Commit adb03115 authored Sep 20, 2016 by Eric Dumazet Committed by David S. Miller Sep 22, 2016

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



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: 04ca6973 ("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>

parent fba12966

net/ipv4/route.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -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);

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