mm/gup: fix wrongly calculated returned value in fault_in_safe_writeable()

commit 8c03ebd7cdc06bd0d2fecb4d1a609ef1dbb7d0aa upstream.

Not like fault_in_readable() or fault_in_writeable(), in
fault_in_safe_writeable() local variable 'start' is increased page by page
to loop till the whole address range is handled.  However, it mistakenly
calculates the size of the handled range with 'uaddr - start'.

Fix it here.

Andreas said:

: In gfs2, fault_in_iov_iter_writeable() is used in
: gfs2_file_direct_read() and gfs2_file_read_iter(), so this potentially
: affects buffered as well as direct reads.  This bug could cause those
: gfs2 functions to spin in a loop.

Link: https://lkml.kernel.org/r/20250410035717.473207-1-bhe@redhat.com
Link: https://lkml.kernel.org/r/20250410035717.473207-2-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Fixes: fe673d3f5bf1 ("mm: gup: make fault_in_safe_writeable() use fixup_user_fault()")
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Yanjun.Zhu <yanjun.zhu@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 69d259f7bf37ecfe0a7e901d3b193f0117fe29a1..29c719b3ab31e10b4dae27077bfdddfbe3a09bf1 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1871,8 +1871,8 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
        } while (start != end);
        mmap_read_unlock(mm);
 
-       if (size > (unsigned long)uaddr - start)
-               return size - ((unsigned long)uaddr - start);
+       if (size > start - (unsigned long)uaddr)
+               return size - (start - (unsigned long)uaddr);
        return 0;
 }
 EXPORT_SYMBOL(fault_in_safe_writeable);