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

jira VULN-71586
cve CVE-2025-38085
commit-author Jann Horn <jannh@google.com>
commit 1013af4
upstream-diff Stable 5.15 backport a3d864c901a300c295692d129159fc3001a56185
  was used for the actual (clean) cherry-pick.

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.

Link: https://lkml.kernel.org/r/20250528-hugetlb-fixes-splitrace-v2-2-1329349bad1a@google.com
Link: https://lkml.kernel.org/r/20250527-hugetlb-fixes-splitrace-v1-2-f4136f5ec58a@google.com
Fixes: 39dde65 ("[PATCH] shared page table for hugetlb page")
	Signed-off-by: Jann Horn <jannh@google.com>
	Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
	Cc: Liam Howlett <liam.howlett@oracle.com>
	Cc: Muchun Song <muchun.song@linux.dev>
	Cc: Oscar Salvador <osalvador@suse.de>
	Cc: Vlastimil Babka <vbabka@suse.cz>
	Cc: <stable@vger.kernel.org>
	Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 1013af4)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

mm/hugetlb.c

@@ -6778,6 +6778,13 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
 		return 0;
 
 	pud_clear(pud);
+	/*
+	 * Once our caller drops the rmap lock, some other process might be
+	 * using this page table as a normal, non-hugetlb page table.
+	 * Wait for pending gup_fast() in other threads to finish before letting
+	 * that happen.
+	 */
+	tlb_remove_table_sync_one();
 	atomic_dec(&virt_to_page(ptep)->pt_share_count);
 	mm_dec_nr_pmds(mm);
 	/*