CVE-2024-36937

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.