powerpc/watchpoint: Fix exception handling for CONFIG_HAVE_HW_BREAKPOINT=N

On powerpc, ptrace watchpoint works in one-shot mode. i.e. kernel
disables event every time it fires and user has to re-enable it.
Also, in case of ptrace watchpoint, kernel notifies ptrace user
before executing instruction.

With CONFIG_HAVE_HW_BREAKPOINT=N, kernel is missing to disable
ptrace event and thus it's causing infinite loop of exceptions.
This is especially harmful when user watches on a data which is
also read/written by kernel, eg syscall parameters. In such case,
infinite exceptions happens in kernel mode which causes soft-lockup.

Fixes: 9422de3e953d ("powerpc: Hardware breakpoints rewrite to handle non DABR breakpoint registers")
Reported-by: Pedro Miraglia Franco de Carvalho <[e-mail]>
Signed-off-by: Ravi Bangoria <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/include/asm/hw_breakpoint.h

···
 	u16		type;
 	u16		len; /* length of the target data symbol */
 	u16		hw_len; /* length programmed in hw */
+	u8		flags;
 };
 
 /* Note: Don't change the first 6 bits below as they are in the same order
···
 #define HW_BRK_TYPE_DABR	(HW_BRK_TYPE_RDWR | HW_BRK_TYPE_TRANSLATE)
 #define HW_BRK_TYPE_PRIV_ALL	(HW_BRK_TYPE_USER | HW_BRK_TYPE_KERNEL | \
 				 HW_BRK_TYPE_HYP)
+
+#define HW_BRK_FLAG_DISABLED	0x1
 
 /* Minimum granularity */
 #ifdef CONFIG_PPC_8xx

arch/powerpc/kernel/process.c

···
 				    (void __user *)address);
 }
 #else	/* !CONFIG_PPC_ADV_DEBUG_REGS */
+
+static void do_break_handler(struct pt_regs *regs)
+{
+	struct arch_hw_breakpoint null_brk = {0};
+	struct arch_hw_breakpoint *info;
+	struct ppc_inst instr = ppc_inst(0);
+	int type = 0;
+	int size = 0;
+	unsigned long ea;
+	int i;
+
+	/*
+	 * If underneath hw supports only one watchpoint, we know it
+	 * caused exception. 8xx also falls into this category.
+	 */
+	if (nr_wp_slots() == 1) {
+		__set_breakpoint(0, &null_brk);
+		current->thread.hw_brk[0] = null_brk;
+		current->thread.hw_brk[0].flags |= HW_BRK_FLAG_DISABLED;
+		return;
+	}
+
+	/* Otherwise findout which DAWR caused exception and disable it. */
+	wp_get_instr_detail(regs, &instr, &type, &size, &ea);
+
+	for (i = 0; i < nr_wp_slots(); i++) {
+		info = &current->thread.hw_brk[i];
+		if (!info->address)
+			continue;
+
+		if (wp_check_constraints(regs, instr, ea, type, size, info)) {
+			__set_breakpoint(i, &null_brk);
+			current->thread.hw_brk[i] = null_brk;
+			current->thread.hw_brk[i].flags |= HW_BRK_FLAG_DISABLED;
+		}
+	}
+}
+
 void do_break (struct pt_regs *regs, unsigned long address,
 		    unsigned long error_code)
 {
···
 
 	if (debugger_break_match(regs))
 		return;
+
+	/*
+	 * We reach here only when watchpoint exception is generated by ptrace
+	 * event (or hw is buggy!). Now if CONFIG_HAVE_HW_BREAKPOINT is set,
+	 * watchpoint is already handled by hw_breakpoint_handler() so we don't
+	 * have to do anything. But when CONFIG_HAVE_HW_BREAKPOINT is not set,
+	 * we need to manually handle the watchpoint here.
+	 */
+	if (!IS_ENABLED(CONFIG_HAVE_HW_BREAKPOINT))
+		do_break_handler(regs);
 
 	/* Deliver the signal to userspace */
 	force_sig_fault(SIGTRAP, TRAP_HWBKPT, (void __user *)address);

arch/powerpc/kernel/ptrace/ptrace-noadv.c

···
 	}
 	return ret;
 #else /* CONFIG_HAVE_HW_BREAKPOINT */
-	if (child->thread.hw_brk[data - 1].address == 0)
+	if (!(child->thread.hw_brk[data - 1].flags & HW_BRK_FLAG_DISABLED) &&
+	    child->thread.hw_brk[data - 1].address == 0)
 		return -ENOENT;
 
 	child->thread.hw_brk[data - 1].address = 0;
 	child->thread.hw_brk[data - 1].type = 0;
+	child->thread.hw_brk[data - 1].flags = 0;
 #endif /* CONFIG_HAVE_HW_BREAKPOINT */
 
 	return 0;