Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper

On 2019-09-25, Linus Torvalds [off-list ref] wrote:

On Wed, Sep 25, 2019 at 10:00 AM Aleksa Sarai [off-list ref] wrote:

quoted

+int is_zeroed_user(const void __user *from, size_t size)

I like how you've done this, but it's buggy and only works on 64-bit.

All the "u64" and "8" cases need to be "unsigned long" and
"sizeof(unsigned long)".

Part of that requirement is:

quoted

+               unsafe_get_user(val, (u64 __user *) from, err_fault);

This part works fine - although 64-bit accesses migth be much more
expensive and the win of unrolling might not be sufficient - but:

quoted

+               if (align) {
+                       /* @from is unaligned. */
+                       val &= ~aligned_byte_mask(align);
+                       align = 0;
+               }

This part fundamentally only works on 'unsigned long'.

Just to make sure I understand, the following diff would this solve the
problem? If so, I'll apply it, and re-send in a few hours.

--8<--------------------------------------------------------------------------

 int is_zeroed_user(const void __user *from, size_t size)
 {
-       u64 val;
-       uintptr_t align = (uintptr_t) from % 8;
+       unsigned long val;
+       uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
 
        if (unlikely(!size))
                return true;

@@ -150,8 +150,8 @@ int is_zeroed_user(const void __user *from, size_t size)
        if (!user_access_begin(from, size))
                return -EFAULT;
 
-       while (size >= 8) {
-               unsafe_get_user(val, (u64 __user *) from, err_fault);
+       while (size >= sizeof(unsigned long)) {
+               unsafe_get_user(val, (unsigned long __user *) from, err_fault);
                if (align) {
                        /* @from is unaligned. */
                        val &= ~aligned_byte_mask(align);

@@ -159,12 +159,12 @@ int is_zeroed_user(const void __user *from, size_t size)
                }
                if (val)
                        goto done;
-               from += 8;
-               size -= 8;
+               from += sizeof(unsigned long);
+               size -= sizeof(unsigned long);
        }
        if (size) {
                /* (@from + @size) is unaligned. */
-               unsafe_get_user(val, (u64 __user *) from, err_fault);
+               unsafe_get_user(val, (unsigned long __user *) from, err_fault);
                val &= aligned_byte_mask(size);
        }

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>