[PATCH 4.9 50/71] inet: use bigger hash table for IP ID generation

Sasha Levin
2021-06-28T15:21:27Z

From: Eric Dumazet <edumazet@google.com>

commit aa6dd211e4b1dde9d5dc25d699d35f789ae7eeba upstream.

In commit 73f156a6e8c1 ("inetpeer: get rid of ip_id_count")
I used a very small hash table that could be abused
by patient attackers to reveal sensitive information.

Switch to a dynamic sizing, depending on RAM size.

Typical big hosts will now use 128x more storage (2 MB)
to get a similar increase in security and reduction
of hash collisions.

As a bonus, use of alloc_large_system_hash() spreads
allocated memory among all NUMA nodes.

Fixes: 73f156a6e8c1 ("inetpeer: get rid of ip_id_count")
Reported-by: Amit Klein <aksecurity@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ipv4/route.c | 42 +++++++++++++++++++++++++++++-------------
1 file changed, 29 insertions(+), 13 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index e9aae4686536..5350e1b61c06 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -70,6 +70,7 @@
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/mm.h>
+#include <linux/bootmem.h>
#include <linux/string.h>
#include <linux/socket.h>
#include <linux/sockios.h>
@@ -463,8 +464,10 @@ static struct neighbour *ipv4_neigh_lookup(const struct dst_entry *dst,
return neigh_create(&arp_tbl, pkey, dev);
}

-#define IP_IDENTS_SZ 2048u
-
+/* Hash tables of size 2048..262144 depending on RAM size.
+ * Each bucket uses 8 bytes.
+ */
+static u32 ip_idents_mask __read_mostly;
static atomic_t *ip_idents __read_mostly;
static u32 *ip_tstamps __read_mostly;

@@ -474,12 +477,16 @@ static u32 *ip_tstamps __read_mostly;
*/
u32 ip_idents_reserve(u32 hash, int segs)
{
- u32 *p_tstamp = ip_tstamps + hash % IP_IDENTS_SZ;
- atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;
- u32 old = ACCESS_ONCE(*p_tstamp);
- u32 now = (u32)jiffies;
+ u32 bucket, old, now = (u32)jiffies;
+ atomic_t *p_id;
+ u32 *p_tstamp;
u32 delta = 0;

+ bucket = hash & ip_idents_mask;
+ p_tstamp = ip_tstamps + bucket;
+ p_id = ip_idents + bucket;
+ old = ACCESS_ONCE(*p_tstamp);
+
if (old != now && cmpxchg(p_tstamp, old, now) == old)
delta = prandom_u32_max(now - old);

@@ -2936,18 +2943,27 @@ struct ip_rt_acct __percpu *ip_rt_acct __read_mostly;

int __init ip_rt_init(void)
{
+ void *idents_hash;
int rc = 0;
int cpu;

- ip_idents = kmalloc(IP_IDENTS_SZ * sizeof(*ip_idents), GFP_KERNEL);
- if (!ip_idents)
- panic("IP: failed to allocate ip_idents\n");
+ /* For modern hosts, this will use 2 MB of memory */
+ idents_hash = alloc_large_system_hash("IP idents",
+ sizeof(*ip_idents) + sizeof(*ip_tstamps),
+ 0,
+ 16, /* one bucket per 64 KB */
+ 0,
+ NULL,
+ &ip_idents_mask,
+ 2048,
+ 256*1024);
+
+ ip_idents = idents_hash;

- prandom_bytes(ip_idents, IP_IDENTS_SZ * sizeof(*ip_idents));
+ prandom_bytes(ip_idents, (ip_idents_mask + 1) * sizeof(*ip_idents));

- ip_tstamps = kcalloc(IP_IDENTS_SZ, sizeof(*ip_tstamps), GFP_KERNEL);
- if (!ip_tstamps)
- panic("IP: failed to allocate ip_tstamps\n");
+ ip_tstamps = idents_hash + (ip_idents_mask + 1) * sizeof(*ip_idents);
+ memset(ip_tstamps, 0, (ip_idents_mask + 1) * sizeof(*ip_tstamps));

for_each_possible_cpu(cpu) {
struct uncached_list *ul = &per_cpu(rt_uncached_list, cpu);
--
2.30.2