debugfs: prevent access to possibly dead file_operations at file open

Nothing prevents a dentry found by path lookup before a return of
__debugfs_remove() to actually get opened after that return. Now, after
the return of __debugfs_remove(), there are no guarantees whatsoever
regarding the memory the corresponding inode's file_operations object
had been kept in.

Since __debugfs_remove() is seldomly invoked, usually from module exit
handlers only, the race is hard to trigger and the impact is very low.

A discussion of the problem outlined above as well as a suggested
solution can be found in the (sub-)thread rooted at

  http://lkml.kernel.org/g/20130401203445.GA20862@ZenIV.linux.org.uk
  ("Yet another pipe related oops.")

Basically, Greg KH suggests to introduce an intermediate fops and
Al Viro points out that a pointer to the original ones may be stored in
->d_fsdata.

Follow this line of reasoning:
- Add SRCU as a reverse dependency of DEBUG_FS.
- Introduce a srcu_struct object for the debugfs subsystem.
- In debugfs_create_file(), store a pointer to the original
  file_operations object in ->d_fsdata.
- Make debugfs_remove() and debugfs_remove_recursive() wait for a
  SRCU grace period after the dentry has been delete()'d and before they
  return to their callers.
- Introduce an intermediate file_operations object named
  "debugfs_open_proxy_file_operations". It's ->open() functions checks,
  under the protection of a SRCU read lock, whether the dentry is still
  alive, i.e. has not been d_delete()'d and if so, tries to acquire a
  reference on the owning module.
  On success, it sets the file object's ->f_op to the original
  file_operations and forwards the ongoing open() call to the original
  ->open().
- For clarity, rename the former debugfs_file_operations to
  debugfs_noop_file_operations -- they are in no way canonical.

The choice of SRCU over "normal" RCU is justified by the fact, that the
former may also be used to protect ->i_private data from going away
during the execution of a file's readers and writers which may (and do)
sleep.

Finally, introduce the fs/debugfs/internal.h header containing some
declarations internal to the debugfs implementation.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 90 insertions, 1 deletions

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index d2ba12e23ed9..736ab3c988f2 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -22,6 +22,9 @@
 #include <linux/slab.h>
 #include <linux/atomic.h>
 #include <linux/device.h>
+#include <linux/srcu.h>
+
+#include "internal.h"
 
 static ssize_t default_read_file(struct file *file, char __user *buf,
 				 size_t count, loff_t *ppos)
@@ -35,13 +38,99 @@ static ssize_t default_write_file(struct file *file, const char __user *buf,
 	return count;
 }
 
-const struct file_operations debugfs_file_operations = {
+const struct file_operations debugfs_noop_file_operations = {
 	.read =		default_read_file,
 	.write =	default_write_file,
 	.open =		simple_open,
 	.llseek =	noop_llseek,
 };
 
+/**
+ * debugfs_use_file_start - mark the beginning of file data access
+ * @dentry: the dentry object whose data is being accessed.
+ * @srcu_idx: a pointer to some memory to store a SRCU index in.
+ *
+ * Up to a matching call to debugfs_use_file_finish(), any
+ * successive call into the file removing functions debugfs_remove()
+ * and debugfs_remove_recursive() will block. Since associated private
+ * file data may only get freed after a successful return of any of
+ * the removal functions, you may safely access it after a successful
+ * call to debugfs_use_file_start() without worrying about
+ * lifetime issues.
+ *
+ * If -%EIO is returned, the file has already been removed and thus,
+ * it is not safe to access any of its data. If, on the other hand,
+ * it is allowed to access the file data, zero is returned.
+ *
+ * Regardless of the return code, any call to
+ * debugfs_use_file_start() must be followed by a matching call
+ * to debugfs_use_file_finish().
+ */
+static int debugfs_use_file_start(const struct dentry *dentry, int *srcu_idx)
+	__acquires(&debugfs_srcu)
+{
+	*srcu_idx = srcu_read_lock(&debugfs_srcu);
+	barrier();
+	if (d_unlinked(dentry))
+		return -EIO;
+	return 0;
+}
+
+/**
+ * debugfs_use_file_finish - mark the end of file data access
+ * @srcu_idx: the SRCU index "created" by a former call to
+ *            debugfs_use_file_start().
+ *
+ * Allow any ongoing concurrent call into debugfs_remove() or
+ * debugfs_remove_recursive() blocked by a former call to
+ * debugfs_use_file_start() to proceed and return to its caller.
+ */
+static void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu)
+{
+	srcu_read_unlock(&debugfs_srcu, srcu_idx);
+}
+
+#define F_DENTRY(filp) ((filp)->f_path.dentry)
+
+#define REAL_FOPS_DEREF(dentry)					\
+	((const struct file_operations *)(dentry)->d_fsdata)
+
+static int open_proxy_open(struct inode *inode, struct file *filp)
+{
+	const struct dentry *dentry = F_DENTRY(filp);
+	const struct file_operations *real_fops = NULL;
+	int srcu_idx, r;
+
+	r = debugfs_use_file_start(dentry, &srcu_idx);
+	if (r) {
+		r = -ENOENT;
+		goto out;
+	}
+
+	real_fops = REAL_FOPS_DEREF(dentry);
+	real_fops = fops_get(real_fops);
+	if (!real_fops) {
+		/* Huh? Module did not clean up after itself at exit? */
+		WARN(1, "debugfs file owner did not clean up at exit: %pd",
+			dentry);
+		r = -ENXIO;
+		goto out;
+	}
+	replace_fops(filp, real_fops);
+
+	if (real_fops->open)
+		r = real_fops->open(inode, filp);
+
+out:
+	fops_put(real_fops);
+	debugfs_use_file_finish(srcu_idx);
+	return r;
+}
+
+const struct file_operations debugfs_open_proxy_file_operations = {
+	.open = open_proxy_open,
+};
+
 static struct dentry *debugfs_create_mode(const char *name, umode_t mode,
 					  struct dentry *parent, void *value,
 				          const struct file_operations *fops,