Re: copy_from_user

[Andrew Morton <akpm@digeo.com>]

Paul Mackerras <paulus@samba.org> wrote:
>
> Some time ago (in the 2.1 series IIRC) we added code to copy_from_user
>  to zero the remainder of the destination buffer if we faulted on the
>  source.  The motive was to eliminate some potential security holes
>  that could arise if callers didn't check the return value from
>  copy_from_user and continued on to pass the contents of the
>  destination buffer back to userspace in one way or another.
> 
>  However, I notice that copy_from_user on i386 in 2.5 doesn't clear the
>  destination if the access_ok() check fails,

This was not deliberate - the memset simply got lost.

It is simple enough to fix.  Do we remember the details of the
security hole?

> or if the size is 1, 2 or 4.

This one is OK - __get_user_asm() does the zeroing in the fixup code.