arm64: ssbd: Introduce thread flag to control userspace mitigation

In order to allow userspace to be mitigated on demand, let's
introduce a new thread flag that prevents the mitigation from
being turned off when exiting to userspace, and doesn't turn
it on on entry into the kernel (with the assumption that the
mitigation is always enabled in the kernel itself).

This will be used by a prctl interface introduced in a later
patch.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 740aa03..cbcf11b 100644 (file)
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -94,6 +94,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 #define TIF_32BIT              22      /* 32bit process */
 #define TIF_SVE                        23      /* Scalable Vector Extension in use */
 #define TIF_SVE_VL_INHERIT     24      /* Inherit sve_vl_onexec across exec */
+#define TIF_SSBD               25      /* Wants SSB mitigation */
 
 #define _TIF_SIGPENDING                (1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED      (1 << TIF_NEED_RESCHED)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index e6f6e23..28ad879 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -147,6 +147,8 @@ alternative_cb      arm64_enable_wa2_handling
 alternative_cb_end
        ldr_this_cpu    \tmp2, arm64_ssbd_callback_required, \tmp1
        cbz     \tmp2, \targ
+       ldr     \tmp2, [tsk, #TSK_TI_FLAGS]
+       tbnz    \tmp2, #TIF_SSBD, \targ
        mov     w0, #ARM_SMCCC_ARCH_WORKAROUND_2
        mov     w1, #\state
 alternative_cb arm64_update_smccc_conduit