FAILED: patch "[PATCH] mm/huge_memory: fix dereferencing invalid pmd migration entry" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x be6e843fc51a584672dfd9c4a6a24c8cb81d5fb7
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025051203-thrift-spool-ebc8@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From be6e843fc51a584672dfd9c4a6a24c8cb81d5fb7 Mon Sep 17 00:00:00 2001
From: Gavin Guo <[email protected]>
Date: Mon, 21 Apr 2025 19:35:36 +0800
Subject: [PATCH] mm/huge_memory: fix dereferencing invalid pmd migration entry

When migrating a THP, concurrent access to the PMD migration entry during a deferred split scan can lead to an invalid address access, as illustrated below. To prevent this invalid access, it is necessary to check the PMD migration entry and return early. In this context, there is no need to use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the equality of the target folio. Since the PMD migration entry is locked, it cannot be served as the target.

Mailing list discussion and explanation from Hugh Dickins: "An anon_vma lookup points to a location which may contain the folio of interest, but might instead contain another folio: and weeding out those other folios is precisely what the "folio != pmd_folio((*pmd)" check (and the "risk of replacing the wrong folio" comment a few lines above it) is for."

BUG: unable to handle page fault for address: ffffea60001db008
CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60
Call Trace:
<TASK>
try_to_migrate_one+0x28c/0x3730
rmap_walk_anon+0x4f6/0x770
unmap_folio+0x196/0x1f0
split_huge_page_to_list_to_order+0x9f6/0x1560
deferred_split_scan+0xac5/0x12a0
shrinker_debugfs_scan_write+0x376/0x470
full_proxy_write+0x15c/0x220
vfs_write+0x2fc/0xcb0
ksys_write+0x146/0x250
do_syscall_64+0x6a/0x120
entry_SYSCALL_64_after_hwframe+0x76/0x7e

The bug is found by syzkaller on an internal kernel, then confirmed on upstream.

index 2a47682d1ab7..47d76d03ce30 100644
@@ -3075,6 +3075,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
 			   pmd_t *pmd, bool freeze, struct folio *folio)
 {
+	bool pmd_migration = is_pmd_migration_entry(*pmd);
+
 	VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
 	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
 	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));

@@ -3085,9 +3087,12 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
 	 * require a folio to check the PMD against. Otherwise, there
 	 * is a risk of replacing the wrong folio.
 	 */
-	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
-	    is_pmd_migration_entry(*pmd)) {
-		if (folio && folio != pmd_folio(*pmd))
+	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) {
+		/*
+		 * Do not apply pmd_folio() to a migration entry; and folio lock
+		 * guarantees that it must be of the wrong folio anyway.
+		 */
+		if (folio && (pmd_migration || folio != pmd_folio(*pmd)))
 			return;
 		__split_huge_pmd_locked(vma, pmd, address, freeze);
 	}

[ Upstream commit be6e843fc51a584672dfd9c4a6a24c8cb81d5fb7 ]

When migrating a THP, concurrent access to the PMD migration entry during a deferred split scan can lead to an invalid address access, as illustrated below. To prevent this invalid access, it is necessary to check the PMD migration entry and return early. In this context, there is no need to use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the equality of the target folio. Since the PMD migration entry is locked, it cannot be served as the target.

Mailing list discussion and explanation from Hugh Dickins: "An anon_vma lookup points to a location which may contain the folio of interest, but might instead contain another folio: and weeding out those other folios is precisely what the "folio != pmd_folio((*pmd)" check (and the "risk of replacing the wrong folio" comment a few lines above it) is for."

BUG: unable to handle page fault for address: ffffea60001db008
CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60
Call Trace:
<TASK>
try_to_migrate_one+0x28c/0x3730
rmap_walk_anon+0x4f6/0x770
unmap_folio+0x196/0x1f0
split_huge_page_to_list_to_order+0x9f6/0x1560
deferred_split_scan+0xac5/0x12a0
shrinker_debugfs_scan_write+0x376/0x470
full_proxy_write+0x15c/0x220
vfs_write+0x2fc/0xcb0
ksys_write+0x146/0x250
do_syscall_64+0x6a/0x120
entry_SYSCALL_64_after_hwframe+0x76/0x7e

The bug is found by syzkaller on an internal kernel, then confirmed on upstream.

---
 mm/huge_memory.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

index f53bc54dacb3..d697d6b73f2c 100644
@@ -2282,6 +2282,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 {
 	spinlock_t *ptl;
 	struct mmu_notifier_range range;
+	bool pmd_migration = is_pmd_migration_entry(*pmd);
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
 				address & HPAGE_PMD_MASK,

@@ -2296,13 +2297,12 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	VM_BUG_ON(freeze && !folio);
 	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
 
-	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
-	    is_pmd_migration_entry(*pmd)) {
+	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) {
 		/*
-		 * It's safe to call pmd_page when folio is set because it's
-		 * guaranteed that pmd is present.
+		 * Do not apply pmd_folio() to a migration entry; and folio lock
+		 * guarantees that it must be of the wrong folio anyway.
 		 */
-		if (folio && folio != page_folio(pmd_page(*pmd)))
+		if (folio && (pmd_migration || folio != page_folio(pmd_page(*pmd))))
 			goto out;
 		__split_huge_pmd_locked(vma, pmd, range.start, freeze);
 	}

base-commit: 58485ff1a74f6c5be9e7c6aafb7293e4337348e7

2.43.0

[ Upstream commit be6e843fc51a584672dfd9c4a6a24c8cb81d5fb7 ]

When migrating a THP, concurrent access to the PMD migration entry during a deferred split scan can lead to an invalid address access, as illustrated below. To prevent this invalid access, it is necessary to check the PMD migration entry and return early. In this context, there is no need to use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the equality of the target folio. Since the PMD migration entry is locked, it cannot be served as the target.

Mailing list discussion and explanation from Hugh Dickins: "An anon_vma lookup points to a location which may contain the folio of interest, but might instead contain another folio: and weeding out those other folios is precisely what the "folio != pmd_folio((*pmd)" check (and the "risk of replacing the wrong folio" comment a few lines above it) is for."

BUG: unable to handle page fault for address: ffffea60001db008
CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60
Call Trace:
<TASK>
try_to_migrate_one+0x28c/0x3730
rmap_walk_anon+0x4f6/0x770
unmap_folio+0x196/0x1f0
split_huge_page_to_list_to_order+0x9f6/0x1560
deferred_split_scan+0xac5/0x12a0
shrinker_debugfs_scan_write+0x376/0x470
full_proxy_write+0x15c/0x220
vfs_write+0x2fc/0xcb0
ksys_write+0x146/0x250
do_syscall_64+0x6a/0x120
entry_SYSCALL_64_after_hwframe+0x76/0x7e

The bug is found by syzkaller on an internal kernel, then confirmed on upstream.

---
 mm/huge_memory.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

index f53bc54dacb3..2c118713f771 100644
@@ -2282,12 +2282,14 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 {
 	spinlock_t *ptl;
 	struct mmu_notifier_range range;
+	bool pmd_migration;
 
 	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
 				address & HPAGE_PMD_MASK,
 				(address & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE);
 	mmu_notifier_invalidate_range_start(&range);
 	ptl = pmd_lock(vma->vm_mm, pmd);
+	pmd_migration = is_pmd_migration_entry(*pmd);
 
 	/*
 	 * If caller asks to setup a migration entry, we need a folio to check

@@ -2296,13 +2298,12 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	VM_BUG_ON(freeze && !folio);
 	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
 
-	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
-	    is_pmd_migration_entry(*pmd)) {
+	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) {
 		/*
-		 * It's safe to call pmd_page when folio is set because it's
-		 * guaranteed that pmd is present.
+		 * Do not apply pmd_folio() to a migration entry; and folio lock
+		 * guarantees that it must be of the wrong folio anyway.
 		 */
-		if (folio && folio != page_folio(pmd_page(*pmd)))
+		if (folio && (pmd_migration || folio != page_folio(pmd_page(*pmd))))
 			goto out;
 		__split_huge_pmd_locked(vma, pmd, range.start, freeze);
 	}

base-commit: 58485ff1a74f6c5be9e7c6aafb7293e4337348e7

2.43.0

Thanks, yes, this new 6.1 version

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected. No action required from the submitter.

The upstream commit SHA1 provided is correct: be6e843fc51a584672dfd9c4a6a24c8cb81d5fb7

Status in newer kernel trees:
6.15.y | Present (exact SHA1)
6.12.y | Present (different SHA1: 6166c3cf4054)
6.6.y | Not found

Note: The patch differs from the upstream commit:
---

1:  be6e843fc51a5 < -:  ------------- mm/huge_memory: fix dereferencing invalid pmd migration entry
-:  ------------- > 1:  7460a8c17e43b mm/huge_memory: fix dereferencing invalid pmd migration entry

---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-6.1.y        |  Success    |  Success   |