BACKPORT: 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>

This is a modification of Mark Rutland's original patch. Guards to check
if CONFIG_THREAD_INFO_IN_TASK is used has been inserted. get_current()
for when CONFIG_THREAD_INFO_IN_TASK is not used has been added to
arch/arm64/include/asm/current.h.

Bug: 38331309
Change-Id: Ic5eae344a7c2baea0864f6ae16be1e9c60c0a74a
(cherry picked from commit c02433dd6de32f042cf3ffe476746b1115b8c096)
Signed-off-by: Zubin Mithra <zsm@google.com>

3 files changed, 44 insertions, 2 deletions

diff --git a/arch/arm64/include/asm/current.h b/arch/arm64/include/asm/current.h
new file mode 100644
index 000000000000..2e61d21294ba
--- /dev/null
+++ b/arch/arm64/include/asm/current.h
@@ -0,0 +1,27 @@
+#ifndef __ASM_CURRENT_H
+#define __ASM_CURRENT_H
+
+#include <linux/compiler.h>
+
+#include <asm/sysreg.h>
+
+#ifndef __ASSEMBLY__
+
+#ifdef CONFIG_THREAD_INFO_IN_TASK
+struct task_struct;
+
+static __always_inline struct task_struct *get_current(void)
+{
+	return (struct task_struct *)read_sysreg(sp_el0);
+}
+#define current get_current()
+#else
+#include <linux/thread_info.h>
+#define get_current() (current_thread_info()->task)
+#define current get_current()
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* __ASM_CURRENT_H */
+
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index d7e851c5bc42..a05033beb2a2 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -66,6 +66,9 @@ asmlinkage void secondary_start_kernel(void);
  */
 struct secondary_data {
 	void *stack;
+#ifdef CONFIG_THREAD_INFO_IN_TASK
+	struct task_struct *task;
+#endif
 };
 extern struct secondary_data secondary_data;
 extern void secondary_entry(void);
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 41e4fd78e345..ec4f8c04aeda 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -47,14 +47,25 @@ typedef unsigned long mm_segment_t;
 struct thread_info {
 	unsigned long		flags;		/* low level flags */
 	mm_segment_t		addr_limit;	/* address limit */
+#ifndef CONFIG_THREAD_INFO_IN_TASK
 	struct task_struct	*task;		/* main task structure */
+#endif
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 	u64			ttbr0;		/* saved TTBR0_EL1 */
 #endif
 	int			preempt_count;	/* 0 => preemptable, <0 => bug */
+#ifndef CONFIG_THREAD_INFO_IN_TASK
 	int			cpu;		/* cpu */
+#endif
 };
 
+#ifdef CONFIG_THREAD_INFO_IN_TASK
+#define INIT_THREAD_INFO(tsk)						\
+{									\
+	.preempt_count	= INIT_PREEMPT_COUNT,				\
+	.addr_limit	= KERNEL_DS,					\
+}
+#else
 #define INIT_THREAD_INFO(tsk)						\
 {									\
 	.task		= &tsk,						\
@@ -63,8 +74,6 @@ struct thread_info {
 	.addr_limit	= KERNEL_DS,					\
 }
 
-#define init_stack		(init_thread_union.stack)
-
 /*
  * how to get the thread information struct from C
  */
@@ -81,6 +90,9 @@ static inline struct thread_info *current_thread_info(void)
 
 	return (struct thread_info *)sp_el0;
 }
+#endif
+
+#define init_stack		(init_thread_union.stack)
 
 #define thread_saved_pc(tsk)	\
 	((unsigned long)(tsk->thread.cpu_context.pc))