arm64: entry: Reword comment about post_ttbr_update_workaround

Commit f167211a93ac upstream.

We don't fully understand the Cavium ThunderX erratum, but it appears
that mapping the kernel as nG can lead to horrible consequences such as
attempting to execute userspace from kernel context. Since kpti isn't
enabled for these CPUs anyway, simplify the comment justifying the lack
of post_ttbr_update_workaround in the exception trampoline.

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 8703f27d7c5db70ffd793a813160f28e810cc1bc)

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 b2e5b78a5e3c79f307595ec3f3952ec2167d41bc..b5063036629d27b5bc1574d91132e0d83fe2f406 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -964,16 +964,9 @@ alternative_else_nop_endif
        orr     \tmp, \tmp, #USER_ASID_FLAG
        msr     ttbr1_el1, \tmp
        /*
-        * We avoid running the post_ttbr_update_workaround here because the
-        * user and kernel ASIDs don't have conflicting mappings, so any
-        * "blessing" as described in:
-        *
-        *   http://lkml.kernel.org/r/56BB848A.6060603@caviumnetworks.com
-        *
-        * will not hurt correctness. Whilst this may partially defeat the
-        * point of using split ASIDs in the first place, it avoids
-        * the hit of invalidating the entire I-cache on every return to
-        * userspace.
+        * We avoid running the post_ttbr_update_workaround here because
+        * it's only needed by Cavium ThunderX, which requires KPTI to be
+        * disabled.
         */
        .endm