Re: [RFC][PATCHES] iov_iter.c rewrite

From: Linus Torvalds
Date: Mon Dec 08 2014 - 14:48:58 EST

On Mon, Dec 8, 2014 at 11:28 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On x86 it does, but I don't see anything obvious in generic version in
> mm/gup.c, so the old code might still have a problem on some architectures.
> What am I missing here?

Hmm. You may be right. The "access_ok()" is supposed to protect
things, but for cases like finit_module() that has explicitly said
"kernel addresses are ok", that check doesn't work.

Maybe something like this..

diff --git a/mm/gup.c b/mm/gup.c
index cd62c8c90d4a..6234b1e6ced9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -951,6 +951,9 @@ int __get_user_pages_fast(unsigned long start,
int nr_pages, int write,
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;

+ if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
+ return 0;
if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
start, len)))
return 0;

Completely untested, obviously. That code isn't even compiled on x86.
Adding linux-arch for more comments.

(Background: the generic non-x86 "get_user_pages_fast()" function
cannot check that the page tables are actually *user* page tables,

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