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    Subject[PATCH 5.10 368/599] af_netlink: Fix shift out of bounds in group mask calculation
    Date
    From: Petr Machata <petrm@nvidia.com>

    [ Upstream commit 0caf6d9922192dd1afa8dc2131abfb4df1443b9f ]

    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>
    Link: https://lore.kernel.org/r/2bef6aabf201d1fc16cca139a744700cff9dcb04.1647527635.git.petrm@nvidia.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/netlink/af_netlink.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
    index e55af5c078ac..f37916156ca5 100644
    --- a/net/netlink/af_netlink.c
    +++ b/net/netlink/af_netlink.c
    @@ -149,6 +149,8 @@ static const struct rhashtable_params netlink_rhashtable_params;

    static inline u32 netlink_group_mask(u32 group)
    {
    + if (group > 32)
    + return 0;
    return group ? 1 << (group - 1) : 0;
    }

    --
    2.34.1


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