[PATCH] sched/core: check format and overflows in cgroup2 cpu.max

[Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>]

Cgroup2 interface for cpu bandwidth limit has some flaws:

- on stack buffer overflow
- no checks for valid format or trailing garbage
- no checks for integer overflows

This patch fixes all these flaws.

Fixes: 0d5936344f30 ("sched: Implement interface for cgroup unified hierarchy")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
---
 kernel/sched/core.c |   29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5f46aa335b28..123b1910bc2e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6947,19 +6947,34 @@ static int __maybe_unused cpu_period_quota_parse(char *buf,
 						 u64 *periodp, u64 *quotap)
 {
 	char tok[21];	/* U64_MAX */
+	int ret, len;
 
-	if (!sscanf(buf, "%s %llu", tok, periodp))
+	if (sscanf(buf, "%20s %n", tok, &len) != 1)
 		return -EINVAL;
 
-	*periodp *= NSEC_PER_USEC;
-
-	if (sscanf(tok, "%llu", quotap))
-		*quotap *= NSEC_PER_USEC;
-	else if (!strcmp(tok, "max"))
+	ret = kstrtou64(tok, 10, quotap);
+	if (ret) {
+		if (strcmp(tok, "max"))
+			return ret;
 		*quotap = RUNTIME_INF;
-	else
+	} else if (*quotap <= U64_MAX / NSEC_PER_USEC) {
+		*quotap *= NSEC_PER_USEC;
+	} else
 		return -EINVAL;
 
+	buf += len;
+	if (*buf) {
+		if (sscanf(buf, "%20s %n", tok, &len) != 1 || buf[len])
+			return -EINVAL;
+		ret = kstrtou64(tok, 10, periodp);
+		if (ret)
+			return ret;
+		if (*periodp > U64_MAX / NSEC_PER_USEC)
+			return -EINVAL;
+	}
+
+	*periodp *= NSEC_PER_USEC;
+
 	return 0;
 }