[PATCH net] fib_trie: correct /proc/net/route for large read buffer

[Jason Baron <jbaron@akamai.com>]

From: Jason Baron <jbaron@akamai.com>

When read() is called on /proc/net/route requesting a size that is one
entry size (128 bytes) less than m->size or greater, the resulting output
has missing and/or duplicate entries. Since m->size is typically PAGE_SIZE,
for a PAGE_SIZE of 4,096 this means that reads requesting more than 3,968
bytes will see bogus output.

For example:

for i in {100..200}; do
	ip route add 192.168.1.$i dev eth0
done
dd if=/proc/net/route of=/tmp/good bs=1024
dd if=/proc/net/route of=/tmp/bad bs=4096

# diff -q /tmp/good /tmp/bad
Files /tmp/good and /tmp/bad differ

I think this has gone unnoticed, since the output of 'netstat -r' and
'route' is generated by reading in 1,024 byte increments and thus not
corrupted. Further, the number of entries in the route table needs to be
sufficiently large in order to trigger the problematic case.

The issue arises because fib_route_get_idx() does not properly handle
the case where pos equals iter->pos. This case only arises when we have
a large read buffer size because we end up re-requesting the last entry
that overflowed m->buf. In the case of a smaller read buffer size,
we don't exceed the size of m->buf, and thus fib_route_get_idx() is called
with pos greater than iter->pos.

Fix by properly handling the iter->pos == pos case.

Fixes: 25b97c016b26 ("ipv4: off-by-one in continuation handling in /proc/net/route")
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jason Baron <jbaron@akamai.com>
---
 net/ipv4/fib_trie.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 31cef3602585..1017533fc75c 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -2411,12 +2411,17 @@ static struct key_vector *fib_route_get_idx(struct fib_route_iter *iter,
 					    loff_t pos)
 {
 	struct key_vector *l, **tp = &iter->tnode;
+	loff_t saved_pos = 0;
 	t_key key;
 
 	/* use cache location of next-to-find key */
 	if (iter->pos > 0 && pos >= iter->pos) {
 		pos -= iter->pos;
 		key = iter->key;
+		if (pos == 0) {
+			saved_pos = iter->pos;
+			key--;
+		}
 	} else {
 		iter->pos = 0;
 		key = 0;
@@ -2436,10 +2441,13 @@ static struct key_vector *fib_route_get_idx(struct fib_route_iter *iter,
 			break;
 	}
 
-	if (l)
+	if (l) {
 		iter->key = key;	/* remember it */
-	else
+		if (saved_pos)
+			iter->pos = saved_pos;
+	} else {
 		iter->pos = 0;		/* forget it */
+	}
 
 	return l;
 }
-- 
2.6.1