lib/idr.c: fix out-of-bounds pointer dereference

I'm working on address sanitizer project for kernel.  Recently we
started experiments with stack instrumentation, to detect out-of-bounds
read/write bugs on stack.

Just after booting I've hit out-of-bounds read on stack in idr_for_each
(and in __idr_remove_all as well):

	struct idr_layer **paa = &pa[0];

	while (id >= 0 && id <= max) {
		...
		while (n < fls(id)) {
			n += IDR_BITS;
			p = *--paa; <--- here we are reading pa[-1] value.
		}
	}

Despite the fact that after this dereference we are exiting out of loop
and never use p, such behaviour is undefined and should be avoided.

Fix this by moving pointer derference to the beggining of the loop,
right before we will use it.

Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Alexey Preobrazhensky <preobr@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/idr.c

@@ -590,26 +590,27 @@ static void __idr_remove_all(struct idr *idp)
 	struct idr_layer **paa = &pa[0];
 
 	n = idp->layers * IDR_BITS;
-	p = idp->top;
+	*paa = idp->top;
 	RCU_INIT_POINTER(idp->top, NULL);
 	max = idr_max(idp->layers);
 
 	id = 0;
 	while (id >= 0 && id <= max) {
+		p = *paa;
 		while (n > IDR_BITS && p) {
 			n -= IDR_BITS;
-			*paa++ = p;
 			p = p->ary[(id >> n) & IDR_MASK];
+			*++paa = p;
 		}
 
 		bt_mask = id;
 		id += 1 << n;
 		/* Get the highest bit that the above add changed from 0->1. */
 		while (n < fls(id ^ bt_mask)) {
-			if (p)
-				free_layer(idp, p);
+			if (*paa)
+				free_layer(idp, *paa);
 			n += IDR_BITS;
-			p = *--paa;
+			--paa;
 		}
 	}
 	idp->layers = 0;
@@ -692,15 +693,16 @@ int idr_for_each(struct idr *idp,
 	struct idr_layer **paa = &pa[0];
 
 	n = idp->layers * IDR_BITS;
-	p = rcu_dereference_raw(idp->top);
+	*paa = rcu_dereference_raw(idp->top);
 	max = idr_max(idp->layers);
 
 	id = 0;
 	while (id >= 0 && id <= max) {
+		p = *paa;
 		while (n > 0 && p) {
 			n -= IDR_BITS;
-			*paa++ = p;
 			p = rcu_dereference_raw(p->ary[(id >> n) & IDR_MASK]);
+			*++paa = p;
 		}
 
 		if (p) {
@@ -712,7 +714,7 @@ int idr_for_each(struct idr *idp,
 		id += 1 << n;
 		while (n < fls(id)) {
 			n += IDR_BITS;
-			p = *--paa;
+			--paa;
 		}
 	}
 
@@ -740,17 +742,18 @@ void *idr_get_next(struct idr *idp, int *nextidp)
 	int n, max;
 
 	/* find first ent */
-	p = rcu_dereference_raw(idp->top);
+	p = *paa = rcu_dereference_raw(idp->top);
 	if (!p)
 		return NULL;
 	n = (p->layer + 1) * IDR_BITS;
 	max = idr_max(p->layer + 1);
 
 	while (id >= 0 && id <= max) {
+		p = *paa;
 		while (n > 0 && p) {
 			n -= IDR_BITS;
-			*paa++ = p;
 			p = rcu_dereference_raw(p->ary[(id >> n) & IDR_MASK]);
+			*++paa = p;
 		}
 
 		if (p) {
@@ -768,7 +771,7 @@ void *idr_get_next(struct idr *idp, int *nextidp)
 		id = round_up(id + 1, 1 << n);
 		while (n < fls(id)) {
 			n += IDR_BITS;
-			p = *--paa;
+			--paa;
 		}
 	}
 	return NULL;