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Date: Tue, 8 Jun 2021 10:03:40 -0700
From: Kees Cook <keescook@chromium.org>
To: Andrea Righi <andrea.righi@canonical.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	regressions@lists.linux.dev
Subject: Re: Regression when writing to /proc/<pid>/attr/
Message-ID: <202106081003.2BECE93@keescook>
References: <20210607142245.eikvyeacqwwu6dn3@wittgenstein>
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In-Reply-To: <YL8Rwb1Pnbdwk0xc@xps-13-7390>

On Tue, Jun 08, 2021 at 08:44:17AM +0200, Andrea Righi wrote:
> On Mon, Jun 07, 2021 at 07:15:36PM -0700, Kees Cook wrote:
> > On Mon, Jun 07, 2021 at 05:02:28PM -0700, Linus Torvalds wrote:
> > > On Mon, Jun 7, 2021 at 4:38 PM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > I'm assuming the issue is the latter (open, drop privs, write). And
> > > > I assume fsuid/fsgid has changed? (i.e. cred_fscmp() couldn't be used
> > > > either?)
> > > 
> > > Hmm. Do we have some place to hide self_exec_id at open time, and then
> > > just verify that it's still the same at IO time?
> > > 
> > > IOW, replace that f_cred comparison with a "self_exec_id has not
> > > changed" comparison instead?
> > 
> > I think we can't use self_exec_id because the original flaw could just
> > be changed to have the parent open two children (the fd opener and the
> > victim), which would have the same self_exec_id.
> > 
> > > Perhaps squirrel it away in file->f_private? Or are we already using
> > > that (didn't check)?
> > 
> > But we can do tracking via file->private_data since the attr files don't
> > use a custom opener. I think an mm_struct comparison is likely what's
> > needed here? (This is actually what several of these special proc files
> > are already doing, but they actually _use_ mm_struct.)
> > 
> > UNTESTED:
> 
> Thanks Kees! Comparing mm_struct makes sense to me. With this applied
> the regression that I was experiencing with containers doesn't happen
> anymore.
> 
> I'll run more stress tests with this, but for now it looks good to me,
> so FWIW:
> 
> Tested-by: Andrea Righi <andrea.righi@canonical.com>

Oh good! Thanks for testing and sorry for the extra work I created! I'll
get this spun up into a proper patch.

-Kees

> 
> > 
> > 
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 58bbf334265b..7118ebe38fa6 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -2674,6 +2674,11 @@ static int proc_pident_readdir(struct file *file, struct dir_context *ctx,
> >  }
> >  
> >  #ifdef CONFIG_SECURITY
> > +static int proc_pid_attr_open(struct inode *inode, struct file *file)
> > +{
> > +	return __mem_open(inode, file, PTRACE_MODE_READ_FSCREDS);
> > +}
> > +
> >  static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,
> >  				  size_t count, loff_t *ppos)
> >  {
> > @@ -2704,7 +2709,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
> >  	int rv;
> >  
> >  	/* A task may only write when it was the opener. */
> > -	if (file->f_cred != current_real_cred())
> > +	if (file->private_data != current->mm)
> >  		return -EPERM;
> >  
> >  	rcu_read_lock();
> > @@ -2754,9 +2759,11 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
> >  }
> >  
> >  static const struct file_operations proc_pid_attr_operations = {
> > +	.open		= proc_pid_attr_open,
> >  	.read		= proc_pid_attr_read,
> >  	.write		= proc_pid_attr_write,
> >  	.llseek		= generic_file_llseek,
> > +	.release	= mem_release,
> >  };
> >  
> >  #define LSM_DIR_OPS(LSM) \
> > 
> > -- 
> > Kees Cook

-- 
Kees Cook