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 caller have been audited for consistency with the documented semantics.

Lightly tested.

Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Huang Ying ying.huang@intel.com Cc: Jonathan Corbet corbet@lwn.net Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Thorsten Leemhuis regressions@leemhuis.info Cc: stable@vger.kernel.org Fixes: 5b65c4677a57 ("mm, x86/mm: Fix performance regression in get_user_pages_fast()") Reported-by: syzbot+6304bf97ef436580fede@syzkaller.appspotmail.com Signed-off-by: Michael S. Tsirkin mst@redhat.com --- 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)) { local_irq_disable();