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Date:   Wed, 31 Mar 2021 23:52:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Lee Duncan <lduncan@...e.com>, Chris Leech <cleech@...hat.com>,
        Adam Nichols <adam@...mm-co.com>,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sysfs: Unconditionally use vmalloc for buffer

On Thu, Apr 01, 2021 at 07:16:56AM +0200, Greg Kroah-Hartman wrote:
> On Wed, Mar 31, 2021 at 07:21:45PM -0700, Kees Cook wrote:
> > The sysfs interface to seq_file continues to be rather fragile
> > (seq_get_buf() should not be used outside of seq_file), as seen with
> > some recent exploits[1]. Move the seq_file buffer to the vmap area
> > (while retaining the accounting flag), since it has guard pages that
> > will catch and stop linear overflows. This seems justified given that
> > sysfs's use of seq_file already uses kvmalloc(), is almost always using
> > a PAGE_SIZE or larger allocation, has normally short-lived allocations,
> > and is not normally on a performance critical path.
> > 
> > Once seq_get_buf() has been removed (and all sysfs callbacks using
> > seq_file directly), this change can also be removed.
> > 
> > [1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> > v3:
> > - Limit to only sysfs (instead of all of seq_file).
> > v2: https://lore.kernel.org/lkml/20210315174851.622228-1-keescook@chromium.org/
> > v1: https://lore.kernel.org/lkml/20210312205558.2947488-1-keescook@chromium.org/
> > ---
> >  fs/sysfs/file.c | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> > 
> > diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> > index 9aefa7779b29..70e7a450e5d1 100644
> > --- a/fs/sysfs/file.c
> > +++ b/fs/sysfs/file.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/mutex.h>
> >  #include <linux/seq_file.h>
> >  #include <linux/mm.h>
> > +#include <linux/vmalloc.h>
> >  
> >  #include "sysfs.h"
> >  
> > @@ -32,6 +33,25 @@ static const struct sysfs_ops *sysfs_file_ops(struct kernfs_node *kn)
> >  	return kobj->ktype ? kobj->ktype->sysfs_ops : NULL;
> >  }
> >  
> > +/*
> > + * To be proactively defensive against sysfs show() handlers that do not
> > + * correctly stay within their PAGE_SIZE buffer, use the vmap area to gain
> > + * the trailing guard page which will stop linear buffer overflows.
> > + */
> > +static void *sysfs_kf_seq_start(struct seq_file *sf, loff_t *ppos)
> > +{
> > +	struct kernfs_open_file *of = sf->private;
> > +	struct kernfs_node *kn = of->kn;
> > +
> > +	WARN_ON_ONCE(sf->buf);
> 
> How can buf ever not be NULL?  And if it is, we will leak memory in the
> next line so we shouldn't have _ONCE, we should always know, but not
> rebooting the machine would be nice.

It should never be possible. I did this because seq_file has some
unusual buf allocation patterns in the kernel, and I liked the cheap
leak check. I use _ONCE because spewing endlessly doesn't help most
cases. And if you want to trigger it again, you don't have to reboot:
https://www.kernel.org/doc/html/latest/admin-guide/clearing-warn-once.html

> 
> > +	sf->buf = __vmalloc(kn->attr.size, GFP_KERNEL_ACCOUNT);
> > +	if (!sf->buf)
> > +		return ERR_PTR(-ENOMEM);
> > +	sf->size = kn->attr.size;
> > +
> > +	return NULL + !*ppos;
> > +}
> 
> Will this also cause the vmalloc fragmentation/abuse that others have
> mentioned as userspace can trigger this?

If I understood the concern correctly, it was about it being a risk for
doing it for all seq_file uses. This version confines the changes to only
sysfs seq_file uses.

> And what code frees it?

The existing hooks to seq_release() handle this already. This kind of
"preallocation" of the seq_file buffer is done in a few places already
(hence my desire for the sanity checking WARN lest future seq_file
semantics change).

-- 
Kees Cook

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