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[ Upstream commit22e5fe2a2a] userfaultfd assumes that the enabled features are set once and never changed after UFFDIO_API ioctl succeeded. However, currently, UFFDIO_API can be called concurrently from two different threads, succeed on both threads and leave userfaultfd's features in non-deterministic state. Theoretically, other uffd operations (ioctl's and page-faults) can be dispatched while adversely affected by such changes of features. Moreover, the writes to ctx->state and ctx->features are not ordered, which can - theoretically, again - let userfaultfd_ioctl() think that userfaultfd API completed, while the features are still not initialized. To avoid races, it is arguably best to get rid of ctx->state. Since there are only 2 states, record the API initialization in ctx->features as the uppermost bit and remove ctx->state. Link: https://lkml.kernel.org/r/20210808020724.1022515-3-namit@vmware.com Fixes:9cd75c3cd4("userfaultfd: non-cooperative: add ability to report non-PF events from uffd descriptor") Signed-off-by: Nadav Amit <namit@vmware.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Peter Xu <peterx@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Sasha Levin <sashal@kernel.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.