[PATCH v7 4/4] arm64/syscalls: Architecture specific pre-usermode check

[Thomas Garnier]

Disable the generic pre-usermode check in favor of an optimized
implementation. This patch adds specific checks on user-mode return path
to make it faster and smaller.

The address limit is checked on each syscall return path to user-mode.
If it was changed, a generic handler is called to stop the kernel on an
explicit check.

Signed-off-by: Thomas Garnier <thgarnie at google.com>
---
Based on next-20170410
---
 arch/arm64/Kconfig        |  1 +
 arch/arm64/kernel/entry.S | 13 +++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 9b8fcab7da56..3f9e8e7d9376 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -24,6 +24,7 @@ config ARM64
 	select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
 	select ARCH_WANT_FRAME_POINTERS
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
+	select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
 	select ARM_AMBA
 	select ARM_ARCH_TIMER
 	select ARM_GIC
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 43512d4d7df2..bdd094c7837a 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -744,6 +744,8 @@ ENDPROC(cpu_switch_to)
 ret_fast_syscall:
 	disable_irq				// disable interrupts
 	str	x0, [sp, #S_X0]			// returned x0
+	ldr	x2, [tsk, #TSK_TI_ADDR_LIMIT]	// check addr limit change
+	tbnz	x2, #63, addr_limit_fail
 	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for syscall tracing
 	and	x2, x1, #_TIF_SYSCALL_WORK
 	cbnz	x2, ret_fast_syscall_trace
@@ -771,6 +773,9 @@ work_pending:
  */
 ret_to_user:
 	disable_irq				// disable interrupts
+	ldr	x2, [tsk, #TSK_TI_ADDR_LIMIT]	// check addr limit change
+	tbnz	x2, #63, addr_limit_fail
+
 	ldr	x1, [tsk, #TSK_TI_FLAGS]
 	and	x2, x1, #_TIF_WORK_MASK
 	cbnz	x2, work_pending
@@ -780,6 +785,14 @@ finish_ret_to_user:
 ENDPROC(ret_to_user)
 
 /*
+ * Address limit was incorrect before returning in user-mode which could be
+ * used to elevate privileges. Call the generic handler to stop the kernel on
+ * the appropriate check. This function does not return.
+ */
+addr_limit_fail:
+	bl address_limit_check_failed
+
+/*
  * This is how we return from a fork.
  */
 ENTRY(ret_from_fork)
-- 
2.12.2.715.g7642488e1d-goog