Al Viro <v...@zeniv.linux.org.uk> writes:
> On Wed, Oct 16, 2013 at 03:01:57PM -0700, Jim Lieb wrote:
>> File servers must do some operations with the credentials of
>> their client. This syscall switches the key credentials similar
>> to nfsd_setuser() in fs/nfsd/auth.c with the capability of retaining a
>> handle to the credentials by way of an fd for an open anonymous file.
>> This makes switching for subsequent operations for that client more
>> efficient.
>
> Yet Another Untyped Multiplexor. Inna bun. Onna stick.
> CMOT Dibbler special...
>
> Switching creds to those of opener of given file descriptor
> is fine, but in any realistic situation you'll get all the real win
> from that - you should cache those fds (which you seem to do), and
> then setuid/etc. is done once per cache miss. Making the magical
> "set them all at once" mess (complete with non-trivial structure,
> 32/64bit compat, etc.) pointless. Moreover, you don't need any magic
> files at all - just set the creds and open /dev/null and there's your fd.
> With proper creds associated with it. While we are at it, just _start_
> with opening /dev/null. With your initial creds. Voila - revert is
> simply switch to that fd's creds.
>
> IOW, you really need only one syscall:
That doesn't look bad but it does need capable(CAP_SETUID) &&
capable(CAP_SETGID) or possibly something a little more refined.
I don't think we want file descriptor passing to all of a sudden become
a grant of privilege, beyond what the passed fd can do.
> SYSCALL_DEFINE1(switch_cred, int, fd)
> {
> struct fd f = fdget(fd);
> if (!f.file)
> return -EBADF;
> put_cred(override_creds(f.file->f_cred);
> fdput(f);
> return 0;
> }
>
> and that's all there is to it.
Eric
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