xdp: use flags field to disambiguate broadcast redirect

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

xdp: use flags field to disambiguate broadcast redirect

When redirecting a packet using XDP, the bpf_redirect_map() helper will set

up the redirect destination information in struct bpf_redirect_info (using

the __bpf_xdp_redirect_map() helper function), and the xdp_do_redirect()

function will read this information after the XDP program returns and pass

the frame on to the right redirect destination.

When using the BPF_F_BROADCAST flag to do multicast redirect to a whole

map, __bpf_xdp_redirect_map() sets the 'map' pointer in struct

bpf_redirect_info to point to the destination map to be broadcast. And

xdp_do_redirect() reacts to the value of this map pointer to decide whether

it's dealing with a broadcast or a single-value redirect. However, if the

destination map is being destroyed before xdp_do_redirect() is called, the

map pointer will be cleared out (by bpf_clear_redirect_map()) without

waiting for any XDP programs to stop running. This causes xdp_do_redirect()

to think that the redirect was to a single target, but the target pointer

is also NULL (since broadcast redirects don't have a single target), so

this causes a crash when a NULL pointer is passed to dev_map_enqueue().

To fix this, change xdp_do_redirect() to react directly to the presence of

the BPF_F_BROADCAST flag in the 'flags' value in struct bpf_redirect_info

to disambiguate between a single-target and a broadcast redirect. And only

read the 'map' pointer if the broadcast flag is set, aborting if that has

been cleared out in the meantime. This prevents the crash, while keeping

the atomic (cmpxchg-based) clearing of the map pointer itself, and without

adding any more checks in the non-broadcast fast path.