[PATCH] idr: fix a critical misallocation bug

From: Tejun Heo
Date: Mon Jan 18 2010 - 20:18:56 EST

Eric Paris located a bug in idr. With IDR_BITS of 6, it grows to
three layers when id 4096 is first allocated. When that happens, idr
wraps incorrectly and searches the idr array ignoring the high bits.
The following test code from Eric demonstrates the bug nicely.

#include <linux/idr.h>
#include <linux/kernel.h>
#include <linux/module.h>

static DEFINE_IDR(test_idr);

int init_module(void)
int ret, forty95, forty96;
void *addr;

/* add 2 entries both with 4095 as the start address */
if (!idr_pre_get(&test_idr, GFP_KERNEL))
return -ENOMEM;
ret = idr_get_new_above(&test_idr, (void *)4095, 4095, &forty95);
if (ret) {
if (ret == -EAGAIN)
goto again1;
return ret;
if (forty95 != 4095)
printk(KERN_ERR "hmmm, forty95=%d\n", forty95);

if (!idr_pre_get(&test_idr, GFP_KERNEL))
return -ENOMEM;
ret = idr_get_new_above(&test_idr, (void *)4096, 4095, &forty96);
if (ret) {
if (ret == -EAGAIN)
goto again2;
return ret;
if (forty96 != 4096)
printk(KERN_ERR "hmmm, forty96=%d\n", forty96);

/* try to find the 2 entries, noticing that 4096 broke */
addr = idr_find(&test_idr, forty95);
if ((int)addr != forty95)
printk(KERN_ERR "hmmm, after find forty95=%d addr=%d\n", forty95, (int)addr);
addr = idr_find(&test_idr, forty96);
if ((int)addr != forty96)
printk(KERN_ERR "hmmm, after find forty96=%d addr=%d\n", forty96, (int)addr);
/* really weird, the entry which should be at 4096 is actually at 0!! */
addr = idr_find(&test_idr, 0);
if ((int)addr)
printk(KERN_ERR "found an entry at id=0 for addr=%d\n", (int)addr);

idr_remove(&test_idr, forty95);
idr_remove(&test_idr, forty96);

return 0;

void cleanup_module(void)

MODULE_AUTHOR("Eric Paris <eparis@xxxxxxxxxx>");
MODULE_DESCRIPTION("Simple idr test");

This happens because when sub_alloc() back tracks it doesn't always do
it step-by-step while the over-the-limit detection assumes
step-by-step backtracking. The logic in sub_alloc() looks like the

clear pa[top level + 1] for end cond detection
l = top level
while (true) {
search for empty slot at this level
if (not found) {
push id to the next possible value
A: if (pa[l] is clear)
failed, return asking caller to grow the tree
if (going up 1 level gives more slots to search)
continue the while loop above with the incremented l
C: goto restart
adjust id accordingly to the found slot
if (l == 0)
return found id;
create lower level if not there yet
record pa[l] and l--

Test A is the fail exit condition but this assumes that failure is
propagated upwared one level at a time but the B optimization path
breaks the assumption and restarts the whole thing with a start value
which is above the possible limit with the current layers.
sub_alloc() assumes the start id value is inside the limit when called
and test A is the only exit condition check, so it ends up searching
for empty slot while ignoring high set bit.

So, for 4095->4096 test, level0 search fails but pa[1] contains a
valid pointer. However, going up 1 level wouldn't give any more empty
slot so it takes C and when the whole thing restarts nobody notices
the high bit set beyond the top level.

This patch fixes the bug by changing the fail exit condition check to
full id limit check.

Based-on-patch-from: Eric Paris <eparis@xxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
I've cc'd people who have touched idr recently. idr code is extremely
fragile and reviews would be very much appreciated.


lib/idr.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/lib/idr.c b/lib/idr.c
index 1cac726..ba7d37c 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -140,8 +140,7 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa)
id = *starting_id;
p = idp->top;
- l = idp->layers;
- pa[l--] = NULL;
+ l = p->layer;
while (1) {
* We run around this while until we reach the leaf node...
@@ -155,8 +154,8 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa)
oid = id;
id = (id | ((1 << (IDR_BITS * l)) - 1)) + 1;

- /* if already at the top layer, we need to grow */
- if (!(p = pa[l])) {
+ /* did id go over the limit? */
+ if (id >= (1 << (idp->layers * IDR_BITS))) {
*starting_id = id;

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