linux-kernel - Re: [PATCH] 9p: cap xattr max size to XATTR_SIZE

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Date: Mon, 4 Mar 2024 08:35:46 -0600
From: Seth Forshee <sforshee@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Christian Schoenebeck <linux_oss@...debyte.com>,
	Eric Van Hensbergen <ericvh@...nel.org>,
	Latchesar Ionkov <lucho@...kov.net>,
	Dominique Martinet <asmadeus@...ewreck.org>, v9fs@...ts.linux.dev,
	linux-kernel@...r.kernel.org, xingwei lee <xrivendell7@...il.com>,
	sam sun <samsun1006219@...il.com>
Subject: Re: [PATCH] 9p: cap xattr max size to XATTR_SIZE_MAX

On Mon, Mar 04, 2024 at 03:19:58PM +0100, Christian Brauner wrote:
> On Mon, Mar 04, 2024 at 02:35:07PM +0100, Christian Schoenebeck wrote:
> > On Monday, March 4, 2024 1:42:43 PM CET Dominique Martinet wrote:
> > > We probably shouldn't ever get an xattr bigger than that, and the current check
> > > of SSIZE_MAX is a bit too large.
> > 
> > Maybe, OTOH e.g. ACLs (dynamic size) are implemented by storing them as xattrs
> > on 9p server as well, and this change somewhat expects server to run Linux as
> > well. So maybe s/XATTR_SIZE_MAX/KMALLOC_MAX_SIZE/ might be more appropriate,
> > considering that this patch is about fixing a potential kmalloc() warning?
> > 
> > Worth to mention in the commit log BTW what the issue was.
> > 
> > /Christian
> 
> So the error is somewhat specific to filesystem capabilities which also
> live in the xattr apis but Seth is working to get rid of them in there.
> 
> They currently use a special api vfs_getxattr_alloc() which is an
> in-kernel api that does a racy query-size+allocate-buffer+retrieve-data
> dance.

Yes, the patches I've sent does use vfs_getxattr_alloc() for fscaps
anymore.

> That api is used for fscaps, security labels, and other xattrs. And that
> api doesn't do any size checks which probably should also be fixed now
> that I write this.
> 
> @Seth?

I agree. I don't see any reason that vfs_getxattr_alloc() shouldn't just
bail out with an error if the size of the xattr is >= XATTR_SIZE_MAX.