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SubjectRe: [PATCH] kmmio/mmiotrace: fix double free of kmmio_fault_pages
On Sat, Jun 05, 2010 at 06:49:42PM +0200, Marcin Slusarz wrote:
> After every iounmap mmiotrace has to free kmmio_fault_pages, but it
> can't do it directly, so it defers freeing by RCU.
>
> It usually works, but when mmiotraced code calls ioremap-iounmap
> multiple times without sleeping between (so RCU won't kick in and
> start freeing) it can be given the same virtual address, so at
> every iounmap mmiotrace will schedule the same pages for release.
> Obviously it will explode on second free.
>
> Fix it by marking kmmio_fault_pages which are scheduled for release
> and not adding them second time.
>

Attached patch for mmiotrace testing module allows to reliably reproduce
the bug. It can be folded into the main patch.

---
diff --git a/arch/x86/mm/testmmiotrace.c b/arch/x86/mm/testmmiotrace.c
index 8565d94..5f0937b 100644
--- a/arch/x86/mm/testmmiotrace.c
+++ b/arch/x86/mm/testmmiotrace.c
@@ -90,6 +90,19 @@ static void do_test(unsigned long size)
iounmap(p);
}

+static void do_test2(void)
+{
+ void __iomem *p;
+ int i;
+
+ for (i = 0; i < 10; ++i) {
+ p = ioremap_nocache(mmio_address, 4096);
+ if (p)
+ iounmap(p);
+ }
+ synchronize_rcu(); /* will freeing work? */
+}
+
static int __init init(void)
{
unsigned long size = (read_far) ? (8 << 20) : (16 << 10);
@@ -104,6 +117,7 @@ static int __init init(void)
"and writing 16 kB of rubbish in there.\n",
size >> 10, mmio_address);
do_test(size);
+ do_test2();
pr_info("All done.\n");
return 0;
}


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