Re: linux-next: Tree for Oct 18 (objtool)

[Peter Zijlstra]

On Fri, Oct 18, 2019 at 08:33:11AM -0700, Randy Dunlap wrote:
> On 10/18/19 12:03 AM, Stephen Rothwell wrote:
> > Hi all,
> > 
> > Changes since 20191017:
> > 
> 
> on x86_64:
> lib/usercopy.o: warning: objtool: check_zeroed_user()+0x35f: call to 
> __ubsan_handle_shift_out_of_bounds() with UACCESS enabled

Blergh... I suppose the below will fix that. I'm a bit conflicted on it
though, the alternative is annotating more ubsan crud.

---
 lib/usercopy.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/lib/usercopy.c b/lib/usercopy.c
index cbb4d9ec00f2..9c5245778dfd 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -49,21 +49,28 @@ EXPORT_SYMBOL(_copy_to_user);
  */
 int check_zeroed_user(const void __user *from, size_t size)
 {
+       unsigned long head_mask, tail_mask;
        unsigned long val;
-       uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+       uintptr_t align;
 
        if (unlikely(size == 0))
                return 1;
 
-       from -= align;
-       size += align;
+       align = (uintptr_t) from % sizeof(unsigned long);
+       if (align) {
+               from -= align;
+               size += align;
+               head_mask = ~aligned_byte_mask(align);
+       }
+
+       tail_mask = aligned_byte_mask(size % sizeof(unsigned long));
 
        if (!user_access_begin(from, size))
                return -EFAULT;
 
        unsafe_get_user(val, (unsigned long __user *) from, err_fault);
        if (align)
-               val &= ~aligned_byte_mask(align);
+               val &= head_mask;
 
        while (size > sizeof(unsigned long)) {
                if (unlikely(val))
@@ -76,7 +83,7 @@ int check_zeroed_user(const void __user *from, size_t size)
        }
 
        if (size < sizeof(unsigned long))
-               val &= aligned_byte_mask(size);
+               val &= tail_mask;
 
 done:
        user_access_end();