[PATCH v2 1/2] mm/page_counter: avoid integer overflow in effective_protection()

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

effective_protection() scales a parent's protection by a ratio of page
counts, e.g. for recursive protection:

	(parent_effective - siblings_protected) * (usage - protected)
		/ (parent_usage - siblings_protected)

The multiply is done at unsigned long width before dividing. On systems
with >= 16TB RAM the product can exceed 2^64 and wrap, giving a bogus
protection value and silently breaking memory.min/low enforcement.

Use mul_u64_u64_div_u64() to multiply in a 128-bit intermediate. Because
usage and parent_usage are not read atomically (a child is charged
before its parent), usage - protected can briefly exceed the divisor,
making the quotient overflow 64 bits and trap (#DE on x86). Cap it so
the ratio stays <= 1.

Reported by the sashiko review tool [1].

[1] https://sashiko.dev/#/patchset/20260826133054.88529-1-ridong.chen@linux.dev?part=1

Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 mm/page_counter.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 661e0f2a5127..e8bd512069c5 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -8,6 +8,7 @@
 #include <linux/page_counter.h>
 #include <linux/atomic.h>
 #include <linux/kernel.h>
+#include <linux/math64.h>
 #include <linux/string.h>
 #include <linux/sched.h>
 #include <linux/bug.h>
@@ -356,7 +357,8 @@ static unsigned long effective_protection(unsigned long usage,
 	 * otherwise get a smaller chunk than what they claimed.
 	 */
 	if (siblings_protected > parent_effective)
-		return protected * parent_effective / siblings_protected;
+		return mul_u64_u64_div_u64(protected, parent_effective,
+					   siblings_protected);
 
 	/*
 	 * Ok, utilized protection of all children is within what the
@@ -397,13 +399,18 @@ static unsigned long effective_protection(unsigned long usage,
 	if (parent_effective > siblings_protected &&
 	    parent_usage > siblings_protected &&
 	    usage > protected) {
-		unsigned long unclaimed;
+		unsigned long unclaimed = parent_effective - siblings_protected;
+		unsigned long unprotected = usage - protected;
+		unsigned long parent_unprotected = parent_usage - siblings_protected;
 
-		unclaimed = parent_effective - siblings_protected;
-		unclaimed *= usage - protected;
-		unclaimed /= parent_usage - siblings_protected;
+		/*
+		 * The usages aren't read atomically, so a child can transiently
+		 * appear to use more than its parent, making the ratio exceed 1
+		 * and the quotient overflow 64 bits (#DE on x86).  Cap it.
+		 */
+		unprotected = min(unprotected, parent_unprotected);
 
-		ep += unclaimed;
+		ep += mul_u64_u64_div_u64(unclaimed, unprotected, parent_unprotected);
 	}
 
 	return ep;
-- 
2.34.1