[PATCH 3.16 046/148] blk-mq: avoid sysfs buffer overflow with too many CPU cores

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ming Lei <ming.lei@xxxxxxxxxx>

commit 8962842ca5abdcf98e22ab3b2b45a103f0408b95 upstream.

It is reported that sysfs buffer overflow can be triggered if the system
has too many CPU cores(>841 on 4K PAGE_SIZE) when showing CPUs of
hctx via /sys/block/$DEV/mq/$N/cpu_list.

Use snprintf to avoid the potential buffer overflow.

This version doesn't change the attribute format, and simply stops
showing CPU numbers if the buffer is going to overflow.

Fixes: 676141e48af7("blk-mq: don't dump CPU -> hw queue map on driver load")
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 block/blk-mq-sysfs.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -226,20 +226,25 @@ static ssize_t blk_mq_hw_sysfs_active_sh
 
 static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
 {
+	const size_t size = PAGE_SIZE - 1;
 	unsigned int i, first = 1;
-	ssize_t ret = 0;
+	int ret = 0, pos = 0;
 
 	for_each_cpu(i, hctx->cpumask) {
 		if (first)
-			ret += sprintf(ret + page, "%u", i);
+			ret = snprintf(pos + page, size - pos, "%u", i);
 		else
-			ret += sprintf(ret + page, ", %u", i);
+			ret = snprintf(pos + page, size - pos, ", %u", i);
+
+		if (ret >= size - pos)
+			break;
 
 		first = 0;
+		pos += ret;
 	}
 
-	ret += sprintf(ret + page, "\n");
-	return ret;
+	ret = snprintf(pos + page, size - pos, "\n");
+	return pos + ret;
 }
 
 static struct blk_mq_ctx_sysfs_entry blk_mq_sysfs_dispatched = {