Fix 'get_user_pages_fast()' with non-page-aligned start address

Alexey Dobriyan reported trouble with LTP with the new fast-gup code,
and Johannes Weiner debugged it to non-page-aligned addresses, where the
new get_user_pages_fast() code would do all the wrong things, including
just traversing past the end of the requested area due to 'addr' never
matching 'end' exactly.

This is not a pretty fix, and we may actually want to move the alignment
into generic code, leaving just the core code per-arch, but Alexey
verified that the vmsplice01 LTP test doesn't crash with this.

Reported-and-tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Debugged-by: Johannes Weiner <hannes@saeurebad.de>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 3085f25b4355..007bb06c7504 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -223,14 +223,17 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
                         struct page **pages)
 {
         struct mm_struct *mm = current->mm;
-        unsigned long end = start + (nr_pages << PAGE_SHIFT);
-        unsigned long addr = start;
+        unsigned long addr, len, end;
         unsigned long next;
         pgd_t *pgdp;
         int nr = 0;
 
+        start &= PAGE_MASK;
+        addr = start;
+        len = (unsigned long) nr_pages << PAGE_SHIFT;
+        end = start + len;
         if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
-                                        start, nr_pages*PAGE_SIZE)))
+                                        start, len)))
                 goto slow_irqon;
 
         /*