arm, hw_breakpoints: Fix racy access to ptrace breakpoints

While the tracer accesses ptrace breakpoints, the child task may
concurrently exit due to a SIGKILL and thus release its breakpoints
at the same time. We can then dereference some freed pointers.

To fix this, hold a reference on the child breakpoints before
manipulating them.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Link: http://lkml.kernel.org/r/1302284067-7860-5-git-send-email-fweisbec@gmail.com

arch/arm/kernel/ptrace.c

@@ -767,12 +767,20 @@ long arch_ptrace(struct task_struct *child, long request,
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 		case PTRACE_GETHBPREGS:
+			if (ptrace_get_breakpoints(child) < 0)
+				return -ESRCH;
+
 			ret = ptrace_gethbpregs(child, addr,
 						(unsigned long __user *)data);
+			ptrace_put_breakpoints(child);
 			break;
 		case PTRACE_SETHBPREGS:
+			if (ptrace_get_breakpoints(child) < 0)
+				return -ESRCH;
+
 			ret = ptrace_sethbpregs(child, addr,
 						(unsigned long __user *)data);
+			ptrace_put_breakpoints(child);
 			break;
 #endif