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0f230bc24b
When we try to unshare a pinned page for a private hugetlb, uffd-wp bit
can get lost during unsharing.
When above condition met, one can lose uffd-wp bit on the privately mapped
hugetlb page. It allows the page to be writable even if it should still be
wr-protected. I assume it can mean data loss.
This should be very rare, only if an unsharing happened on a private
hugetlb page with uffd-wp protected (e.g. in a child which shares the
same page with parent with UFFD_FEATURE_EVENT_FORK enabled).
When I wrote the reproducer (provided in the last patch) I needed to
use the newest gup_test cmd introduced by David to trigger it because I
don't even know another way to do a proper RO longerm pin.
Besides that, it needs a bunch of other conditions all met:
(1) hugetlb being mapped privately,
(2) userfaultfd registered with WP and EVENT_FORK,
(3) the user app fork()s, then,
(4) RO longterm pin onto a wr-protected anonymous page.
If it's not impossible to hit in production I'd say extremely rare.
Link: https://lkml.kernel.org/r/20230417195317.898696-3-peterx@redhat.com
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Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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There are various text files in the Documentation/ subdirectory,
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