Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps

[Robert Foss]

On 2016-08-09 03:24 PM, Jann Horn wrote:
> On Tue, Aug 09, 2016 at 12:05:43PM -0400, robert.foss@xxxxxxxxxxxxx wrote:
> > From: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> >
> > This is based on earlier work by Thiago Goncales. It implements a new
> > per process proc file which summarizes the contents of the smaps file
> > but doesn't display any addresses.  It gives more detailed information
> > than statm like the PSS (proprotional set size).  It differs from the
> > original implementation in that it doesn't use the full blown set of
> > seq operations, uses a different termination condition, and doesn't
> > displayed "Locked" as that was broken on the original implemenation.
> >
> > This new proc file provides information faster than parsing the potentially
> > huge smaps file.
> >
> > Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> >
> > Tested-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
> > Signed-off-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
>
>
> > +static int totmaps_proc_show(struct seq_file *m, void *data)
> > +{
> > +	struct proc_maps_private *priv = m->private;
> > +	struct mm_struct *mm;
> > +	struct vm_area_struct *vma;
> > +	struct mem_size_stats *mss_sum = priv->mss;
> > +
> > +	/* reference to priv->task already taken */
> > +	/* but need to get the mm here because */
> > +	/* task could be in the process of exiting */
>
> Can you please elaborate on this? My understanding here is that you
> intend for the caller to be able to repeatedly read the same totmaps
> file with pread() and still see updated information after the target
> process has called execve() and be able to detect process death
> (instead of simply seeing stale values). Is that accurate?
>
> I would prefer it if you could grab a reference to the mm_struct
> directly at open time.

Sonny, do you know more about the above comment?

> > +	mm = get_task_mm(priv->task);
> > +	if (!mm || IS_ERR(mm))
> > +		return -EINVAL;
>
> get_task_mm() doesn't return error codes, and all other callers just
> check whether the return value is NULL.

I'll have that fixed in v2, thanks for spotting it!

> > +	down_read(&mm->mmap_sem);
> > +	hold_task_mempolicy(priv);
> > +
> > +	for (vma = mm->mmap; vma != priv->tail_vma; vma = vma->vm_next) {
> > +		struct mem_size_stats mss;
> > +		struct mm_walk smaps_walk = {
> > +			.pmd_entry = smaps_pte_range,
> > +			.mm = vma->vm_mm,
> > +			.private = &mss,
> > +		};
> > +
> > +		if (vma->vm_mm && !is_vm_hugetlb_page(vma)) {
> > +			memset(&mss, 0, sizeof(mss));
> > +			walk_page_vma(vma, &smaps_walk);
> > +			add_smaps_sum(&mss, mss_sum);
> > +		}
> > +	}
>
> Errrr... what? You accumulate values from mem_size_stats items into a
> struct mss_sum that is associated with the struct file? So when you
> read the file the second time, you get the old values plus the new ones?
> And when you read the file in parallel, you get inconsistent values?
>
> For most files in procfs, the behavior is that you can just call
> pread(fd, buf, sizeof(buf), 0) on the same fd again and again, giving
> you the current values every time, without mutating state. I strongly
> recommend that you get rid of priv->mss and just accumulate the state
> in a local variable (maybe one on the stack).

So a simple "static struct mem_size_stats" in totmaps_proc_show() would 
be a better solution?

> > @@ -836,6 +911,50 @@ static int tid_smaps_open(struct inode *inode, struct file *file)
> >  	return do_maps_open(inode, file, &proc_tid_smaps_op);
> >  }
> >
> > +static int totmaps_open(struct inode *inode, struct file *file)
> > +{
> > +	struct proc_maps_private *priv;
> > +	int ret = -ENOMEM;
> > +	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> > +	if (priv) {
> > +		priv->mss = kzalloc(sizeof(*priv->mss), GFP_KERNEL);
> > +		if (!priv->mss)
> > +			return -ENOMEM;
>
> Memory leak: If the first allocation works and the second one doesn't, this
> doesn't free the first allocation.
>
> Please change this to use the typical goto pattern for error handling.

Fix will be implemented in v2.

> > +
> > +		/* we need to grab references to the task_struct */
> > +		/* at open time, because there's a potential information */
> > +		/* leak where the totmaps file is opened and held open */
> > +		/* while the underlying pid to task mapping changes */
> > +		/* underneath it */
>
> Nit: That's not how comments are done in the kernel. Maybe change this to
> a normal block comment instead of one block comment per line?

I'm not sure how that one slipped by, but I'll change it in v2.

> > +		priv->task = get_pid_task(proc_pid(inode), PIDTYPE_PID);
>
> `get_pid_task(proc_pid(inode), PIDTYPE_PID)` is exactly the definition
> of get_proc_task(inode), maybe use that instead?

Will do. v2 will fix this.

> > +		if (!priv->task) {
> > +			kfree(priv->mss);
> > +			kfree(priv);
> > +			return -ESRCH;
> > +		}
> > +
> > +		ret = single_open(file, totmaps_proc_show, priv);
> > +		if (ret) {
> > +			put_task_struct(priv->task);
> > +			kfree(priv->mss);
> > +			kfree(priv);
> > +		}
> > +	}
> > +	return ret;
> > +}
>
> Please change this method to use the typical goto pattern for error
> handling. IMO repeating the undo steps in all error cases makes
> mistakes (like the one above) more likely and increases the amount
> of redundant code.

Agreed. Change queued for v2.

> Also: The smaps file is only accessible to callers with
> PTRACE_MODE_READ privileges on the target task. Your thing doesn't
> do any access checks, neither in the open handler nor in the read
> handler. Can you give an analysis of why it's okay to expose this
> data? As far as I can tell, without spending a lot of time thinking
> about it, this kind of data looks like it might potentially be
> useful for side-channel information leaks or so.

I think it should require the same permissions as smaps, so changing the 
code to require PTRACE_MODE_READ privileges is most likely a good idea. 
I'll have a look at it for v2.