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Re: [Cluster-devel] [RFC PATCH 5/5] gfs2: Add xreaddir file operation and supporting functions



On Tue, 29 Jul 2014 18:25:57 -0400 (EDT)
Abhijith Das <adas redhat com> wrote:

> > > +	if ((xc->xc_xattr_mask & XSTAT_XATTR_ALL) &&
> > > +		lxd->xd_blob.xb_xattr_count) {  
> > 
> > How can that be right?  lxd is __user, it doesn't seem right to be
> > dereferencing it directly...?  
> 
> Wouldn't the call to access_ok() at the start of the syscall take care of this? All the
> __user pointers point to areas within the user supplied buffer buf and overflow past the
> end of the buffer for the last lxd is checked for.

No, dereferencing user-space pointers in the kernel is never OK.  What
if user space remapped that page after the access_ok() call?  You need
to use copy_*_user() to get at user-space structures from the kernel.

jon


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