xen-netback: fix debugfs write length check

Enlarge buffer size and check input length properly, so that we don't
misuse -ENOSPC.

Note that command like "kickXXXX" is still allowed, that's one patch for
another day if we really want to be very strict on this.

Reported-by: SeeChen Ng <seechen81@gmail.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Zoltan Kiss <zoltan.kiss@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index 580517d857bf..4c9041e60e7d 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -116,6 +116,7 @@ static int xenvif_read_io_ring(struct seq_file *m, void *v)
 }
 
 #define XENVIF_KICK_STR "kick"
+#define BUFFER_SIZE     32
 
 static ssize_t
 xenvif_write_io_ring(struct file *filp, const char __user *buf, size_t count,
@@ -124,22 +125,24 @@ xenvif_write_io_ring(struct file *filp, const char __user *buf, size_t count,
         struct xenvif_queue *queue =
                 ((struct seq_file *)filp->private_data)->private;
         int len;
-        char write[sizeof(XENVIF_KICK_STR)];
+        char write[BUFFER_SIZE];
 
         /* don't allow partial writes and check the length */
         if (*ppos != 0)
                 return 0;
-        if (count < sizeof(XENVIF_KICK_STR) - 1)
+        if (count >= sizeof(write))
                 return -ENOSPC;
 
         len = simple_write_to_buffer(write,
-                                     sizeof(write),
+                                     sizeof(write) - 1,
                                      ppos,
                                      buf,
                                      count);
         if (len < 0)
                 return len;
 
+        write[len] = '\0';
+
         if (!strncmp(write, XENVIF_KICK_STR, sizeof(XENVIF_KICK_STR) - 1))
                 xenvif_interrupt(0, (void *)queue);
         else {