Re: [PATCH] lib: idr: fix out-of-bounds pointer dereference

Lai Jiangshan
2014-06-24T01:25:34Z

On 06/23/2014 09:37 PM, Andrey Ryabinin wrote:
> 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.
> }
> }

I prefer to moving the loop-exit-condition-checking code down:

if (id < 0 || id > max) /* only idr_get_next() needs this branch */
return ...

for (;;) { /* move out from here */
...
increase the @id;
if (id < 0 || id > max) /* move to here */
break;
while (n < fls(id)) {
...
}
}

>
> 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>
> ---
> lib/idr.c | 25 ++++++++++++++-----------
> 1 file changed, 14 insertions(+), 11 deletions(-)
>
> diff --git a/lib/idr.c b/lib/idr.c
> index 39158ab..50be3fa 100644
> --- a/lib/idr.c
> +++ b/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;