Fix rmmod/read/write races in /proc entries

Fix following races:
===========================================
1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
   meanwhile. Or, more generically, system call done on /proc file, method
   supplied by module is called, module dissapeares meanwhile.

   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*
==========================================
2. bogo-revoke aka proc_kill_inodes()

  remove_proc_entry		vfs_read
  proc_kill_inodes		[check ->f_op validness]
				[check ->f_op->read validness]
				[verify_area, security permissions checks]
	->f_op = NULL;
				if (file->f_op->read)
					/* ->f_op dereference, boom */

NOTE, NOTE, NOTE: file_operations are proxied for regular files only. Let's
see how this scheme behaves, then extend if needed for directories.
Directories creators in /proc only set ->owner for them, so proxying for
directories may be unneeded.

NOTE, NOTE, NOTE: methods being proxied are ->llseek, ->read, ->write,
->poll, ->unlocked_ioctl, ->ioctl, ->compat_ioctl, ->open, ->release.
If your in-tree module uses something else, yell on me. Full audit pending.

[akpm@linux-foundation.org: build fix]
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 31 insertions, 1 deletions

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 8a40e15f5ec..4f8e53568b2 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -20,6 +20,7 @@
 #include <linux/namei.h>
 #include <linux/bitops.h>
 #include <linux/spinlock.h>
+#include <linux/completion.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -613,6 +614,9 @@ static struct proc_dir_entry *proc_create(struct proc_dir_entry **parent,
 	ent->namelen = len;
 	ent->mode = mode;
 	ent->nlink = nlink;
+	ent->pde_users = 0;
+	spin_lock_init(&ent->pde_unload_lock);
+	ent->pde_unload_completion = NULL;
  out:
 	return ent;
 }
@@ -734,9 +738,35 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 		de = *p;
 		*p = de->next;
 		de->next = NULL;
+
+		spin_lock(&de->pde_unload_lock);
+		/*
+		 * Stop accepting new callers into module. If you're
+		 * dynamically allocating ->proc_fops, save a pointer somewhere.
+		 */
+		de->proc_fops = NULL;
+		/* Wait until all existing callers into module are done. */
+		if (de->pde_users > 0) {
+			DECLARE_COMPLETION_ONSTACK(c);
+
+			if (!de->pde_unload_completion)
+				de->pde_unload_completion = &c;
+
+			spin_unlock(&de->pde_unload_lock);
+			spin_unlock(&proc_subdir_lock);
+
+			wait_for_completion(de->pde_unload_completion);
+
+			spin_lock(&proc_subdir_lock);
+			goto continue_removing;
+		}
+		spin_unlock(&de->pde_unload_lock);
+
+continue_removing:
 		if (S_ISDIR(de->mode))
 			parent->nlink--;
-		proc_kill_inodes(de);
+		if (!S_ISREG(de->mode))
+			proc_kill_inodes(de);
 		de->nlink = 0;
 		WARN_ON(de->subdir);
 		if (!atomic_read(&de->count))