[PATCH] parisc: Fix kernel panic due invalid values of IAOQ0 or IAOQ1

[Helge Deller <deller@gmx.de>]

On parisc the privilege level of a process is stored in the lowest two bits of
the instruction pointers (IAOQ0 and IAOQ1). On Linux we use privilege level 0
for the kernel and privilege level 3 for user-space. So userspace should not be
allowed to modify IAOQ0 or IAOQ1 of a ptraced process to change it's privilege
level to e.g. 0 to try to gain kernel privileges.

This patch prevents such modifications by always setting the two lowest bits to
one (which relates to privilege level 3 for user-space) if IAOQ0 or IAOQ1 are
modified via ptrace calls.

Fixes: https://bugs.gentoo.org/481768
Reported-by: Jeroen Roovers <jer@gentoo.org>
Cc: <stable@vger.kernel.org>

diff --git a/arch/parisc/kernel/ptrace.c b/arch/parisc/kernel/ptrace.c
index a3d2fb4e6dd2..8ecd41938709 100644
--- a/arch/parisc/kernel/ptrace.c
+++ b/arch/parisc/kernel/ptrace.c
@@ -167,6 +175,9 @@ long arch_ptrace(struct task_struct *child, long request,
 		if ((addr & (sizeof(unsigned long)-1)) ||
 		     addr >= sizeof(struct pt_regs))
 			break;
+		if (addr == PT_IAOQ0 || addr == PT_IAOQ1) {
+			data |= 3; /* ensure userspace privilege */
+		}
 		if ((addr >= PT_GR1 && addr <= PT_GR31) ||
 				addr == PT_IAOQ0 || addr == PT_IAOQ1 ||
 				(addr >= PT_FR0 && addr <= PT_FR31 + 4) ||
@@ -281,6 +292,9 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
 			addr = translate_usr_offset(addr);
 			if (addr >= sizeof(struct pt_regs))
 				break;
+			if (addr == PT_IAOQ0 || addr == PT_IAOQ1) {
+				data |= 3; /* ensure userspace privilege */
+			}
 			if (addr >= PT_FR0 && addr <= PT_FR31 + 4) {
 				/* Special case, fp regs are 64 bits anyway */
 				*(__u64 *) ((char *) task_regs(child) + addr) = data;