From: Eric Dumazet <edumazet@...>

commit adb03115f4590baa280ddc440a8eff08a6be0cb7 upstream.

Jiri Pirko reported an UBSAN warning happening in ip_idents_reserve()

[] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
[] signed integer overflow:
[] -2117905507 + -695755206 cannot be represented in type 'int'

Since we do not have uatomic_add_return() yet, use atomic_cmpxchg()
so that the arithmetics can be done using unsigned int.

Fixes: 04ca6973f7c1 ("ip: make IP identifiers less predictable")
Signed-off-by: Eric Dumazet <edumazet@...>
Reported-by: Jiri Pirko <jiri@...>
Signed-off-by: David S. Miller <davem@...>
Signed-off-by: Ben Hutchings <ben.hutchings@...>
---
net/ipv4/route.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index b050cf980a57..a0270af75fa5 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -476,12 +476,18 @@ u32 ip_idents_reserve(u32 hash, int segs)
atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;
u32 old = ACCESS_ONCE(*p_tstamp);
u32 now = (u32)jiffies;
- u32 delta = 0;
+ u32 new, delta = 0;

if (old != now && cmpxchg(p_tstamp, old, now) == old)
delta = prandom_u32_max(now - old);

- return atomic_add_return(segs + delta, p_id) - segs;
+ /* Do not use atomic_add_return() as it makes UBSAN unhappy */
+ do {
+ old = (u32)atomic_read(p_id);
+ new = old + delta + segs;
+ } while (atomic_cmpxchg(p_id, old, new) != old);
+
+ return new - segs;
}
EXPORT_SYMBOL(ip_idents_reserve);

--
2.10.2



--
Ben Hutchings
Software Developer, Codethink Ltd.