Re: [PATCH v3] sysfs: Unconditionally use vmalloc for buffer

[Kees Cook <keescook@chromium.org>]

On Thu, Apr 01, 2021 at 09:14:25AM +0200, Michal Hocko wrote:
> On Wed 31-03-21 19:21:45, 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.
> 
> I thought the previous discussion has led to a conclusion that the
> preferred way is to disallow direct seq_file buffer usage. But this is
> obviously up to sysfs maintainers. I am happy you do not want to spread
> this out to all seq_file users anymore.

Yeah; I still want to remove external seq_get_buf(), but that'll take
time. I'll be doing the work, though, since I still need access to
f_cred for show() access control checks.

> > 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.
> 
> Let me clarify on this, because this is not quite right. kvmalloc vs
> vmalloc (both with GFP_KERNEL) on PAGE_SIZE are two different beasts.
> The first one is almost always going to use kmalloc because the page
> allocator almost never fails those requests.

Yes, good point. I will adjust my changelog.

Thanks!

-Kees

> 
> > 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@chromium.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);
> > +	sf->buf = __vmalloc(kn->attr.size, GFP_KERNEL_ACCOUNT);
> > +	if (!sf->buf)
> > +		return ERR_PTR(-ENOMEM);
> > +	sf->size = kn->attr.size;
> > +
> > +	return NULL + !*ppos;
> > +}
> > +
> >  /*
> >   * Reads on sysfs are handled through seq_file, which takes care of hairy
> >   * details like buffering and seeking.  The following function pipes
> > @@ -206,14 +226,17 @@ static const struct kernfs_ops sysfs_file_kfops_empty = {
> >  };
> >  
> >  static const struct kernfs_ops sysfs_file_kfops_ro = {
> > +	.seq_start	= sysfs_kf_seq_start,
> >  	.seq_show	= sysfs_kf_seq_show,
> >  };
> >  
> >  static const struct kernfs_ops sysfs_file_kfops_wo = {
> > +	.seq_start	= sysfs_kf_seq_start,
> >  	.write		= sysfs_kf_write,
> >  };
> >  
> >  static const struct kernfs_ops sysfs_file_kfops_rw = {
> > +	.seq_start	= sysfs_kf_seq_start,
> >  	.seq_show	= sysfs_kf_seq_show,
> >  	.write		= sysfs_kf_write,
> >  };
> > -- 
> > 2.25.1
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Kees Cook