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de31b3cd70
The FRED RSP0 MSR is only used for delivering events when running
userspace. Linux leverages this property to reduce expensive MSR
writes and optimize context switches. The kernel only writes the
MSR when about to run userspace *and* when the MSR has actually
changed since the last time userspace ran.
This optimization is implemented by maintaining a per-CPU cache of
FRED RSP0 and then checking that against the value for the top of
current task stack before running userspace.
However cpu_init_fred_exceptions() writes the MSR without updating
the per-CPU cache. This means that the kernel might return to
userspace with MSR_IA32_FRED_RSP0==0 when it needed to point to the
top of current task stack. This would induce a double fault (#DF),
which is bad.
A context switch after cpu_init_fred_exceptions() can paper over
the issue since it updates the cached value. That evidently
happens most of the time explaining how this bug got through.
Fix the bug through resynchronizing the FRED RSP0 MSR with its
per-CPU cache in cpu_init_fred_exceptions().
Fixes:
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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.