[PATCH] Fix rmmod/read/write races in /proc entries

[Alexey Dobriyan <adobriyan@openvz.org>]

--------------------------------------------------------
pde = create_proc_entry()
if (!pde)
	return -ENOMEM;
pde->write_proc = ...;
				open
				write
				copy_from_user
pde = create_proc_entry();
if (!pde) {
	remove_proc_entry();
	return -ENOMEM;
	/* module unloaded */
}
				*boom*
--------------------------------------------------------
pde = create_proc_entry();
if (pde) {
	/* which dereferences ->data */
	pde->write_proc = ...;
				open
				write
	pde->data = ...;
}
--------------------------------------------------------

The following plan is going to be executed (as per Al Viro's explanations):

PDE gets atomic counter counting reads and writes in progress
via ->read_proc, ->write_proc, ->get_info . Generic proc code will bump
PDE's counter before calling into module-specific method and decrement
it after it returns.

remove_proc_entry() will wait until all readers and writers are done.
To do this reliably it will set ->proc_fops to NULL and generic proc
code won't call into module it it sees NULL ->proc_fops.

This patch implements part above. So far, no changes in modules code
required. To proceed, lets look into ->proc_fops values during and after PDE
creation:

	pde = create_proc_entry();
	if (pde)
		pde->proc_fops = ...;

proc_create() create empty PDE;		(->proc_fops is NULL)
proc_register() glues PDE to /proc	(->proc_fops is NULL)
proc_register() sets ->proc_fops to default	(->proc_fops valid)
[ module sets ->proc_fops to it's own	(->proc_fops) valid ]

Observation: ->proc_fops is not NULL, when create_proc_entry() exits.

Next set of races come into play:

	pde = create_proc_entry();
	if (pde) {
		pde->read_proc = ...;
		pde->data = ...;
	}

Almost all ->read_proc, ->write_proc callbacks assume that ->data is valid when
they're called. They cast ->data and dereference it.

To fix this we need a way to indicate that PDE is not readable and writeable.
->proc_fops nicely fits, because modules setting ->proc_fops only don't need
changes at all. create_proc_entry() will exit with NULL ->proc_fops and helpers
will be needed sometimes to set it.

1. Module sets ->proc_fops only
	no changes
2. Module sets ->data and ->proc_fops
	use a helper to set ->data before ->proc_fops and a barrier.
2. Module uses only create_proc_read_entry()
	changes only in create_proc_read_entry();
3. Module uses only create_proc_info_entry()
	changes only in create_proc_info_entry();
4. Module sets combination of ->data, ->read_proc, ->write_proc
	use helper which will set fields, barrier and sets default ->proc_fops.
	the best name I've come up so far is

	void set_proc_entry_data_rw(struct proc_dir_entry *, void *, read_proc_t, write_proc_t);

Helper(s) will be introduced, then create_proc_entry() will start exiting with
NULL ->proc_fops. After that use of helper(s) will become mandatory. Grepping
for offenders will be easy (read_proc, ... are good names). ->data is bad
name, however, after helpers will be plugged we can rename ->data to
->pde_data and it'll become good name.

Sorry, for somewhat chaotic explanations, please, comment on patch and RFC.

Signed-off-by: Alexey Dobriyan <adobriyan@openvz.org>
---

 fs/proc/generic.c       |   32 ++++++++++++++++++++++++++++----
 include/linux/proc_fs.h |    2 ++
 2 files changed, 30 insertions(+), 4 deletions(-)

--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -19,6 +19,7 @@ #include <linux/init.h>
 #include <linux/idr.h>
 #include <linux/namei.h>
 #include <linux/bitops.h>
+#include <linux/delay.h>
 #include <linux/spinlock.h>
 #include <asm/uaccess.h>
 
@@ -76,6 +77,12 @@ proc_file_read(struct file *file, char _
 	if (!(page = (char*) __get_free_page(GFP_KERNEL)))
 		return -ENOMEM;
 
+	if (!dp->proc_fops)
+		goto out_free;
+	atomic_inc(&dp->pde_users);
+	if (!dp->proc_fops)
+		goto out_dec;
+
 	while ((nbytes > 0) && !eof) {
 		count = min_t(size_t, PROC_BLOCK_SIZE, nbytes);
 
@@ -195,6 +202,9 @@ proc_file_read(struct file *file, char _
 		buf += n;
 		retval += n;
 	}
+out_dec:
+	atomic_dec(&dp->pde_users);
+out_free:
 	free_page((unsigned long) page);
 	return retval;
 }
@@ -205,14 +215,20 @@ proc_file_write(struct file *file, const
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
 	struct proc_dir_entry * dp;
+	ssize_t rv;
 	
 	dp = PDE(inode);
 
-	if (!dp->write_proc)
+	if (!dp->write_proc || !dp->proc_fops)
 		return -EIO;
 
-	/* FIXME: does this routine need ppos?  probably... */
-	return dp->write_proc(file, buffer, count, dp->data);
+	rv = -EIO;
+	atomic_inc(&dp->pde_users);
+	if (dp->proc_fops)
+		/* FIXME: does this routine need ppos?  probably... */
+		rv = dp->write_proc(file, buffer, count, dp->data);
+	atomic_dec(&dp->pde_users);
+	return rv;
 }
 
 
@@ -717,12 +733,20 @@ void remove_proc_entry(const char *name,
 	if (!parent && xlate_proc_name(name, &parent, &fn) != 0)
 		goto out;
 	len = strlen(fn);
-
+again:
 	spin_lock(&proc_subdir_lock);
 	for (p = &parent->subdir; *p; p=&(*p)->next ) {
 		if (!proc_match(len, fn, *p))
 			continue;
 		de = *p;
+
+		de->proc_fops = NULL;
+		if (atomic_read(&de->pde_users) > 0) {
+			spin_unlock(&proc_subdir_lock);
+			msleep(1);
+			goto again;
+		}
+
 		*p = de->next;
 		de->next = NULL;
 		if (S_ISDIR(de->mode))
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -66,6 +66,8 @@ struct proc_dir_entry {
 	atomic_t count;		/* use count */
 	int deleted;		/* delete flag */
 	void *set;
+	atomic_t pde_users;	/* number of readers + number of writers via
+				 * ->read_proc, ->write_proc, ->get_info */
 };
 
 struct kcore_list {