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authorNicolai Stange <nicstange@gmail.com>2017-10-31 00:15:48 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2017-11-07 20:25:02 +0100
commite9117a5a4bf65d8e99f060d356a04d27a60b436d (patch)
treebd7ad76ed4c2f538f13ab38afb0dda9478ad044e
parent7c8d469877b16d2c1cecf101a0abb7b218db85bc (diff)
debugfs: implement per-file removal protection
Since commit 49d200deaa68 ("debugfs: prevent access to removed files' private data"), accesses to a file's private data are protected from concurrent removal by covering all file_operations with a SRCU read section and sychronizing with those before returning from debugfs_remove() by means of synchronize_srcu(). As pointed out by Johannes Berg, there are debugfs files with forever blocking file_operations. Their corresponding SRCU read side sections would block any debugfs_remove() forever as well, even unrelated ones. This results in a livelock. Because a remover can't cancel any indefinite blocking within foreign files, this is a problem. Resolve this by introducing support for more granular protection on a per-file basis. This is implemented by introducing an 'active_users' refcount_t to the per-file struct debugfs_fsdata state. At file creation time, it is set to one and a debugfs_remove() will drop that initial reference. The new debugfs_file_get() and debugfs_file_put(), intended to be used in place of former debugfs_use_file_start() and debugfs_use_file_finish(), increment and decrement it respectively. Once the count drops to zero, debugfs_file_put() will signal a completion which is possibly being waited for from debugfs_remove(). Thus, as long as there is a debugfs_file_get() not yet matched by a corresponding debugfs_file_put() around, debugfs_remove() will block. Actual users of debugfs_use_file_start() and -finish() will get converted to the new debugfs_file_get() and debugfs_file_put() by followup patches. Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private data") Reported-by: Johannes Berg <johannes@sipsolutions.net> Signed-off-by: Nicolai Stange <nicstange@gmail.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--fs/debugfs/file.c48
-rw-r--r--fs/debugfs/inode.c29
-rw-r--r--fs/debugfs/internal.h2
-rw-r--r--include/linux/debugfs.h11
4 files changed, 84 insertions, 6 deletions
diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index b6f5ddab66bf..6644bfdea2f8 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -109,6 +109,54 @@ const struct file_operations *debugfs_real_fops(const struct file *filp)
}
EXPORT_SYMBOL_GPL(debugfs_real_fops);
+/**
+ * debugfs_file_get - mark the beginning of file data access
+ * @dentry: the dentry object whose data is being accessed.
+ *
+ * Up to a matching call to debugfs_file_put(), 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_file_get() 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.
+ */
+int debugfs_file_get(struct dentry *dentry)
+{
+ struct debugfs_fsdata *fsd = dentry->d_fsdata;
+
+ /* Avoid starvation of removers. */
+ if (d_unlinked(dentry))
+ return -EIO;
+
+ if (!refcount_inc_not_zero(&fsd->active_users))
+ return -EIO;
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(debugfs_file_get);
+
+/**
+ * debugfs_file_put - mark the end of file data access
+ * @dentry: the dentry object formerly passed to
+ * debugfs_file_get().
+ *
+ * Allow any ongoing concurrent call into debugfs_remove() or
+ * debugfs_remove_recursive() blocked by a former call to
+ * debugfs_file_get() to proceed and return to its caller.
+ */
+void debugfs_file_put(struct dentry *dentry)
+{
+ struct debugfs_fsdata *fsd = dentry->d_fsdata;
+
+ if (refcount_dec_and_test(&fsd->active_users))
+ complete(&fsd->active_users_drained);
+}
+EXPORT_SYMBOL_GPL(debugfs_file_put);
+
static int open_proxy_open(struct inode *inode, struct file *filp)
{
const struct dentry *dentry = F_DENTRY(filp);
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index a9c3d3e9af39..6449eb935540 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -374,6 +374,7 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
inode->i_fop = proxy_fops;
fsd->real_fops = real_fops;
+ refcount_set(&fsd->active_users, 1);
dentry->d_fsdata = fsd;
d_instantiate(dentry, inode);
@@ -631,18 +632,34 @@ struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
}
EXPORT_SYMBOL_GPL(debugfs_create_symlink);
+static void __debugfs_remove_file(struct dentry *dentry, struct dentry *parent)
+{
+ struct debugfs_fsdata *fsd;
+
+ simple_unlink(d_inode(parent), dentry);
+ d_delete(dentry);
+ fsd = dentry->d_fsdata;
+ init_completion(&fsd->active_users_drained);
+ if (!refcount_dec_and_test(&fsd->active_users))
+ wait_for_completion(&fsd->active_users_drained);
+}
+
static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
{
int ret = 0;
if (simple_positive(dentry)) {
dget(dentry);
- if (d_is_dir(dentry))
- ret = simple_rmdir(d_inode(parent), dentry);
- else
- simple_unlink(d_inode(parent), dentry);
- if (!ret)
- d_delete(dentry);
+ if (!d_is_reg(dentry)) {
+ if (d_is_dir(dentry))
+ ret = simple_rmdir(d_inode(parent), dentry);
+ else
+ simple_unlink(d_inode(parent), dentry);
+ if (!ret)
+ d_delete(dentry);
+ } else {
+ __debugfs_remove_file(dentry, parent);
+ }
dput(dentry);
}
return ret;
diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h
index 512601eed3ce..0eea99432840 100644
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -21,6 +21,8 @@ extern const struct file_operations debugfs_full_proxy_file_operations;
struct debugfs_fsdata {
const struct file_operations *real_fops;
+ refcount_t active_users;
+ struct completion active_users_drained;
};
#endif /* _DEBUGFS_INTERNAL_H_ */
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index cbee5f4a02a3..3b914d588148 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -98,6 +98,9 @@ void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu);
const struct file_operations *debugfs_real_fops(const struct file *filp)
__must_hold(&debugfs_srcu);
+int debugfs_file_get(struct dentry *dentry);
+void debugfs_file_put(struct dentry *dentry);
+
ssize_t debugfs_attr_read(struct file *file, char __user *buf,
size_t len, loff_t *ppos);
ssize_t debugfs_attr_write(struct file *file, const char __user *buf,
@@ -236,6 +239,14 @@ static inline void debugfs_use_file_finish(int srcu_idx)
__releases(&debugfs_srcu)
{ }
+static inline int debugfs_file_get(struct dentry *dentry)
+{
+ return 0;
+}
+
+static inline void debugfs_file_put(struct dentry *dentry)
+{ }
+
static inline ssize_t debugfs_attr_read(struct file *file, char __user *buf,
size_t len, loff_t *ppos)
{