On 08/19/2016 07:25 AM, Michael Kerrisk (man-pages) wrote:
The limit checking in pipe_set_size() (used by fcntl(F_SETPIPE_SZ))
has the following problems:
[...]
@@ -1030,6 +1030,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
{
struct pipe_buffer *bufs;
unsigned int size, nr_pages;
+ long ret = 0;
size = round_pipe_size(arg);
nr_pages = size >> PAGE_SHIFT;
@@ -1037,13 +1038,26 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
if (!nr_pages)
return -EINVAL;
- if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size)
- return -EPERM;
+ account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
- if ((too_many_pipe_buffers_hard(pipe->user) ||
- too_many_pipe_buffers_soft(pipe->user)) &&
- !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
- return -EPERM;
+ /*
+ * 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;
+ goto out_revert_acct;
+ } else if ((too_many_pipe_buffers_hard(pipe->user) ||
+ too_many_pipe_buffers_soft(pipe->user)) &&
+ !capable(CAP_SYS_RESOURCE) &&
+ !capable(CAP_SYS_ADMIN)) {
+ ret = -EPERM;
+ goto out_revert_acct;
+ }
+ }
I'm slightly worried about not checking arg/nr_pages before we pass it
on to account_pipe_buffers().
The potential problem happens if the user passes a very large number
which will overflow pipe->user->pipe_bufs.
On 32-bit, sizeof(int) == sizeof(long), so if they pass arg = INT_MAX
then round_pipe_size() returns INT_MAX. Although it's true that the
accounting is done in terms of pages and not bytes, so you'd need on the
order of (1 << 13) = 8192 processes hitting the limit at the same time
in order to make it overflow, which seems a bit unlikely.
(See https://lkml.org/lkml/2016/8/12/215 for another discussion on the
limit checking)
Is there any reason why we couldn't do the (size > pipe_max_size) check
before calling account_pipe_buffers()?
Vegard
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