arm64: split thread_info from task stack

This patch moves arm64's struct thread_info from the task stack into
task_struct. This protects thread_info from corruption in the case of
stack overflows, and makes its address harder to determine if stack
addresses are leaked, making a number of attacks more difficult. Precise
detection and handling of overflow is left for subsequent patches.

Largely, this involves changing code to store the task_struct in sp_el0,
and acquire the thread_info from the task struct. Core code now
implements current_thread_info(), and as noted in <linux/sched.h> this
relies on offsetof(task_struct, thread_info) == 0, enforced by core
code.

This change means that the 'tsk' register used in entry.S now points to
a task_struct, rather than a thread_info as it used to. To make this
clear, the TI_* field offsets are renamed to TSK_TI_*, with asm-offsets
appropriately updated to account for the structural change.

Userspace clobbers sp_el0, and we can no longer restore this from the
stack. Instead, the current task is cached in a per-cpu variable that we
can safely access from early assembly as interrupts are disabled (and we
are thus not preemptible).

Both secondary entry and idle are updated to stash the sp and task
pointer separately.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 332e33193ccf..eaafb253bbfa 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -428,7 +428,8 @@ ENDPROC(__create_page_tables)
 __primary_switched:
 	adrp	x4, init_thread_union
 	add	sp, x4, #THREAD_SIZE
-	msr	sp_el0, x4			// Save thread_info
+	adr_l	x5, init_task
+	msr	sp_el0, x5			// Save thread_info
 
 	adr_l	x8, vectors			// load VBAR_EL1 with virtual
 	msr	vbar_el1, x8			// vector table address
@@ -699,10 +700,10 @@ __secondary_switched:
 	isb
 
 	adr_l	x0, secondary_data
-	ldr	x0, [x0, #CPU_BOOT_STACK]	// get secondary_data.stack
-	mov	sp, x0
-	and	x0, x0, #~(THREAD_SIZE - 1)
-	msr	sp_el0, x0			// save thread_info
+	ldr	x1, [x0, #CPU_BOOT_STACK]	// get secondary_data.stack
+	mov	sp, x1
+	ldr	x2, [x0, #CPU_BOOT_TASK]
+	msr	sp_el0, x2
 	mov	x29, #0
 	b	secondary_start_kernel
 ENDPROC(__secondary_switched)