Re: [PATCH 2/6] m68k: call find_vma with the mmap_sem held in sys_cacheflush()

[Geert Uytterhoeven]

Hi David,

On Mon, Apr 21, 2014 at 12:28 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> On Sun, 2014-04-20 at 10:04 +0200, Geert Uytterhoeven wrote:
>> On Sun, Apr 20, 2014 at 4:26 AM, Davidlohr Bueso <davidlohr@xxxxxx> 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@xxxxxx>
>> > Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
>> > Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
>> > ---
>> >  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?
>
> I don't think so. afaict the vma is only searched to check upon validity
> for the address being passed. Once the sem is dropped, the call doesn't
> do absolutely anything else with the returned vma.

The function indeed doesn't do anything anymore with the vma itself, but
it does do something with the addr/len pair, which may no longer match
with the vma if it changes after the up_read(). I.e. the address may no longer
be valid when the cache is actually flushed.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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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