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

From: Peter Zijlstra <[email-masked]>
Date: Mon, 21 Oct 2019 14:35:49 +0200

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();