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

Catalin Marinas
2013-03-27T11:09:38Z

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 <will.deacon@arm.com>
> ---
> 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