ARM: save and reset the address limit when entering an exception

When we enter an exception, the current address limit should not apply
to the exception context: if the exception context wishes to access
kernel space via the user accessors (eg, perf code), it must explicitly
request such access.

Acked-by: Will Deacon <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/include/asm/ptrace.h

···
 struct svc_pt_regs {
 	struct pt_regs regs;
 	u32 dacr;
-	u32 unused;
+	u32 addr_limit;
 };
 
 #define to_svc_pt_regs(r) container_of(r, struct svc_pt_regs, regs)

arch/arm/kernel/asm-offsets.c

···
   DEFINE(S_OLD_R0,		offsetof(struct pt_regs, ARM_ORIG_r0));
   DEFINE(PT_REGS_SIZE,		sizeof(struct pt_regs));
   DEFINE(SVC_DACR,		offsetof(struct svc_pt_regs, dacr));
+  DEFINE(SVC_ADDR_LIMIT,	offsetof(struct svc_pt_regs, addr_limit));
   DEFINE(SVC_REGS_SIZE,		sizeof(struct svc_pt_regs));
   BLANK();
 #ifdef CONFIG_CACHE_L2X0

arch/arm/kernel/entry-armv.S

···
 	@
 	stmia	r7, {r2 - r6}
 
+	get_thread_info tsk
+	ldr	r0, [tsk, #TI_ADDR_LIMIT]
+	mov	r1, #TASK_SIZE
+	str	r1, [tsk, #TI_ADDR_LIMIT]
+	str	r0, [sp, #SVC_ADDR_LIMIT]
+
 	uaccess_save r0
 	.if \uaccess
 	uaccess_disable r0
···
 	irq_handler
 
 #ifdef CONFIG_PREEMPT
-	get_thread_info tsk
 	ldr	r8, [tsk, #TI_PREEMPT]		@ get preempt count
 	ldr	r0, [tsk, #TI_FLAGS]		@ get flags
 	teq	r8, #0				@ if preempt count != 0

arch/arm/kernel/entry-header.S

···
 	blne	trace_hardirqs_off
 #endif
 	.endif
+	ldr	r1, [sp, #SVC_ADDR_LIMIT]
 	uaccess_restore
+	str	r1, [tsk, #TI_ADDR_LIMIT]
 
 #ifndef CONFIG_THUMB2_KERNEL
 	@ ARM mode SVC restore
···
 	@ on the stack remains correct).
 	@
 	.macro  svc_exit_via_fiq
+	ldr	r1, [sp, #SVC_ADDR_LIMIT]
 	uaccess_restore
+	str	r1, [tsk, #TI_ADDR_LIMIT]
 #ifndef CONFIG_THUMB2_KERNEL
 	@ ARM mode restore
 	mov	r0, sp

arch/arm/kernel/process.c

···
 	unsigned long flags;
 	char buf[64];
 #ifndef CONFIG_CPU_V7M
-	unsigned int domain;
+	unsigned int domain, fs;
 #ifdef CONFIG_CPU_SW_DOMAIN_PAN
 	/*
 	 * Get the domain register for the parent context. In user
 	 * mode, we don't save the DACR, so lets use what it should
 	 * be. For other modes, we place it after the pt_regs struct.
 	 */
-	if (user_mode(regs))
+	if (user_mode(regs)) {
 		domain = DACR_UACCESS_ENABLE;
-	else
+		fs = get_fs();
+	} else {
 		domain = to_svc_pt_regs(regs)->dacr;
+		fs = to_svc_pt_regs(regs)->addr_limit;
+	}
 #else
 	domain = get_domain();
+	fs = get_fs();
 #endif
 #endif
 
···
 		if ((domain & domain_mask(DOMAIN_USER)) ==
 		    domain_val(DOMAIN_USER, DOMAIN_NOACCESS))
 			segment = "none";
-		else if (get_fs() == get_ds())
+		else if (fs == get_ds())
 			segment = "kernel";
 		else
 			segment = "user";