+ mm-page_counter-reject-empty-string-in-page_counter_memparse.patch added to mm-new branch

+ mm-page_counter-reject-empty-string-in-page_counter_memparse.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: page_counter: reject empty string in page_counter_memparse()
has been added to the -mm mm-new branch.  Its filename is
     mm-page_counter-reject-empty-string-in-page_counter_memparse.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_counter-reject-empty-string-in-page_counter_memparse.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.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

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and is updated there most days

------------------------------------------------------
From: Tao Cui <cuitao@kylinos.cn>
Subject: mm: page_counter: reject empty string in page_counter_memparse()
Date: Mon, 17 Aug 2026 12:26:52 +0800

memparse() consumes no characters on an empty input and leaves the end
pointer at the terminating NUL.  The only validation in
page_counter_memparse() checks for trailing characters, so an empty input
slips through and the limit becomes 0.

All limit write callbacks of the memory controller strstrip() the input
before calling this helper, so a script that writes an unset variable hits
this path:

  LIMIT=
  echo "$LIMIT" > $CG/memory.max
  echo $?
  0
  cat $CG/memory.max
  0

Nothing reports the mistake: the limit is now 0 and the OOM killer goes
after every task in the cgroup.  The same happens for memory.min,
memory.low, memory.high, memory.swap.high, memory.swap.max and
memory.zswap.max, where 0 silently removes the protection or disables swap
and zswap.

Reject the input when no characters were consumed, which is the one case
the trailing-character check cannot catch.

Link: https://lore.kernel.org/20260817042652.74136-1-cui.tao@linux.dev
Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_counter.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/page_counter.c~mm-page_counter-reject-empty-string-in-page_counter_memparse
+++ a/mm/page_counter.c
@@ -281,7 +281,7 @@ int page_counter_memparse(const char *bu
 	}
 
 	bytes = memparse(buf, &end);
-	if (*end != '\0')
+	if (*end != '\0' || end == buf)
 		return -EINVAL;
 
 	*nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);
_

Patches currently in -mm which might be from cuitao@kylinos.cn are

docs-cgroup-document-empty-write-behavior-of-memory-limit-knobs.patch
mm-page_counter-reject-empty-string-in-page_counter_memparse.patch