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

Aleksa Sarai
2019-09-25T17:20:49Z

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On 2019-09-25, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, Sep 25, 2019 at 10:00 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
> >
> > +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:
>
> > + 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:
>
> > + 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/>

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