[PATCH 4.15 47/53] get_user_pages_fast(): return -EFAULT on access_ok failure

From: Greg Kroah-Hartman
Date: Tue Apr 17 2018 - 12:05:41 EST

4.15-stable review patch. If anyone has any objections, please let me know.


From: Michael S. Tsirkin <mst@xxxxxxxxxx>

commit c61611f70958d86f659bca25c02ae69413747a8d upstream.

get_user_pages_fast is supposed to be a faster drop-in equivalent of
get_user_pages. As such, callers expect it to return a negative return
code when passed an invalid address, and never expect it to return 0
when passed a positive number of pages, since its documentation says:

* Returns number of pages pinned. This may be fewer than the number
* requested. If nr_pages is 0 or negative, returns 0. If no pages
* were pinned, returns -errno.

When get_user_pages_fast fall back on get_user_pages this is exactly
what happens. Unfortunately the implementation is inconsistent: it
returns 0 if passed a kernel address, confusing callers: for example,
the following is pretty common but does not appear to do the right thing
with a kernel address:

ret = get_user_pages_fast(addr, 1, writeable, &page);
if (ret < 0)
return ret;

Change get_user_pages_fast to return -EFAULT when supplied a kernel
address to make it match expectations.

All callers have been audited for consistency with the documented

Link: http://lkml.kernel.org/r/1522962072-182137-4-git-send-email-mst@xxxxxxxxxx
Fixes: 5b65c4677a57 ("mm, x86/mm: Fix performance regression in get_user_pages_fast()")
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Reported-by: syzbot+6304bf97ef436580fede@xxxxxxxxxxxxxxxxxxxxxxxxx
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Thorsten Leemhuis <regressions@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

mm/gup.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1816,9 +1816,12 @@ int get_user_pages_fast(unsigned long st
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;

+ if (nr_pages <= 0)
+ return 0;
if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
(void __user *)start, len)))
- return 0;
+ return -EFAULT;

if (gup_fast_permitted(start, nr_pages, write)) {