netdev - Re: [PATCH net-next v2 3/3] netlink: Fix wraparound of sk->sk_rmem

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Message-ID: <20250619061427.1202690-1-kuni1840@gmail.com>
Date: Wed, 18 Jun 2025 23:13:02 -0700
From: Kuniyuki Iwashima <kuni1840@...il.com>
To: jbaron@...mai.com
Cc: davem@...emloft.net,
	edumazet@...gle.com,
	horms@...nel.org,
	kuba@...nel.org,
	kuniyu@...gle.com,
	netdev@...r.kernel.org,
	pabeni@...hat.com
Subject: Re: [PATCH net-next v2 3/3] netlink: Fix wraparound of sk->sk_rmem_alloc

From: Jason Baron <jbaron@...mai.com>
Date: Wed, 18 Jun 2025 19:13:23 -0400
> For netlink sockets, when comparing allocated rmem memory with the
> rcvbuf limit, the comparison is done using signed values. This means
> that if rcvbuf is near INT_MAX, then sk->sk_rmem_alloc may become
> negative in the comparison with rcvbuf which will yield incorrect
> results.
> 
> This can be reproduced by using the program from SOCK_DIAG(7) with
> some slight modifications. First, setting sk->sk_rcvbuf to INT_MAX
> using SO_RCVBUFFORCE and then secondly running the "send_query()"
> in a loop while not calling "receive_responses()". In this case,
> the value of sk->sk_rmem_alloc will continuously wrap around
> and thus more memory is allocated than the sk->sk_rcvbuf limit.
> This will eventually fill all of memory leading to an out of memory
> condition with skbs filling up the slab.
> 
> Let's fix this in a similar manner to:
> commit 5a465a0da13e ("udp: Fix multiple wraparounds of sk->sk_rmem_alloc.")
> 
> As noted in that fix, if there are multiple threads writing to a
> netlink socket it's possible to slightly exceed rcvbuf value. But as
> noted this avoids an expensive 'atomic_add_return()' for the common
> case.

This was because UDP RX path is the fast path, but netlink isn't.
Also, it's common for UDP that multiple packets for the same socket
are processed concurrently, and 850cbaddb52d dropped lock_sock from
the path.