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Documentation/arch/x86: Hide clearcpuid=

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This option was never meant to be used in production because it solely
clears the X86_FEATURE kernel-internal representation of what CPUID bits
it has detected and doesn't do any *proper* feature disablement like
clearing CR4.CET in the user shadow stack case, for example.

So remove its documentation so that it doesn't get used in production
and people get silly ideas. It is meant strictly for debugging; and if
a chicken bit for properly disabling a feature is warranted, then that
would need proper enablement.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Mathias Krause <minipli@grsecurity.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/20260520202508.160112-1-bp@kernel.org
This commit is contained in:
Borislav Petkov (AMD) 2026-05-20 13:25:07 -07:00 committed by Ingo Molnar
parent e7ae89a0c9
commit 80501dff81
2 changed files with 4 additions and 18 deletions
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18
Documentation/admin-guide/kernel-parameters.txt
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@ -789,24 +789,6 @@ Kernel parameters
cio_ignore= [S390]
See Documentation/arch/s390/common_io.rst for details.
clearcpuid=X[,X...] [X86]
Disable CPUID feature X for the kernel. See
arch/x86/include/asm/cpufeatures.h for the valid bit
numbers X. Note the Linux-specific bits are not necessarily
stable over kernel options, but the vendor-specific
ones should be.
X can also be a string as appearing in the flags: line
in /proc/cpuinfo which does not have the above
instability issue. However, not all features have names
in /proc/cpuinfo.
Note that using this option will taint your kernel.
Also note that user programs calling CPUID directly
or using the feature without checking anything
will still see it. This just prevents it from
being used by the kernel or shown in /proc/cpuinfo.
Also note the kernel might malfunction if you disable
some critical bits.
clk_ignore_unused
[CLK]
Prevents the clock framework from automatically gating

4
Documentation/arch/x86/cpuinfo.rst
View File

@ -187,6 +187,10 @@ to disable features using the feature number as defined in
Protection can be disabled using clearcpuid=514. The number 514 is calculated
from #define X86_FEATURE_UMIP (16*32 + 2).
DO NOT USE this cmdline option in production - it is meant to be used only as
a quick'n'dirty debugging aid to rule out a feature-enabling code is the
culprit. If you use it, it'll taint the kernel.
In addition, there exists a variety of custom command-line parameters that
disable specific features. The list of parameters includes, but is not limited
to, nofsgsbase, nosgx, noxsave, etc. 5-level paging can also be disabled using