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commit35dcb3ac66upstream. Per-vcpu flags are updated using a non-atomic RMW operation. Which means it is possible to get preempted between the read and write operations. Another interesting thing to note is that preemption also updates flags, as we have some flag manipulation in both the load and put operations. It is thus possible to lose information communicated by either load or put, as the preempted flag update will overwrite the flags when the thread is resumed. This is specially critical if either load or put has stored information which depends on the physical CPU the vcpu runs on. This results in really elusive bugs, and kudos must be given to Mostafa for the long hours of debugging, and finally spotting the problem. Fix it by disabling preemption during the RMW operation, which ensures that the state stays consistent. Also upgrade vcpu_get_flag path to use READ_ONCE() to make sure the field is always atomically accessed. Fixes:e87abb73e5("KVM: arm64: Add helpers to manipulate vcpu flags among a set") Reported-by: Mostafa Saleh <smostafa@google.com> Signed-off-by: Marc Zyngier <maz@kernel.org> Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20230418125737.2327972-1-maz@kernel.org Signed-off-by: Oliver Upton <oliver.upton@linux.dev> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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 Restructured Text 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.