Re: [PATCH 2/2] pipe: make pipe user buffer limit checks more precise

Michael Kerrisk (man-pages)
2016-08-17T08:02:31Z

Hi Vegard,

On 08/17/2016 10:00 AM, Vegard Nossum wrote:
> On 08/16/2016 10:21 PM, Michael Kerrisk (man-pages) wrote:
>>>> @@ -1132,8 +1136,8 @@ long pipe_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
>>>> if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size) {
>>>> ret = -EPERM;
>>>> goto out;
>>>> - } else if ((too_many_pipe_buffers_hard(pipe->user) ||
>>>> - too_many_pipe_buffers_soft(pipe->user)) &&
>>>> + } else if ((too_many_pipe_buffers_hard(pipe->user, nr_pages) ||
>>>> + too_many_pipe_buffers_soft(pipe->user, nr_pages)) &&
>>>> !capable(CAP_SYS_RESOURCE) &&
>>>> !capable(CAP_SYS_ADMIN)) {
>>>> ret = -EPERM;
>>>>
>>>
>>> Isn't there also a race where two or more concurrent pipe()/fnctl()
>>> calls can together push us over the limits before the accounting is done?
>>
>> I guess there is!
>>
>>> I think there really ought to be a check after doing the accounting if
>>> we really want to be meticulous here.
>>
>> Let me confirm what I understand from your comment: because of the race,
>> then a user could subvert the checks and allocate an arbitrary amount
>> of kernel memory for pipes. Right?
>>
>> I'm not sure what you mean by "a check after doing the accounting". Is not the
>> only solution here some kind of lock around the check+accounting steps?
>
> Instead of doing atomic_long_read() in the check + atomic_long_add() for
> accounting we could do a single speculative atomic_long_add_return() and
> then if it goes above the limit we can lower it again with atomic_sub()
> when aborting the operation (if it doesn't go above the limit we don't
> need to do anything).

So, would that mean something like the following (where I've moved
some checks from pipe_fcntl() to pipe_set_size()):

8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---

long pipe_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct pipe_inode_info *pipe;
long ret;

pipe = get_pipe_info(file);
if (!pipe)
return -EBADF;

__pipe_lock(pipe);

switch (cmd) {
case F_SETPIPE_SZ: {
unsigned int size, nr_pages;

size = round_pipe_size(arg);
nr_pages = size >> PAGE_SHIFT;

ret = -EINVAL;
if (!nr_pages)
goto out;

ret = pipe_set_size(pipe, nr_pages);
break;
}
case F_GETPIPE_SZ:
ret = pipe->buffers * PAGE_SIZE;
break;
default:
ret = -EINVAL;
break;
}

out:
__pipe_unlock(pipe);
return ret;
}

/*...*/

static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long nr_pages)
{
struct pipe_buffer *bufs;
unsigned int size;
long ret = 0;

size = nr_pages * PAGE_SIZE;
account_pipe_buffers(pipe, pipe->buffers, nr_pages);

/*
* If trying to increase the pipe capacity, check that an
* unprivileged user is not trying to exceed various limits.
* (Decreasing the pipe capacity is always permitted, even
* if the user is currently over a limit.)
*/
if (nr_pages > pipe->buffers) {
if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size) {
ret = -EPERM;
} else if ((too_many_pipe_buffers_hard(pipe->user, 0) ||
too_many_pipe_buffers_soft(pipe->user, 0)) &&
!capable(CAP_SYS_RESOURCE) &&
!capable(CAP_SYS_ADMIN)) {
ret = -EPERM;
}
}

/*
* If we exceeded a limit, revert the accounting and go no further
*/
if (ret) {
account_pipe_buffers(pipe, nr_pages, pipe->buffers);
return ret;
}

/*
* We can shrink the pipe, if arg >= pipe->nrbufs. Since we don't
* expect a lot of shrink+grow operations, just free and allocate
* again like we would do for growing. If the pipe currently
* contains more buffers than arg, then return busy.
*/
if (nr_pages < pipe->nrbufs)
return -EBUSY;

bufs = kcalloc(nr_pages, sizeof(*bufs),
GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
if (unlikely(!bufs))
return -ENOMEM;

/*
* The pipe array wraps around, so just start the new one at zero
* and adjust the indexes.
*/
if (pipe->nrbufs) {
unsigned int tail;
unsigned int head;

tail = pipe->curbuf + pipe->nrbufs;
if (tail < pipe->buffers)
tail = 0;
else
tail &= (pipe->buffers - 1);

head = pipe->nrbufs - tail;
if (head)
memcpy(bufs, pipe->bufs + pipe->curbuf, head * sizeof(struct pipe_buffer));
if (tail)
memcpy(bufs + head, pipe->bufs, tail * sizeof(struct pipe_buffer));
}

pipe->curbuf = 0;
kfree(pipe->bufs);
pipe->bufs = bufs;
pipe->buffers = nr_pages;
return nr_pages * PAGE_SIZE;
}

8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---8x---

Seem okay? Probably, some analogous fix is required in alloc_pipe_info()
when creating a pipe(?).

Thanks,

Michael