mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race huge_pmd_unshare() drops a reference on a page table that may have previously been shared across processes, potentially turning it into a normal page table used in another process in which unrelated VMAs can afterwards be installed. If this happens in the middle of a concurrent gup_fast(), gup_fast() could end up walking the page tables of another process. While I don't see any way in which that immediately leads to kernel memory corruption, it is really weird and unexpected. Fix it with an explicit broadcast IPI through tlb_remove_table_sync_one(), just like we do in khugepaged when removing page tables for a THP collapse.

References

• https://git.kernel.org/stable/c/952596b08c74e8fe9e2883d1dc8a8f54a37384ec
• https://git.kernel.org/stable/c/a3d864c901a300c295692d129159fc3001a56185
• https://git.kernel.org/stable/c/b7754d3aa7bf9f62218d096c0c8f6c13698fac8b
• https://git.kernel.org/stable/c/fe684290418ef9ef76630072086ee530b92f02b8
• https://git.kernel.org/stable/c/034a52b5ef57c9c8225d94e9067f3390bb33922f
• https://git.kernel.org/stable/c/a6bfeb97941a9187833b526bc6cc4ff5706d0ce9
• https://git.kernel.org/stable/c/1013af4f585fccc4d3e5c5824d174de2257f7d6d
• https://project-zero.issues.chromium.org/issues/420715744