[PATCH 3/8] idr: fix NULL pointer dereference when ida_remove(unallocated_id)

[Lai Jiangshan]

If the ida has at least one existed_id, and when an special unallocated_id
is passed to the ida_remove(), the system will crash because it hits NULL
pointer dereference.

This special unallocated_id is that it shares the same lowest idr layer with
an exsited_id, but the idr slot is different(if the unallocated_id is allocated).
In this case the supposed idr slot of the unallocated_id is NULL,
It means @bitmap == NULL, and when the code dereference it, it crash the kernel.

See the test code:

static void test3(void)
{
	int id;
	DEFINE_IDA(test_ida);

	printk(KERN_INFO "Start test3\n");
	if (ida_pre_get(&test_ida, GFP_KERNEL) < 0) return;
	if (ida_get_new(&test_ida,  &id) < 0) return;
	ida_remove(&test_ida, 4000); /* bug: null deference here */
	printk(KERN_INFO "End of test3\n");
}

It only happens when unallocated_id, it is caller's fault. It is not
a bug. But it is better to add the proper check and complains instead
of crashing the kernel.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
 lib/idr.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/lib/idr.c b/lib/idr.c
index a28036d..d200d67 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -1045,7 +1045,7 @@ void ida_remove(struct ida *ida, int id)
 	__clear_bit(n, p->bitmap);
 
 	bitmap = (void *)p->ary[n];
-	if (!test_bit(offset, bitmap->bitmap))
+	if (!bitmap || !test_bit(offset, bitmap->bitmap))
 		goto err;
 
 	/* update bitmap and remove it if empty */
-- 
1.7.4.4

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