What does "kernel tainted" mean?

Question

My OS is Fedora 17. Recently, kernel tainted warning "kernel bug at kernel/auditsc.c:1772!-abrt" occurs: This problem should not be reported (it is likely a known problem). A kernel problem occurred, but your kernel has been tainted (flags:GD). Kernel maintainers are unable to diagnose tainted reports.

Then, I get the following:

# cat /proc/sys/kernel/tainted
128

# dmesg | grep -i taint
[ 8306.955523] Pid: 4511, comm: chrome Tainted: G      D      3.9.10-100.fc17.i686.PAE #1 Dell Inc. 
[ 8307.366310] Pid: 4571, comm: chrome Tainted: G      D      3.9.10-100.fc17.i686.PAE #1 Dell Inc. 

It seems that the value "128" is much serious: 128 – The system has died.

How about this warning? Since chrome is flagged as the "Tainted" source, anybody also meet this matter?

Answer 1

To (over) simplify, 'tainted' means that the kernel is in a state other than what it would be in if it were built fresh from the open source origin and used in a way that it had been intended. It is a way of flagging a kernel to warn people (e.g., developers) that there may be unknown reasons for it to be unreliable, and that debugging it may be difficult or impossible.

In this case, 'GD' means that all modules are licensed as GPL or compatible (ie not proprietary), and that a crash or BUG() occurred.

The reasons are listed below:

See: oops-tracing.txt

---------------------------------------------------------------------------
Tainted kernels:

Some oops reports contain the string 'Tainted: ' after the program
counter. This indicates that the kernel has been tainted by some
mechanism.  The string is followed by a series of position-sensitive
characters, each representing a particular tainted value.

  1: 'G' if all modules loaded have a GPL or compatible license, 'P' if
     any proprietary module has been loaded.  Modules without a
     MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
     insmod as GPL compatible are assumed to be proprietary.

  2: 'F' if any module was force loaded by "insmod -f", ' ' if all
     modules were loaded normally.

  3: 'S' if the oops occurred on an SMP kernel running on hardware that
     hasn't been certified as safe to run multiprocessor.
     Currently this occurs only on various Athlons that are not
     SMP capable.

  4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all
     modules were unloaded normally.

  5: 'M' if any processor has reported a Machine Check Exception,
     ' ' if no Machine Check Exceptions have occurred.

  6: 'B' if a page-release function has found a bad page reference or
     some unexpected page flags.

  7: 'U' if a user or user application specifically requested that the
     Tainted flag be set, ' ' otherwise.

  8: 'D' if the kernel has died recently, i.e. there was an OOPS or BUG.

  9: 'A' if the ACPI table has been overridden.

 10: 'W' if a warning has previously been issued by the kernel.
     (Though some warnings may set more specific taint flags.)

 11: 'C' if a staging driver has been loaded.

 12: 'I' if the kernel is working around a severe bug in the platform
     firmware (BIOS or similar).

 13: 'O' if an externally-built ("out-of-tree") module has been loaded.

 14: 'E' if an unsigned module has been loaded in a kernel supporting
     module signature.

 15: 'L' if a soft lockup has previously occurred on the system.

 16: 'K' if the kernel has been live patched.

The primary reason for the 'Tainted: ' string is to tell kernel
debuggers if this is a clean kernel or if anything unusual has
occurred.  Tainting is permanent: even if an offending module is
unloaded, the tainted value remains to indicate that the kernel is not
trustworthy.

Answer 2

Also showing numbers for the content of /proc/sys/kernel/tainted file:

Non-zero if the kernel has been tainted. Numeric values, which can be
ORed together. The letters are seen in "Tainted" line of Oops reports.

     1 (P):  A module with a non-GPL license has been loaded, this
             includes modules with no license.
             Set by modutils >= 2.4.9 and module-init-tools.
     2 (F): A module was force loaded by insmod -f.
            Set by modutils >= 2.4.9 and module-init-tools.
     4 (S): Unsafe SMP processors: SMP with CPUs not designed for SMP.
     8 (R): A module was forcibly unloaded from the system by rmmod -f.
    16 (M): A hardware machine check error occurred on the system.
    32 (B): A bad page was discovered on the system.
    64 (U): The user has asked that the system be marked "tainted". This
            could be because they are running software that directly modifies
            the hardware, or for other reasons.
   128 (D): The system has died.
   256 (A): The ACPI DSDT has been overridden with one supplied by the user
            instead of using the one provided by the hardware.
   512 (W): A kernel warning has occurred.
  1024 (C): A module from drivers/staging was loaded.
  2048 (I): The system is working around a severe firmware bug.
  4096 (O): An out-of-tree module has been loaded.
  8192 (E): An unsigned module has been loaded in a kernel supporting module
            signature.
 16384 (L): A soft lockup has previously occurred on the system.
 32768 (K): The kernel has been live patched.
 65536 (X): Auxiliary taint, defined and used by for distros.
131072 (T): The kernel was built with the struct randomization plugin.

Source: https://www.kernel.org/doc/Documentation/sysctl/kernel.txt

Credit: https://askubuntu.com/questions/248470/what-does-the-kernel-taint-value-mean