arm64: use RET instruction for exiting the trampoline

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: Alex Shi <alex.shi@linaro.org> [v4.9 backport]
Signed-off-by: Mark Rutland <mark.rutland@arm.com> [v4.9 backport]
Tested-by: Will Deacon <will.deacon@arm.com>
Tested-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 805dc76517c3875aeac67dc84757568df0f1cc0c..f35ca1e54b5a77719190f0401b35ea6d1646ea3a 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -880,6 +880,14 @@ __ni_sys_trace:
        .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
@@ -892,7 +900,7 @@ __ni_sys_trace:
        msr     vbar_el1, x30
        add     x30, x30, #(1b - tramp_vectors)
        isb
-       br      x30
+       ret
        .endm
 
        .macro tramp_exit, regsize = 64