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: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 68a65ce7039aa247b911e00ed5c743db35167781)

CVE-2017-5753
CVE-2017-5715
CVE-2017-5754

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index d79bc9c5c500583f7fbe1d3c5236aac481cf8466..c12db966ab85aa03897ef3a947f85c394cd0f95d 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -983,6 +983,14 @@ 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
@@ -995,7 +1003,7 @@ alternative_insn isb, nop, ARM64_WORKAROUND_QCOM_FALKOR_E1003
        msr     vbar_el1, x30
        add     x30, x30, #(1b - tramp_vectors)
        isb
-       br      x30
+       ret
        .endm
 
        .macro tramp_exit, regsize = 64