[kernel-hardening] [PATCH v8 4/4] arm64/syscalls: Optimize address limit check

Thomas Garnier
2017-04-26T18:34:25Z

Disable the generic address limit check in favor of an architecture
specific optimized implementation.

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@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
---
Based on next-20170426
---
arch/arm64/Kconfig | 1 +
arch/arm64/kernel/entry.S | 12 ++++++++++++
2 files changed, 13 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 3dcd7ec69bca..fe9466d3bf94 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -25,6 +25,7 @@ config ARM64
select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
select ARCH_WANT_FRAME_POINTERS
select ARCH_HAS_UBSAN_SANITIZE_ALL
+ select ADDR_LIMIT_CHECK
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..c895c4402d32 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 +773,8 @@ 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 +784,14 @@ 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:
+ b addr_limit_check_failed
+
+/*
* This is how we return from a fork.
*/
ENTRY(ret_from_fork)
--
2.13.0.rc0.306.g87b477812d-goog