drm/v3d: Reject empty multisync extension to prevent infinite loop

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

drm/v3d: Reject empty multisync extension to prevent infinite loop

v3d_get_extensions() walks a userspace-provided singly-linked list of

ioctl extensions without any bound on the chain length. A local user

can craft a self-referential extension (ext->next == &ext) with zero

in_sync_count and out_sync_count, which bypasses the existing duplicate-

extension guard:

if (se->in_sync_count || se->out_sync_count)

return -EINVAL;

The guard never fires because v3d_get_multisync_post_deps() returns

immediately when count is zero, leaving both fields at zero on every

iteration. The result is an infinite loop in kernel context, blocking

the calling thread and pegging a CPU core indefinitely.

Fix this by rejecting a multisync extension where both in_sync_count

and out_sync_count are zero in v3d_get_multisync_submit_deps(). An

empty multisync carries no synchronization information and serves no

useful purpose, so returning -EINVAL for such an extension is the

correct defense against this attack vector.