[PATCH 4/4] ipv4: Silence intentional wrapping addition

[Kees Cook]

The overflow sanitizer quickly noticed what appears to have been an old
sore spot involving intended wrap around:

[   22.192362] ------------[ cut here ]------------
[   22.193329] UBSAN: signed-integer-overflow in ../arch/x86/include/asm/atomic.h:85:11
[   22.194844] 1469769800 + 1671667352 cannot be represented in type 'int'
[   22.195975] CPU: 2 PID: 2260 Comm: nmbd Not tainted 6.7.0 #1
[   22.196927] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
[   22.198231] Call Trace:
[   22.198641]  <TASK>
[   22.198641]  dump_stack_lvl+0x64/0x80
[   22.199533]  handle_overflow+0x152/0x1a0
[   22.200382]  __ip_select_ident+0xe3/0x100

Explicitly mark ip_select_ident() as performing wrapping signed
arithmetic. Update the passed type as a u32 since that is how it is used
(it is either u16 or a literal "1" in callers, but used with a wrapping
int, so it's actually a u32). Update the comment to mention annotation
instead of -fno-strict-overflow, which is no longer the issue.

Signed-off-by: Kees Cook <keescook at chromium.org>
---
Cc: Jakub Kicinski <kuba at kernel.org>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: David Ahern <dsahern at kernel.org>
Cc: Eric Dumazet <edumazet at google.com>
Cc: Paolo Abeni <pabeni at redhat.com>
Cc: netdev at vger.kernel.org
---
 include/net/ip.h |  4 ++--
 net/ipv4/route.c | 10 +++++-----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 25cb688bdc62..09d502a0ae30 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -537,10 +537,10 @@ void ip_dst_metrics_put(struct dst_entry *dst)
 		kfree(p);
 }
 
-void __ip_select_ident(struct net *net, struct iphdr *iph, int segs);
+void __ip_select_ident(struct net *net, struct iphdr *iph, u32 segs);
 
 static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb,
-					struct sock *sk, int segs)
+					struct sock *sk, u32 segs)
 {
 	struct iphdr *iph = ip_hdr(skb);
 
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index c8f76f56dc16..400e7a16fdba 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -458,7 +458,7 @@ static u32 *ip_tstamps __read_mostly;
  * if one generator is seldom used. This makes hard for an attacker
  * to infer how many packets were sent between two points in time.
  */
-static u32 ip_idents_reserve(u32 hash, int segs)
+static __signed_wrap u32 ip_idents_reserve(u32 hash, u32 segs)
 {
 	u32 bucket, old, now = (u32)jiffies;
 	atomic_t *p_id;
@@ -473,14 +473,14 @@ static u32 ip_idents_reserve(u32 hash, int segs)
 	if (old != now && cmpxchg(p_tstamp, old, now) == old)
 		delta = get_random_u32_below(now - old);
 
-	/* If UBSAN reports an error there, please make sure your compiler
-	 * supports -fno-strict-overflow before reporting it that was a bug
-	 * in UBSAN, and it has been fixed in GCC-8.
+	/* If UBSAN reports an error here, please make sure your arch's
+	 * atomic_add_return() implementation has been annotated with
+	 * __signed_wrap or uses wrapping_add() internally.
 	 */
 	return atomic_add_return(segs + delta, p_id) - segs;
 }
 
-void __ip_select_ident(struct net *net, struct iphdr *iph, int segs)
+void __ip_select_ident(struct net *net, struct iphdr *iph, u32 segs)
 {
 	u32 hash, id;
 
-- 
2.34.1