linux-kernel - Re: [PATCH] lib/test_vmalloc.c: avoid garbage in page array

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Message-ID: <CA+KHdyVSD=vHh-WQxS_PrJ8GnBQZGFPgcS5AkmkyoUxpe_ziEw@mail.gmail.com>
Date:   Thu, 25 May 2023 22:04:11 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] lib/test_vmalloc.c: avoid garbage in page array

On Wed, May 24, 2023 at 10:24 AM Lorenzo Stoakes <lstoakes@...il.com> wrote:
>
> It turns out that alloc_pages_bulk_array() does not treat the page_array
> parameter as an output parameter, but rather reads the array and skips any
> entries that have already been allocated.
>
> This is somewhat unexpected and breaks this test, as we allocate the pages
> array uninitialised on the assumption it will be overwritten.
>
> As a result, the test was referencing uninitialised data and causing the
> PFN to not be valid and thus a WARN_ON() followed by a null pointer deref
> and panic.
>
> In addition, this is an array of pointers not of struct page objects, so we
> need only allocate an array with elements of pointer size.
>
> We solve both problems by simply using kcalloc() and referencing
> sizeof(struct page *) rather than sizeof(struct page).
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  lib/test_vmalloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
> index 9dd9745d365f..3718d9886407 100644
> --- a/lib/test_vmalloc.c
> +++ b/lib/test_vmalloc.c
> @@ -369,7 +369,7 @@ vm_map_ram_test(void)
>         int i;
>
>         map_nr_pages = nr_pages > 0 ? nr_pages:1;
> -       pages = kmalloc(map_nr_pages * sizeof(struct page), GFP_KERNEL);
> +       pages = kcalloc(map_nr_pages, sizeof(struct page *), GFP_KERNEL);
>         if (!pages)
>                 return -1;
>
> --
> 2.40.1
>
Uh.. :)

Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>

--
Uladzislau Rezki