Subject: [PATCH 2/3] ARM: cacheflush: don't bother rounding to nearest vma

From: [email-masked] (Catalin Marinas)
Date: Wed, 27 Mar 2013 11:09:38 +0000

On Mon, Mar 25, 2013 at 06:18:05PM +0000, Will Deacon wrote:
> do_cache_op finds the lowest VMA contained in the specified address
> range and rounds the range to cover only the mapped addresses.
> 
> Since commit 4542b6a0fa6b ("ARM: 7365/1: drop unused parameter from
> flush_cache_user_range") the VMA is not used for anything else in this
> code and seeing as the low-level cache flushing routines return -EFAULT
> if the address is not valid, there is no need for this range truncation.
> 
> This patch removes the VMA handling code from the cacheflushing syscall.
> 
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  arch/arm/kernel/traps.c | 17 +----------------
>  1 file changed, 1 insertion(+), 16 deletions(-)
> 
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index 1c08911..da5e268 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -509,25 +509,10 @@ static int bad_syscall(int n, struct pt_regs *regs)
>  static inline int
>  do_cache_op(unsigned long start, unsigned long end, int flags)
>  {
> -	struct mm_struct *mm = current->active_mm;
> -	struct vm_area_struct *vma;
> -
>  	if (end < start || flags)
>  		return -EINVAL;
>  
> -	down_read(&mm->mmap_sem);
> -	vma = find_vma(mm, start);
> -	if (vma && vma->vm_start < end) {
> -		if (start < vma->vm_start)
> -			start = vma->vm_start;
> -		if (end > vma->vm_end)
> -			end = vma->vm_end;
> -
> -		up_read(&mm->mmap_sem);
> -		return flush_cache_user_range(start, end);
> -	}
> -	up_read(&mm->mmap_sem);
> -	return -EINVAL;
> +	return flush_cache_user_range(start, end);

While this would work, it introduces a possibility of DoS where an
application passes bigger valid range (kernel linear mapping) and the
kernel code would not be preempted (CONFIG_PREEMPT disabled). IIRC,
that's why Russell reject such patch a while back.

-- 
Catalin