Vulnerability CVE-2022-48700
Published: 2024-05-03

Description:
In the Linux kernel, the following vulnerability has been resolved:

vfio/type1: Unpin zero pages

There's currently a reference count leak on the zero page. We increment
the reference via pin_user_pages_remote(), but the page is later handled
as an invalid/reserved page, therefore it's not accounted against the
user and not unpinned by our put_pfn().

Introducing special zero page handling in put_pfn() would resolve the
leak, but without accounting of the zero page, a single user could
still create enough mappings to generate a reference count overflow.

The zero page is always resident, so for our purposes there's no reason
to keep it pinned. Therefore, add a loop to walk pages returned from
pin_user_pages_remote() and unpin any zero pages.

 References:
https://git.kernel.org/stable/c/578d644edc7d2c1ff53f7e4d0a25da473deb4a03
https://git.kernel.org/stable/c/5321908ef74fb593e0dbc8737d25038fc86c9986
https://git.kernel.org/stable/c/5d721bf222936f5cf3ee15ced53cc483ecef7e46
https://git.kernel.org/stable/c/873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4
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