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.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ief67c5d2c48cbc8cf8506a79e396d0d36df2fd51
Git-Commit: 5baaf50e
Git-repo: git://android.googlesource.com/kernel/common.git


Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>