+ percpu-add-basic-double-free-check.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: percpu: add basic double free check
has been added to the -mm mm-new branch.  Its filename is
     percpu-add-basic-double-free-check.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/percpu-add-basic-double-free-check.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
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------------------------------------------------------
From: Dennis Zhou <dennis@kernel.org>
Subject: percpu: add basic double free check
Date: Thu, 15 Jan 2026 18:32:16 -0800

This adds a basic double free check by validating the first bit of the
allocation in alloc_map and bound_map are set.  If the alloc_map bit is
not set, then this means the area is currently unallocated.  If the
bound_map bit is not set, then we are not freeing from the beginning of
the allocation.

This is a respin of [1] adding the requested changes from me and
Christoph.

Link: https://lore.kernel.org/linux-mm/20250904143514.Yk6Ap-jy@linutronix.de/ [1]
Link: https://lkml.kernel.org/r/20260116023216.14515-1-dennis@kernel.org
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Chistoph Lameter <cl@linux.com>
Cc: Chris Mason <clm@meta.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/percpu.c |   23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

--- a/mm/percpu.c~percpu-add-basic-double-free-check
+++ a/mm/percpu.c
@@ -79,6 +79,7 @@
 #include <linux/mutex.h>
 #include <linux/percpu.h>
 #include <linux/pfn.h>
+#include <linux/ratelimit.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/vmalloc.h>
@@ -1276,18 +1277,24 @@ static int pcpu_alloc_area(struct pcpu_c
 static int pcpu_free_area(struct pcpu_chunk *chunk, int off)
 {
 	struct pcpu_block_md *chunk_md = &chunk->chunk_md;
+	int region_bits = pcpu_chunk_map_bits(chunk);
 	int bit_off, bits, end, oslot, freed;
 
 	lockdep_assert_held(&pcpu_lock);
-	pcpu_stats_area_dealloc(chunk);
 
 	oslot = pcpu_chunk_slot(chunk);
 
 	bit_off = off / PCPU_MIN_ALLOC_SIZE;
+	if (unlikely(bit_off < 0 || bit_off >= region_bits))
+		return 0;
+
+	/* check double free */
+	if (!test_bit(bit_off, chunk->alloc_map) ||
+	    !test_bit(bit_off, chunk->bound_map))
+		return 0;
 
 	/* find end index */
-	end = find_next_bit(chunk->bound_map, pcpu_chunk_map_bits(chunk),
-			    bit_off + 1);
+	end = find_next_bit(chunk->bound_map, region_bits, bit_off + 1);
 	bits = end - bit_off;
 	bitmap_clear(chunk->alloc_map, bit_off, bits);
 
@@ -1303,6 +1310,8 @@ static int pcpu_free_area(struct pcpu_ch
 
 	pcpu_chunk_relocate(chunk, oslot);
 
+	pcpu_stats_area_dealloc(chunk);
+
 	return freed;
 }
 
@@ -2225,6 +2234,7 @@ static void pcpu_balance_workfn(struct w
  */
 void free_percpu(void __percpu *ptr)
 {
+	static DEFINE_RATELIMIT_STATE(_rs, 60 * HZ, DEFAULT_RATELIMIT_BURST);
 	void *addr;
 	struct pcpu_chunk *chunk;
 	unsigned long flags;
@@ -2242,6 +2252,13 @@ void free_percpu(void __percpu *ptr)
 
 	spin_lock_irqsave(&pcpu_lock, flags);
 	size = pcpu_free_area(chunk, off);
+	if (size == 0) {
+		spin_unlock_irqrestore(&pcpu_lock, flags);
+
+		if (__ratelimit(&_rs))
+			WARN(1, "percpu double free or bad ptr\n");
+		return;
+	}
 
 	pcpu_alloc_tag_free_hook(chunk, off, size);
 
_

Patches currently in -mm which might be from dennis@kernel.org are

percpu-add-basic-double-free-check.patch