Re: [PATCH 7/7] pipe: read buffer limits atomically

Kees Cook
2018-01-09T22:27:46Z

On Sun, Jan 7, 2018 at 9:35 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> The pipe buffer limits are accessed without any locking, and may be
> changed at any time by the sysctl handlers. In theory this could cause
> problems for expressions like the following:
>
> pipe_user_pages_hard && user_bufs > pipe_user_pages_hard
>
> ... since the assembly code might reference the 'pipe_user_pages_hard'
> memory location multiple times, and if the admin removes the limit by
> setting it to 0, there is a very brief window where processes could
> incorrectly observe the limit to be exceeded.
>
> Fix this by loading the limits with READ_ONCE() prior to use.
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> fs/pipe.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 774cafd947dc..2e2349602815 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -605,12 +605,16 @@ static unsigned long account_pipe_buffers(struct user_struct *user,
>
> static bool too_many_pipe_buffers_soft(unsigned long user_bufs)
> {
> - return pipe_user_pages_soft && user_bufs > pipe_user_pages_soft;
> + unsigned long soft_limit = READ_ONCE(pipe_user_pages_soft);
> +
> + return soft_limit && user_bufs > soft_limit;
> }
>
> static bool too_many_pipe_buffers_hard(unsigned long user_bufs)
> {
> - return pipe_user_pages_hard && user_bufs > pipe_user_pages_hard;
> + unsigned long hard_limit = READ_ONCE(pipe_user_pages_hard);
> +
> + return hard_limit && user_bufs > hard_limit;
> }
>
> static bool is_unprivileged_user(void)
> @@ -624,13 +628,14 @@ struct pipe_inode_info *alloc_pipe_info(void)
> unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
> struct user_struct *user = get_current_user();
> unsigned long user_bufs;
> + unsigned int max_size = READ_ONCE(pipe_max_size);
>
> pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
> if (pipe == NULL)
> goto out_free_uid;
>
> - if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
> - pipe_bufs = pipe_max_size >> PAGE_SHIFT;
> + if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
> + pipe_bufs = max_size >> PAGE_SHIFT;
>
> user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>
> --
> 2.15.1
>

--
Kees Cook
Pixel Security