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").