[PATCH 1/2] debugfs: prevent access to possibly dead file_operations at file open

Nicolai Stange
2015-11-30T23:21:38Z

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.
- In __debugfs_remove(), clear out that dentry->d_fsdata member again.
debugfs_remove() and debugfs_remove_recursive() wait for a SRCU grace
period before returning to their caller.
- Introduce an intermediate file_operations object named
"debugfs_proxy_file_operations". It's ->open() functions checks,
under the protection of a SRCU read lock, whether the "original"
file_operations pointed to by ->d_fsdata is still valid 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.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
Applicable to the Linus tree.
The second of the two patches depends on the first one

fs/debugfs/file.c | 29 ++++++++++++++++++++++++++++-
fs/debugfs/inode.c | 16 +++++++++++++---
include/linux/debugfs.h | 6 +++++-
lib/Kconfig.debug | 1 +
4 files changed, 47 insertions(+), 5 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index d2ba12e..67df2c9 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -35,13 +35,40 @@ 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,
};

+static int proxy_open(struct inode *inode, struct file *filp)
+{
+ struct dentry * const dentry = filp->f_path.dentry;
+ const struct file_operations __rcu *rcu_real_fops;
+ const struct file_operations *real_fops;
+ int srcu_idx;
+
+ srcu_idx = srcu_read_lock(&debugfs_srcu);
+ rcu_real_fops = (const struct file_operations __rcu *)dentry->d_fsdata;
+ real_fops = srcu_dereference(rcu_real_fops, &debugfs_srcu);
+ real_fops = fops_get(real_fops);
+ srcu_read_unlock(&debugfs_srcu, srcu_idx);
+
+ if (!real_fops)
+ return -ENOENT;
+
+ replace_fops(filp, real_fops);
+ if (filp->f_op->open)
+ return filp->f_op->open(inode, filp);
+
+ return 0;
+}
+
+const struct file_operations debugfs_proxy_file_operations = {
+ .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,
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index b7fcc0d..8ae2e1a 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -30,6 +30,8 @@

#define DEBUGFS_DEFAULT_MODE 0700

+DEFINE_SRCU(debugfs_srcu);
+
static struct vfsmount *debugfs_mount;
static int debugfs_mount_count;
static bool debugfs_registered;
@@ -340,8 +342,12 @@ struct dentry *debugfs_create_file(const char *name, umode_t mode,
return failed_creating(dentry);

inode->i_mode = mode;
- inode->i_fop = fops ? fops : &debugfs_file_operations;
inode->i_private = data;
+
+ inode->i_fop = fops ? &debugfs_proxy_file_operations
+ : &debugfs_noop_file_operations;
+ rcu_assign_pointer(dentry->d_fsdata, (void *)fops);
+
d_instantiate(dentry, inode);
fsnotify_create(d_inode(dentry->d_parent), dentry);
return end_creating(dentry);
@@ -523,10 +529,12 @@ static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)

if (simple_positive(dentry)) {
dget(dentry);
- if (d_is_dir(dentry))
+ if (d_is_dir(dentry)) {
ret = simple_rmdir(d_inode(parent), dentry);
- else
+ } else {
simple_unlink(d_inode(parent), dentry);
+ rcu_assign_pointer(dentry->d_fsdata, NULL);
+ }
if (!ret)
d_delete(dentry);
dput(dentry);
@@ -565,6 +573,7 @@ void debugfs_remove(struct dentry *dentry)
mutex_unlock(&d_inode(parent)->i_mutex);
if (!ret)
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
+ synchronize_srcu(&debugfs_srcu);
}
EXPORT_SYMBOL_GPL(debugfs_remove);

@@ -642,6 +651,7 @@ void debugfs_remove_recursive(struct dentry *dentry)
if (!__debugfs_remove(child, parent))
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
mutex_unlock(&d_inode(parent)->i_mutex);
+ synchronize_srcu(&debugfs_srcu);
}
EXPORT_SYMBOL_GPL(debugfs_remove_recursive);

diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index 19c066d..f8c7494 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -19,6 +19,7 @@
#include <linux/seq_file.h>

#include <linux/types.h>
+#include <linux/srcu.h>

struct device;
struct file_operations;
@@ -44,7 +45,10 @@ extern struct dentry *arch_debugfs_dir;
#if defined(CONFIG_DEBUG_FS)

/* declared over in file.c */
-extern const struct file_operations debugfs_file_operations;
+extern const struct file_operations debugfs_noop_file_operations;
+extern const struct file_operations debugfs_proxy_file_operations;
+
+extern struct srcu_struct debugfs_srcu;

struct dentry *debugfs_create_file(const char *name, umode_t mode,
struct dentry *parent, void *data,
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 8c15b29..3929878 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -257,6 +257,7 @@ config PAGE_OWNER

config DEBUG_FS
bool "Debug Filesystem"
+ select SRCU
help
debugfs is a virtual file system that kernel developers use to put
debugging files into. Enable this option to be able to read and
--
2.6.3