Commit 1013af4f authored by Jann Horn's avatar Jann Horn Committed by Andrew Morton
Browse files

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.

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: 39dde65c ("[PATCH] shared page table for hugetlb page")
Signed-off-by: default avatarJann Horn <jannh@google.com>
Reviewed-by: default avatarLorenzo 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 081056dc

mm/hugetlb.c

+7 −0
Original line number Diff line number Diff line
@@ -7629,6 +7629,13 @@ 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();

	ptdesc_pmd_pts_dec(virt_to_ptdesc(ptep));

	mm_dec_nr_pmds(mm);

	return 1;