Re: memory leak in alloc_huge_page

[Mike Kravetz <mike.kravetz@oracle.com>]

On 12/01/2015 06:04 AM, Dmitry Vyukov wrote:
> Hello,
> 
> The following program leaks memory:
> 
> // autogenerated by syzkaller (http://github.com/google/syzkaller)
> #include <syscall.h>
> #include <string.h>
> #include <stdint.h>
> 
> #define SYS_mlock2 325
> 
> int main()
> {
>         syscall(SYS_mmap, 0x20000000ul, 0x1000ul, 0x3ul, 0x45031ul,
> 0xfffffffffffffffful, 0x0ul);
>         syscall(SYS_mlock2, 0x20000000ul, 0x1000ul, 0x1ul, 0, 0, 0);
>         return 0;
> }
> 
> unreferenced object 0xffff88002eaafd88 (size 32):
>   comm "a.out", pid 5063, jiffies 4295774645 (age 15.810s)
>   hex dump (first 32 bytes):
>     28 e9 4e 63 00 88 ff ff 28 e9 4e 63 00 88 ff ff  (.Nc....(.Nc....
>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>   backtrace:
>     [<     inline     >] kmalloc include/linux/slab.h:458
>     [<ffffffff815efa64>] region_chg+0x2d4/0x6b0 mm/hugetlb.c:398
>     [<ffffffff815f0c63>] __vma_reservation_common+0x2c3/0x390 mm/hugetlb.c:1791
>     [<     inline     >] vma_needs_reservation mm/hugetlb.c:1813
>     [<ffffffff815f658e>] alloc_huge_page+0x19e/0xc70 mm/hugetlb.c:1845
>     [<     inline     >] hugetlb_no_page mm/hugetlb.c:3543
>     [<ffffffff815fc561>] hugetlb_fault+0x7a1/0x1250 mm/hugetlb.c:3717
>     [<ffffffff815fd349>] follow_hugetlb_page+0x339/0xc70 mm/hugetlb.c:3880
>     [<ffffffff815a2bb2>] __get_user_pages+0x542/0xf30 mm/gup.c:497
>     [<ffffffff815a400e>] populate_vma_page_range+0xde/0x110 mm/gup.c:919
>     [<ffffffff815a4207>] __mm_populate+0x1c7/0x310 mm/gup.c:969
>     [<ffffffff815b74f1>] do_mlock+0x291/0x360 mm/mlock.c:637
>     [<     inline     >] SYSC_mlock2 mm/mlock.c:658
>     [<ffffffff815b7a4b>] SyS_mlock2+0x4b/0x70 mm/mlock.c:648
> 
> If this program run in a loop number of objects in kmalloc-32 slab
> indeed grows infinitely.
> 
> On commit 31ade3b83e1821da5fbb2f11b5b3d4ab2ec39db8 (Nov 29).
> 
> There seems to be another leak if nrg is not NULL on this path, but
> it's not what happens in my case since the WARNING does not fire.
> Still something to fix:
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 827bb02..e97a31b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -372,8 +372,10 @@ retry_locked:
>                 spin_unlock(&resv->lock);
> 
>                 trg = kmalloc(sizeof(*trg), GFP_KERNEL);
> -               if (!trg)
> +               if (!trg) {
> +                       WARN_ON(nrg != NULL);
>                         return -ENOMEM;
> +               }
> 
>                 spin_lock(&resv->lock);
>                 list_add(&trg->link, &resv->region_cache);
> 
> 

Thanks Dmitry,

If nrg is not NULL, then it was added to the resv map and 'should' be
free'ed when the map is free'ed.  This is not optimal, but I do not
think it would lead to a leak.  I'll take a close look at this code
with an emphasis on the leak you discovered.

-- 
Mike Kravetz

> Thanks
> 

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