arm64: ssbd: Introduce thread flag to control userspace mitigation

commit 9dd9614f5476687abbff8d4b12cd08ae70d7c2ad upstream.

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>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index ddded64..fc786d3 100644 (file)
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -92,6 +92,7 @@ void arch_setup_new_exec(void);
 #define TIF_RESTORE_SIGMASK    20
 #define TIF_SINGLESTEP         21
 #define TIF_32BIT              22      /* 32bit process */
+#define TIF_SSBD               23      /* 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 5a58597..c1ffa95 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