The patch titled
Subject: mm/hugetlb: restore the reservation if needed
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-hugetlb-restore-the-reservation-if-needed.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-restore-the-reservation-if-needed.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: Breno Leitao <leitao@xxxxxxxxxx>
Subject: mm/hugetlb: restore the reservation if needed
Date: Wed, 17 Jan 2024 09:10:57 -0800
Currently there is a bug that a huge page could be stolen, and when the
original owner tries to fault in it, it causes a page fault.
You can achieve that by:
1) Creating a single page
echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
2) mmap() the page above with MAP_HUGETLB into (void *ptr1).
* This will mark the page as reserved
3) touch the page, which causes a page fault and allocates the page
* This will move the page out of the free list.
* It will also unreserved the page, since there is no more free
page
4) madvise(MADV_DONTNEED) the page
* This will free the page, but not mark it as reserved.
5) Allocate a secondary page with mmap(MAP_HUGETLB) into (void *ptr2).
* it should fail, but, since there is no more available page.
* But, since the page above is not reserved, this mmap() succeed.
6) Faulting at ptr1 will cause a SIGBUS
* it will try to allocate a huge page, but there is none
available
A full reproducer is in selftest. See
https://lore.kernel.org/all/20240105155419.1939484-1-leitao@xxxxxxxxxx/
Fix this by restoring the reserved page if necessary. If the page being
unmapped has HPAGE_RESV_OWNER set, and needs a reservation, set the
restore_reserve flag, which will move the page from free to reserved.
Link: https://lkml.kernel.org/r/20240117171058.2192286-1-leitao@xxxxxxxxxx
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Suggested-by: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/hugetlb.c | 10 ++++++++++
1 file changed, 10 insertions(+)
--- a/mm/hugetlb.c~mm-hugetlb-restore-the-reservation-if-needed
+++ a/mm/hugetlb.c
@@ -5677,6 +5677,16 @@ void __unmap_hugepage_range(struct mmu_g
hugetlb_count_sub(pages_per_huge_page(h), mm);
hugetlb_remove_rmap(page_folio(page));
+ if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) &&
+ vma_needs_reservation(h, vma, start)) {
+ /*
+ * Restore the reservation if needed, otherwise the
+ * backing page could be stolen by someone.
+ */
+ folio_set_hugetlb_restore_reserve(page_folio(page));
+ vma_add_reservation(h, vma, address);
+ }
+
spin_unlock(ptl);
tlb_remove_page_size(tlb, page, huge_page_size(h));
/*
_
Patches currently in -mm which might be from leitao@xxxxxxxxxx are
mm-hugetlb-restore-the-reservation-if-needed.patch
selftests-mm-run_vmtestssh-add-hugetlb_madv_vs_map.patch
selftests-mm-new-test-that-steals-pages.patch
