udp: Fix multiple wraparounds of sk->sk_rmem_alloc.

In the Linux kernel, the following vulnerability has been resolved:

udp: Fix multiple wraparounds of sk->sk_rmem_alloc.

__udp_enqueue_schedule_skb() has the following condition:

if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
goto drop;

sk->sk_rcvbuf is initialised by net.core.rmem_default and later can
be configured by SO_RCVBUF, which is limited by net.core.rmem_max,
or SO_RCVBUFFORCE.

If we set INT_MAX to sk->sk_rcvbuf, the condition is always false
as sk->sk_rmem_alloc is also signed int.

Then, the size of the incoming skb is added to sk->sk_rmem_alloc
unconditionally.

This results in integer overflow (possibly multiple times) on
sk->sk_rmem_alloc and allows a single socket to have skb up to
net.core.udp_mem[1].

For example, if we set a large value to udp_mem[1] and INT_MAX to
sk->sk_rcvbuf and flood packets to the socket, we can see multiple
overflows:

# cat /proc/net/sockstat | grep UDP:
UDP: inuse 3 mem 7956736 <-- (7956736 << 12) bytes > INT_MAX * 15
^- PAGE_SHIFT
# ss -uam
State Recv-Q ...
UNCONN -1757018048 ... <-- flipping the sign repeatedly
skmem:(r2537949248,rb2147483646,t0,tb212992,f1984,w0,o0,bl0,d0)

Previously, we had a boundary check for INT_MAX, which was removed by
commit 6a1f12dd85a8 ("udp: relax atomic operation on sk->sk_rmem_alloc").

A complete fix would be to revert it and cap the right operand by
INT_MAX:

rmem = atomic_add_return(size, &sk->sk_rmem_alloc);
if (rmem > min(size + (unsigned int)sk->sk_rcvbuf, INT_MAX))
goto uncharge_drop;

but we do not want to add the expensive atomic_add_return() back just
for the corner case.

Casting rmem to unsigned int prevents multiple wraparounds, but we still
allow a single wraparound.

# cat /proc/net/sockstat | grep UDP:
UDP: inuse 3 mem 524288 <-- (INT_MAX + 1) >> 12

# ss -uam
State Recv-Q ...
UNCONN -2147482816 ... <-- INT_MAX + 831 bytes
skmem:(r2147484480,rb2147483646,t0,tb212992,f3264,w0,o0,bl0,d14468947)

So, let's define rmem and rcvbuf as unsigned int and check skb->truesize
only when rcvbuf is large enough to lower the overflow possibility.

Note that we still have a small chance to see overflow if multiple skbs
to the same socket are processed on different core at the same time and
each size does not exceed the limit but the total size does.

Note also that we must ignore skb->truesize for a small buffer as
explained in commit 363dc73acacb ("udp: be less conservative with
sock rmem accounting").