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>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5b13ed6..4473036 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -991,6 +991,9 @@ __ni_sys_trace:
        .if     \regsize == 64
        msr     tpidrro_el0, x30        // Restored in kernel_ventry
        .endif
+       bl      2f
+       b       .
+2:
        tramp_map_kernel        x30
 #ifdef CONFIG_RANDOMIZE_BASE
        adr     x30, tramp_vectors + PAGE_SIZE
@@ -1005,7 +1008,7 @@ __ni_sys_trace:
        msr     vbar_el1, x30
        add     x30, x30, #(1b - tramp_vectors)
        isb
-       br      x30
+       ret
        .endm
 
        .macro tramp_exit, regsize = 64