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net: get rid of an signed integer overflow in ip_idents_reserve()

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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: 04ca6973f7 ("ip: make IP identifiers less predictable")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2016-09-20 18:06:17 -07:00 committed by David S. Miller
parent fba1296624
commit adb03115f4
1 changed files with 8 additions and 2 deletions
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10
net/ipv4/route.c
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@ -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);