Re: check capabilities in open()

Al Viro
2010-07-24T18:23:59Z

On Sat, Jul 24, 2010 at 08:07:01PM +0400, Vasiliy Kulikov wrote:
> Hi,
>
> I've found that some drivers check process capabilities via capable() in
> open(), not in ioctl()/write()/etc.
>
> I cannot find answer in POSIX, but IMO process expects that file
> descriptors of priviledged user and file descriptors of the same
> file/device are the same in priviledge aspect. Driver should deny/allow
> open() and deny/allow ioctl() based on user priviledges. The path how
> the process gained this fd doesn't matter.
>
> So I think these 2 examples should be equal:
>
> 1) root process opened the file and then dropped its priviledges
>
> 2) nonroot process opened the file

They most certainly should _not_. Consider the following mechanism:
process A authenticates itself to process B
B is convinced to open a file that wouldn't be readable for A.
B passes descriptor to A.
A reads from it.
You are breaking that.