Yup. E.g. years ago Huawei tried to merge something in the kernel that had a glaring security hole, many speculated that it was a deliberate attempt to add an exploit to the kernel.

It was immediately spotted before it even got close to being merged, and of course it got rejected.

The likes of Google, Microsoft, RedHat/IBM, Intel, AMD, Chronos group, etc are always investigating what other companies are trying to implement into the kernel. They obviously won’t stand for any dodgy stuff from another company being injected into the kernel.

Everything is highly scrutinised, not just by the kernel maintainers, but also by the contributors.