arm64: use RET instruction for exiting the trampoline

Speculation attacks against the entry trampoline can potentially resteer
the speculative instruction stream through the indirect branch and into
arbitrary gadgets within the kernel.

This patch defends against these attacks by forcing a misprediction
through the return stack: a dummy BL instruction loads an entry into
the stack, so that the predicted program flow of the subsequent RET
instruction is to a branch-to-self instruction which is finally resolved
as a branch to the kernel vectors with speculation suppressed.

Change-Id: I79a6fe1e02f68860aac1b59508c565b8273825ec
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Git-commit: 75359368
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>