linux-kernel - Re: [PATCH 2/6] m68k: call find_vma with the mmap_sem held in sys

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Date:	Sun, 20 Apr 2014 10:04:02 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, zeus@....org,
	Aswin Chandramouleeswaran <aswin@...com>,
	Linux MM <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH 2/6] m68k: call find_vma with the mmap_sem held in sys_cacheflush()

Hi David,

On Sun, Apr 20, 2014 at 4:26 AM, Davidlohr Bueso <davidlohr@...com> wrote:
> Performing vma lookups without taking the mm->mmap_sem is asking
> for trouble. While doing the search, the vma in question can be
> modified or even removed before returning to the caller. Take the
> lock (shared) in order to avoid races while iterating through
> the vmacache and/or rbtree.

Thanks for your patch!

> This patch is completely *untested*.
>
> Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Cc: linux-m68k@...ts.linux-m68k.org
> ---
>  arch/m68k/kernel/sys_m68k.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/arch/m68k/kernel/sys_m68k.c b/arch/m68k/kernel/sys_m68k.c
> index 3a480b3..d2263a0 100644
> --- a/arch/m68k/kernel/sys_m68k.c
> +++ b/arch/m68k/kernel/sys_m68k.c
> @@ -376,7 +376,6 @@ cache_flush_060 (unsigned long addr, int scope, int cache, unsigned long len)
>  asmlinkage int
>  sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
>  {
> -       struct vm_area_struct *vma;
>         int ret = -EINVAL;
>
>         if (scope < FLUSH_SCOPE_LINE || scope > FLUSH_SCOPE_ALL ||
> @@ -389,16 +388,23 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
>                 if (!capable(CAP_SYS_ADMIN))
>                         goto out;
>         } else {
> +               struct vm_area_struct *vma;
> +               bool invalid;
> +
> +               /* Check for overflow.  */
> +               if (addr + len < addr)
> +                       goto out;
> +
>                 /*
>                  * Verify that the specified address region actually belongs
>                  * to this process.
>                  */
> -               vma = find_vma (current->mm, addr);
>                 ret = -EINVAL;
> -               /* Check for overflow.  */
> -               if (addr + len < addr)
> -                       goto out;
> -               if (vma == NULL || addr < vma->vm_start || addr + len > vma->vm_end)
> +               down_read(&current->mm->mmap_sem);
> +               vma = find_vma(current->mm, addr);
> +               invalid = !vma || addr < vma->vm_start || addr + len > vma->vm_end;
> +               up_read(&current->mm->mmap_sem);
> +               if (invalid)
>                         goto out;
>         }

Shouldn't the up_read() be moved to the end of the function?
The vma may still be modified or destroyed between the call to find_vma(),
and the actual cache flush?

Am I missing something?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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