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test_progs and test_verifier binaries execute unpriv tests under the
following conditions:
- unpriv BPF is enabled;
- CPU mitigations are enabled (see [1] for details).
The detection of the "mitigations enabled" state is performed by
unpriv_helpers.c:get_mitigations_off() via inspecting kernel boot
command line, looking for a parameter "mitigations=off".
Such detection scheme won't work for certain configurations,
e.g. when CONFIG_CPU_MITIGATIONS is disabled and boot parameter is
not supplied.
Miss-detection leads to test_progs executing tests meant to be run
only with mitigations enabled, e.g.
verifier_and.c:known_subreg_with_unknown_reg(), and reporting false
failures.
Internally, verifier sets bpf_verifier_env->bypass_spec_{v1,v4}
basing on the value returned by kernel/cpu.c:cpu_mitigations_off().
This function is backed by a variable kernel/cpu.c:cpu_mitigations.
This state is not fully introspect-able via sysfs. The closest proxy
is /sys/devices/system/cpu/vulnerabilities/spectre_v1, but it reports
"vulnerable" state only if mitigations are disabled *and* current cpu
is vulnerable, while verifier does not check cpu state.
There are only two ways the kernel/cpu.c:cpu_mitigations can be set:
- via boot parameter;
- via CONFIG_CPU_MITIGATIONS option.
This commit updates unpriv_helpers.c:get_mitigations_off() to scan
/boot/config-$(uname -r) and /proc/config.gz for
CONFIG_CPU_MITIGATIONS value in addition to boot command line check.
Tested using the following configurations:
- mitigations enabled (unpriv tests are enabled)
- mitigations disabled via boot cmdline (unpriv tests skipped)
- mitigations disabled via CONFIG_CPU_MITIGATIONS
(unpriv tests skipped)
[1] https://lore.kernel.org/bpf/20231025031144.5508-1-laoar.shao@gmail.com/
Reported-by: Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250617005710.1066165-2-eddyz87@gmail.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.