Re: [PATCH] mpt3sas: downgrade full copy_from_user to access_ok check

[Al Viro]

On Tue, Sep 19, 2017 at 11:11:11PM -0400, Meng Xu wrote:
> Since right after the user copy, we are going to
> memset(&karg, 0, sizeof(karg)), I guess an access_ok check is enough?

access_ok() is *NOT* "will copy_from_user() succeed?"  Not even close.
On a bunch of architectures (sparc64, for one) access_ok() is always
true.

All it does is checking that address is not a kernel one - e.g. on
i386 anything in range 0..3Gb qualifies.  Whether anything's mapped
at that address or not.

Why bother with that copy_from_user() at all?  The same ioctl()
proceeds to copy_to_user() on exact same range; all you get from
it is "if the area passed by caller is writable, but not readable,
fail with -EFAULT".  Who cares?

Just drop that copy_from_user() completely.  Anything access_ok()
might've caught will be caught by copy_to_user() anyway.