+ mm-hugetlb-fix-huge_pmd_unshare-vs-gup-fast-race.patch added to mm-hotfixes-unstable branch

+ mm-hugetlb-fix-huge_pmd_unshare-vs-gup-fast-race.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-hugetlb-fix-huge_pmd_unshare-vs-gup-fast-race.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-fix-huge_pmd_unshare-vs-gup-fast-race.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Jann Horn <jannh@google.com>
Subject: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race
Date: Tue, 27 May 2025 23:23:54 +0200

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/20250527-hugetlb-fixes-splitrace-v1-2-f4136f5ec58a@google.com
Fixes: 39dde65c9940 ("[PATCH] shared page table for hugetlb page")
Signed-off-by: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/mm/hugetlb.c~mm-hugetlb-fix-huge_pmd_unshare-vs-gup-fast-race
+++ a/mm/hugetlb.c
@@ -7627,6 +7627,13 @@ int huge_pmd_unshare(struct mm_struct *m
 		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;
_

Patches currently in -mm which might be from jannh@google.com are

mm-hugetlb-unshare-page-tables-during-vma-split-not-before.patch
mm-hugetlb-fix-huge_pmd_unshare-vs-gup-fast-race.patch