linux-kernel - Re: [PATCH 1/1] fs: xattr: Don't display attributes without read access

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Date:	Wed, 19 Feb 2014 06:43:46 +0800
From:	Fabian Frederick <fabf@...net.be>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] fs: xattr: Don't display attributes without read
 access

On Tue, 18 Feb 2014 15:16:50 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Sun, 16 Feb 2014 20:31:01 +0800 Fabian Frederick <fabf@...net.be> wrote:
> 
> > Any user can display extented attribute names without read
> > access.
> > 
> > eg: attr -l <filename>
> > 
> > This patch checks inode_permission in listxattr common
> > function before executing vfs_listxattr.
> > 
> > ...
> >
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -543,6 +543,10 @@ listxattr(struct dentry *d, char __user *list, size_t size)
> >  	char *klist = NULL;
> >  	char *vlist = NULL;	/* If non-NULL, we used vmalloc() */
> >  
> > +	error = inode_permission(d->d_inode, MAY_READ);
> > +	if (error)
> > +		return error;
> > +
> >  	if (size) {
> >  		if (size > XATTR_LIST_MAX)
> >  			size = XATTR_LIST_MAX;
> 
> erk.  Doesn't this mean that if existing userspace is relying on the
> current behaviour, this patch will cause breakage?
> 
IMHO userspace applications already receive weird results in that case.

Without read permission, attr -l receives attribute names which means
it tries lgetxattr on those attributes where result is EACCESS :
"Attribute <attribute name> has -1 byte" !!!

Besides, is it semantically correct for a user to have access to
 a part of "file content" without read access ?
 
With that patch, attr -l <filename> displays

attr_list: Permission denied
Could not list "(null)" for <filename>

On the other hand, when stracing that situation, 
I do see attr making more noise than usual ie it's opening all attr.mo 
twice so I guess I should return something else than "permission denied" to avoid problems in userspace ...

Fabian

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