Re: [PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 17 Apr 2025 13:44:48 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> > @@ -808,7 +809,9 @@ static __always_inline char *test_string(char *str)
> >         kstr = ubuf->buffer;
> >
> >         /* For safety, do not trust the string pointer */
> > -       if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
> > +       cnt = strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE);
> > +       /* Return null if empty string or error */
> > +       if (cnt <= 1)
> >                 return NULL;  
> 
> I wouldn't touch this part and leave it up to Steven to fix (if he
> agrees it needs fixing). Current logic seems wrong already, as it
> won't correctly handle -EFAULT. And, on the other hand, there is
> nothing wrong or special about empty string, so I don't think it needs
> special handling. Let's drop these changes in trace_events_filter.c?

Bah, it is wrong. I don't usually use filtering on strings much, but come
to think of it, the last time I tried, it didn't work, but I found another
way to get what I was looking for, and didn't look deeper into it.

I only care if it faulted or not. I don't care about it just copying zero
bytes. It should have been:

	if (strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE) < 0)

> 
> >         return kstr;
> >  }
> > @@ -818,6 +821,7 @@ static __always_inline char *test_ustring(char *str)
> >         struct ustring_buffer *ubuf;
> >         char __user *ustr;
> >         char *kstr;
> > +       int cnt;
> >
> >         if (!ustring_per_cpu)
> >                 return NULL;
> > @@ -827,7 +831,9 @@ static __always_inline char *test_ustring(char *str)
> >
> >         /* user space address? */
> >         ustr = (char __user *)str;
> > -       if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))

This is broken too.

As this isn't relying on the other change in this patch, I'll just fix it
myself. I'm getting a pull request ready anyway.

Thanks!

-- Steve


> > +       cnt = strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE);
> > +       /* Return null if empty string or error */
> > +       if (cnt <= 1)
> >                 return NULL;  
> 
> ditto
> 
> >