[PATCH] blk-throttle: verify format of bandwidth limit and detect overflows

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Unlike to memory cgroup blkio throttler does not support value suffixes.

It silently ignores everything after last digit. For example this command
will set rate limit 1 byte per second rather than 1 megabyte per second:

# echo "7:0 1M" > blkio.throttle.read_bps_device
# cat blkio.throttle.read_bps_device
7:0 1

Cgroup2 interface has the same flaw:

# echo "7:0 rbps=1M" > io.max
# cat io.max
7:0 rbps=1 wbps=max riops=max wiops=max

Also sscanf does not care much about overflows.

This patch uses modern function kstrtou64 for parsing.
It rejects trailing garbage and detects integer overflows.

Also this patch handles iops limit overflows for cgroup-v1 in the same as
cgroup-v2: limits >= UINT_MAX becomes unlimited.

Fixes: 2ee867dcfa2e ("blkcg: implement interface for the unified hierarchy")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 block/blk-throttle.c |   40 ++++++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 1b97a73d2fb1..ce00060b3a48 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1433,8 +1433,10 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	if (ret)
 		return ret;
 
-	ret = -EINVAL;
-	if (sscanf(ctx.body, "%llu", &v) != 1)
+	/* remove trailing spaces, kstrto* are strict about them */
+	ctx.body = strim(ctx.body);
+	ret = kstrtou64(ctx.body, 10, &v);
+	if (ret)
 		goto out_finish;
 	if (!v)
 		v = U64_MAX;
@@ -1444,7 +1446,8 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	if (is_u64)
 		*(u64 *)((void *)tg + of_cft(of)->private) = v;
 	else
-		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
+		*(unsigned int *)((void *)tg + of_cft(of)->private) =
+							min_t(u64, v, UINT_MAX);
 
 	tg_conf_updated(tg, false);
 	ret = 0;
@@ -1609,23 +1612,25 @@ static ssize_t tg_set_limit(struct kernfs_open_file *of,
 	idle_time = tg->idletime_threshold_conf;
 	latency_time = tg->latency_target_conf;
 	while (true) {
-		char tok[27];	/* wiops=18446744073709551616 */
-		char *p;
-		u64 val = U64_MAX;
-		int len;
-
-		if (sscanf(ctx.body, "%26s%n", tok, &len) != 1)
-			break;
-		if (tok[0] == '\0')
-			break;
-		ctx.body += len;
+		char tok[8];	/* "latency" */
+		char buf[21];	/* U64_MAX */
+		int end;
+		u64 val;
 
 		ret = -EINVAL;
-		p = tok;
-		strsep(&p, "=");
-		if (!p || (sscanf(p, "%llu", &val) != 1 && strcmp(p, "max")))
+		if (sscanf(ctx.body, "%7[^=]=%20s %n", tok, buf, &end) != 2)
 			goto out_finish;
 
+		/* skip this field and trailing spaces */
+		ctx.body += end;
+
+		ret = kstrtou64(buf, 10, &val);
+		if (ret) {
+			if (strcmp(buf, "max"))
+				goto out_finish;
+			val = U64_MAX;
+		}
+
 		ret = -ERANGE;
 		if (!val)
 			goto out_finish;
@@ -1645,6 +1650,9 @@ static ssize_t tg_set_limit(struct kernfs_open_file *of,
 			latency_time = val;
 		else
 			goto out_finish;
+
+		if (!*ctx.body)
+			break;
 	}
 
 	tg->bps_conf[READ][index] = v[0];