[PATCH net-next] fs: nfsd: Fix buffer overflow in write_pool_threads()

[David Laight <david.laight.linux@gmail.com>]

write_pool_threads() writes the number of threads in each pool into a
caller-supplied 'almost PAGE_SIZE' buffer.
If there are enough pools to overflow the buffer the code continues
writing beynd its end.

Fix the overflow check so that it actually works.

Fixes: eed2965af1bae "knfsd: allow admin to set nthreads per node"
Signed-off-by: David Laight <david.laight.linux@gmail.com>
---

I'm pretty sure this is 'root only' code.
So you'd have to try very hard to actually get the overflow.

 fs/nfsd/nfsctl.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 39e7012a60d8..b74048aa2402 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -483,8 +483,7 @@ static ssize_t write_pool_threads(struct file *file, char *buf, size_t size)
 		 * file, sorry.  Report zero threads.
 		 */
 		mutex_unlock(&nfsd_mutex);
-		strcpy(buf, "0\n");
-		return strlen(buf);
+		return strscpy(buf, "0\n", SIMPLE_TRANSACTION_LIMIT);
 	}
 
 	nthreads = kzalloc_objs(int, npools);
@@ -523,13 +522,14 @@ static ssize_t write_pool_threads(struct file *file, char *buf, size_t size)
 
 	mesg = buf;
 	size = SIMPLE_TRANSACTION_LIMIT;
-	for (i = 0; i < npools && size > 0; i++) {
-		snprintf(mesg, size, "%d%c", nthreads[i], (i == npools-1 ? '\n' : ' '));
-		len = strlen(mesg);
+	for (i = 0; i < npools; i++) {
+		len = scnprintf(mesg, size, "%d ", nthreads[i]);
 		size -= len;
 		mesg += len;
 	}
 	rv = mesg - buf;
+	if (rv != SIMPLE_TRANSACTION_LIMIT - 1)
+		msg[-1] = '\n';
 out_free:
 	kfree(nthreads);
 	mutex_unlock(&nfsd_mutex);
-- 
2.39.5