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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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