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

commit 8c03ebd
Author: Baoquan He <bhe@redhat.com>
Date:   Thu Apr 10 11:57:14 2025 +0800

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

    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: fe673d3 ("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>

JIRA: https://issues.redhat.com/browse/RHEL-77742
Signed-off-by: Nico Pache <npache@redhat.com>

Author: PacheNico

mm/gup.c

@@ -2210,8 +2210,8 @@ 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);