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

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