ipv4: ARP neigh procfs buffer overflow

If arp_format_neigh_entry() can be called with n->dev->addr_len == 0, then a
write to hbuffer[-1] occurs.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index c29d75d8f1b1..090e9991ac2a 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -1304,7 +1304,9 @@ static void arp_format_neigh_entry(struct seq_file *seq,
                 hbuffer[k++] = hex_asc_lo(n->ha[j]);
                 hbuffer[k++] = ':';
         }
-        hbuffer[--k] = 0;
+        if (k != 0)
+                --k;
+        hbuffer[k] = 0;
 #if defined(CONFIG_AX25) || defined(CONFIG_AX25_MODULE)
         }
 #endif