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    Subject[PATCH 4.15 033/202] [Variant 3/Meltdown] arm64: use RET instruction for exiting the trampoline
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Will Deacon <will.deacon@arm.com>


    Commit be04a6d1126b upstream.

    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: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm64/kernel/entry.S | 10 +++++++++-
    1 file changed, 9 insertions(+), 1 deletion(-)

    --- a/arch/arm64/kernel/entry.S
    +++ b/arch/arm64/kernel/entry.S
    @@ -1029,6 +1029,14 @@ alternative_else_nop_endif
    .if \regsize == 64
    msr tpidrro_el0, x30 // Restored in kernel_ventry
    .endif
    + /*
    + * Defend against branch aliasing attacks by pushing a dummy
    + * entry onto the return stack and using a RET instruction to
    + * enter the full-fat kernel vectors.
    + */
    + bl 2f
    + b .
    +2:
    tramp_map_kernel x30
    #ifdef CONFIG_RANDOMIZE_BASE
    adr x30, tramp_vectors + PAGE_SIZE
    @@ -1041,7 +1049,7 @@ alternative_insn isb, nop, ARM64_WORKARO
    msr vbar_el1, x30
    add x30, x30, #(1b - tramp_vectors)
    isb
    - br x30
    + ret
    .endm

    .macro tramp_exit, regsize = 64

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