FAILED: patch "[PATCH] arm64: mm: Fix the lockless page-table walk in show_pte()" 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 a77644d009dece1104b6fcc6e322b0e4503db0d6
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090940-catering-debug-33e5@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From a77644d009dece1104b6fcc6e322b0e4503db0d6 Mon Sep 17 00:00:00 2001
From: Karl Mehltretter <[email protected]>
Date: Fri, 28 Aug 2026 19:41:31 +0200
Subject: [PATCH] arm64: mm: Fix the lockless page-table walk in show_pte()

show_pte() walks page tables locklessly and can run with interrupts enabled. A concurrent teardown can free a table page while it is being walked. It can also clear a parent entry after show_pte() checked it; the regular pXd_offset() helpers then reread the cleared entry and can derive a bogus lower-level pointer and fault again.

Use the lockless offset helpers with the saved parent entries, as gup_fast() does, and pass the saved PMD to pte_offset_map().

For task page tables, arm64 selects MMU_GATHER_RCU_TABLE_FREE. Disable local interrupts around the walk to hold off RCU-deferred table frees and block the tlb_remove_table_sync_one() IPI until the walk is finished.

Place the IRQ guard after the header print. This does not make the output a consistent snapshot, but prevents the task page-table walk from dereferencing a released table page or deriving a pointer from a different parent value.

index 0b52557652be..75c3e463df2e 100644
@@ -16,6 +16,7 @@
 #include <linux/mm.h>
 #include <linux/hardirq.h>
 #include <linux/init.h>
+#include <linux/irqflags.h>
 #include <linux/kasan.h>
 #include <linux/kprobes.h>
 #include <linux/uaccess.h>

@@ -154,6 +155,9 @@ static void show_pte(unsigned long addr)
 	pr_alert("%s pgtable: %luk pages, %llu-bit VAs, pgdp=%016lx\n",
 		 mm == &init_mm ? "swapper" : "user", PAGE_SIZE / SZ_1K,
 		 vabits_actual, mm_to_pgd_phys(mm));
+
+	guard(irqsave)();
+
 	pgdp = pgd_offset(mm, addr);
 	pgd = READ_ONCE(*pgdp);
 	pr_alert("[%016lx] pgd=%016llx", addr, pgd_val(pgd));

@@ -167,25 +171,25 @@ static void show_pte(unsigned long addr)
 		if (pgd_none(pgd) || pgd_bad(pgd))
 			break;
 
-		p4dp = p4d_offset(pgdp, addr);
+		p4dp = p4d_offset_lockless(pgdp, pgd, addr);
 		p4d = READ_ONCE(*p4dp);
 		pr_cont(", p4d=%016llx", p4d_val(p4d));
 		if (p4d_none(p4d) || p4d_bad(p4d))
 			break;
 
-		pudp = pud_offset(p4dp, addr);
+		pudp = pud_offset_lockless(p4dp, p4d, addr);
 		pud = READ_ONCE(*pudp);
 		pr_cont(", pud=%016llx", pud_val(pud));
 		if (pud_none(pud) || pud_bad(pud))
 			break;
 
-		pmdp = pmd_offset(pudp, addr);
+		pmdp = pmd_offset_lockless(pudp, pud, addr);
 		pmd = READ_ONCE(*pmdp);
 		pr_cont(", pmd=%016llx", pmd_val(pmd));
 		if (pmd_none(pmd) || pmd_bad(pmd))
 			break;
 
-		ptep = pte_offset_map(pmdp, addr);
+		ptep = pte_offset_map(&pmd, addr);
 		if (!ptep)
 			break;
 

From: Hugh Dickins <[email protected]>

[ Upstream commit 52924726f4c06b3f77de75fb665e6efd8e777d07 ]

In rare transient cases, not yet made possible, pte_offset_map() and pte_offset_map_lock() may not find a page table: handle appropriately.

Stable-dep-of: a77644d009de ("arm64: mm: Fix the lockless page-table walk in show_pte()")

---
 arch/arm64/mm/fault.c | 3 +++
 1 file changed, 3 insertions(+)

index 0776c98ad27f1..c4120bd80a9b4 100644
@@ -177,6 +177,9 @@ static void show_pte(unsigned long addr)
 			break;
 
 		ptep = pte_offset_map(pmdp, addr);
+		if (!ptep)
+			break;
+
 		pte = READ_ONCE(*ptep);
 		pr_cont(", pte=%016llx", pte_val(pte));
 		pte_unmap(ptep);

2.53.0

From: Karl Mehltretter <[email protected]>

[ Upstream commit a77644d009dece1104b6fcc6e322b0e4503db0d6 ]

show_pte() walks page tables locklessly and can run with interrupts enabled. A concurrent teardown can free a table page while it is being walked. It can also clear a parent entry after show_pte() checked it; the regular pXd_offset() helpers then reread the cleared entry and can derive a bogus lower-level pointer and fault again.

Use the lockless offset helpers with the saved parent entries, as gup_fast() does, and pass the saved PMD to pte_offset_map().

For task page tables, arm64 selects MMU_GATHER_RCU_TABLE_FREE. Disable local interrupts around the walk to hold off RCU-deferred table frees and block the tlb_remove_table_sync_one() IPI until the walk is finished.

Place the IRQ guard after the header print. This does not make the output a consistent snapshot, but prevents the task page-table walk from dereferencing a released table page or deriving a pointer from a different parent value.

---
 arch/arm64/mm/fault.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

index c4120bd80a9b4..5bd5e10c49f18 100644
@@ -15,6 +15,7 @@
 #include <linux/mm.h>
 #include <linux/hardirq.h>
 #include <linux/init.h>
+#include <linux/irqflags.h>
 #include <linux/kasan.h>
 #include <linux/kprobes.h>
 #include <linux/uaccess.h>

@@ -145,6 +146,9 @@ static void show_pte(unsigned long addr)
 	pr_alert("%s pgtable: %luk pages, %llu-bit VAs, pgdp=%016lx\n",
 		 mm == &init_mm ? "swapper" : "user", PAGE_SIZE / SZ_1K,
 		 vabits_actual, mm_to_pgd_phys(mm));
+
+	guard(irqsave)();
+
 	pgdp = pgd_offset(mm, addr);
 	pgd = READ_ONCE(*pgdp);
 	pr_alert("[%016lx] pgd=%016llx", addr, pgd_val(pgd));

@@ -158,25 +162,25 @@ static void show_pte(unsigned long addr)
 		if (pgd_none(pgd) || pgd_bad(pgd))
 			break;
 
-		p4dp = p4d_offset(pgdp, addr);
+		p4dp = p4d_offset_lockless(pgdp, pgd, addr);
 		p4d = READ_ONCE(*p4dp);
 		pr_cont(", p4d=%016llx", p4d_val(p4d));
 		if (p4d_none(p4d) || p4d_bad(p4d))
 			break;
 
-		pudp = pud_offset(p4dp, addr);
+		pudp = pud_offset_lockless(p4dp, p4d, addr);
 		pud = READ_ONCE(*pudp);
 		pr_cont(", pud=%016llx", pud_val(pud));
 		if (pud_none(pud) || pud_bad(pud))
 			break;
 
-		pmdp = pmd_offset(pudp, addr);
+		pmdp = pmd_offset_lockless(pudp, pud, addr);
 		pmd = READ_ONCE(*pmdp);
 		pr_cont(", pmd=%016llx", pmd_val(pmd));
 		if (pmd_none(pmd) || pmd_bad(pmd))
 			break;
 
-		ptep = pte_offset_map(pmdp, addr);
+		ptep = pte_offset_map(&pmd, addr);
 		if (!ptep)
 			break;
 

2.53.0