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Subject[PATCH v2 2/3] gup: return -EFAULT on access_ok failure
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

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 caller have been audited for consistency with the documented

Lightly tested.

Cc: Kirill A. Shutemov <>
Cc: Andrew Morton <>
Cc: Huang Ying <>
Cc: Jonathan Corbet <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Cc: Thorsten Leemhuis <>
Fixes: 5b65c4677a57 ("mm, x86/mm: Fix performance regression in get_user_pages_fast()")
Signed-off-by: Michael S. Tsirkin <>
mm/gup.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 6afae32..8f3a064 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1806,9 +1806,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
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)) {
 \ /
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