such as
135 scsi/snic on something else than x86_64, scsi/ips on non
136 x86/x86_64/itanium, have broken firmware settings for the
137 irqchip/irq-gic on arm64 ...).
138 - x86/x86_64: Microcode late loading is dangerous and will result in
139 tainting the kernel. It requires that all CPUs rendezvous to make sure
140 the update happens when the system is as quiescent as possible. However,
141 a higher priority MCE/SMI/NMI can move control flow away from that
142 rendezvous and interrupt the update, which can be detrimental to the
143 machine.
144
145 3) ``R`` if a module was force unloaded by ``rmmod -f``, ``' '`` if all
146 modules were unloaded normally.
147
148 4) ``M`` if any processor has reported a Machine Check Exception,
149 ``' '`` if no Machine Check Exceptions have occurred.
150
151 5) ``B`` If a page-release function has found a bad page reference or some
152 unexpected page flags. This indicates a hardware problem or a kernel bug;
153 there should be other information in the log indicating why this tainting
154 occurred.
155
156 6) ``U`` if a user or user application specifically requested that the
157 Tainted flag be set, ``' '`` otherwise.
158
159 7) ``D`` if the kernel has died recently, i.e. there was an OOPS or BUG.
160
161 8) ``A`` if an ACPI table has been overridden.
162
163 9) ``W`` if a warning has previously been issued by the kernel.
164 (Though some warnings may set more specific taint flags.)
165
166 10) ``C`` if a staging driver has been loaded.
167
168 11) ``I`` if the kernel is working around a severe bug in the platform
169 firmware (BIOS or similar).
170
171 12) ``O`` if an externally-built ("out-of-tree") module has been loaded.
172
173 13) ``E`` if an unsigned module has been loaded in a kernel supporting
174 module signature.
175
176 14) ``L`` if a soft lockup has previously occurred on the system.
177
178 15) ``K`` if the kernel has been live patched.
179
180 16) ``X`` Auxiliary taint, defined for and used by Linux distributors.
181
182 17) ``T`` Kernel was build with the randstruct plugin, which can intentionally
183 produce extremely unusual kernel structure layouts (even performance
184 pathological ones), which is important to know when debugging. Set at
185 build time.
186
187 18) ``N`` if an in-kernel test, such as a KUnit test, has been run.
188
189 19) ``J`` if userspace opened /dev/fwctl/* and performed a FWTCL_RPC_DEBUG_WRITE
190 to use the devices debugging features. Device debugging features could
191 cause the device to malfunction in undefined ways.