Re: [PATCH net] af_netlink: Fix shift out of bounds in group mask calculation

[Ido Schimmel <idosch@idosch.org>]

On Thu, Mar 17, 2022 at 03:53:06PM +0100, Petr Machata wrote:
> When a netlink message is received, netlink_recvmsg() fills in the address
> of the sender. One of the fields is the 32-bit bitfield nl_groups, which
> carries the multicast group on which the message was received. The least
> significant bit corresponds to group 1, and therefore the highest group
> that the field can represent is 32. Above that, the UB sanitizer flags the
> out-of-bounds shift attempts.
> 
> Which bits end up being set in such case is implementation defined, but
> it's either going to be a wrong non-zero value, or zero, which is at least
> not misleading. Make the latter choice deterministic by always setting to 0
> for higher-numbered multicast groups.
> 
> To get information about membership in groups >= 32, userspace is expected
> to use nl_pktinfo control messages[0], which are enabled by NETLINK_PKTINFO
> socket option.
> [0] https://lwn.net/Articles/147608/
> 
> The way to trigger this issue is e.g. through monitoring the BRVLAN group:
> 
> 	# bridge monitor vlan &
> 	# ip link add name br type bridge
> 
> Which produces the following citation:
> 
> 	UBSAN: shift-out-of-bounds in net/netlink/af_netlink.c:162:19
> 	shift exponent 32 is too large for 32-bit type 'int'
> 
> Fixes: f7fa9b10edbb ("[NETLINK]: Support dynamic number of multicast groups per netlink family")
> Signed-off-by: Petr Machata <petrm@nvidia.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>