+ mm-percpu-add-a-simple-double-free-check-for-per-cpu-memory.patch added to mm-new branch

+ mm-percpu-add-a-simple-double-free-check-for-per-cpu-memory.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/percpu: add a simple double-free check for per-CPU memory
has been added to the -mm mm-new branch.  Its filename is
     mm-percpu-add-a-simple-double-free-check-for-per-cpu-memory.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-percpu-add-a-simple-double-free-check-for-per-cpu-memory.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
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

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------------------------------------------------------
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: mm/percpu: add a simple double-free check for per-CPU memory
Date: Thu, 4 Sep 2025 16:35:14 +0200

The free path clears the allocation bits in pcpu_chunk::alloc_map.  A
simple double free check would be to check if the bits, which are about to
be cleared, are already cleared.

Check if the bit is already cleared. Issue a warning and abort free in
that case.

Link: https://lkml.kernel.org/r/20250904143514.Yk6Ap-jy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Christoph Lameter (Ampere) <cl@gentwo.org>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/percpu.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/mm/percpu.c~mm-percpu-add-a-simple-double-free-check-for-per-cpu-memory
+++ a/mm/percpu.c
@@ -1276,7 +1276,7 @@ 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 bit_off, bits, end, oslot, freed;
+	int bit_off, bits, end, oslot, freed, free_bit;
 
 	lockdep_assert_held(&pcpu_lock);
 	pcpu_stats_area_dealloc(chunk);
@@ -1289,6 +1289,11 @@ static int pcpu_free_area(struct pcpu_ch
 	end = find_next_bit(chunk->bound_map, pcpu_chunk_map_bits(chunk),
 			    bit_off + 1);
 	bits = end - bit_off;
+
+	free_bit = find_next_bit(chunk->alloc_map, end, bit_off);
+	if (WARN(free_bit != bit_off, "Trying to free already free memory"))
+		return 0;
+
 	bitmap_clear(chunk->alloc_map, bit_off, bits);
 
 	freed = bits * PCPU_MIN_ALLOC_SIZE;
_

Patches currently in -mm which might be from bigeasy@linutronix.de are

mm-percpu-add-a-simple-double-free-check-for-per-cpu-memory.patch