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authorNicolai Stange <nicstange@gmail.com>2016-03-22 16:11:13 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-04-13 00:14:21 +0300
commit9fd4dcece43a53e5a9e65a973df5693702ee6401 (patch)
tree70fe5fc79019aec56775a74d71c26094e211abd5 /fs/debugfs/inode.c
parent3a3a5fece6f28c14d3d05c74fb7696412e53a067 (diff)
downloadlinux-9fd4dcece43a53e5a9e65a973df5693702ee6401.tar.xz
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>
Diffstat (limited to 'fs/debugfs/inode.c')
-rw-r--r--fs/debugfs/inode.c13
1 files changed, 12 insertions, 1 deletions
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index b1e7f35f3cd4..2905dd160575 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -27,9 +27,14 @@
#include <linux/parser.h>
#include <linux/magic.h>
#include <linux/slab.h>
+#include <linux/srcu.h>
+
+#include "internal.h"
#define DEBUGFS_DEFAULT_MODE 0700
+DEFINE_SRCU(debugfs_srcu);
+
static struct vfsmount *debugfs_mount;
static int debugfs_mount_count;
static bool debugfs_registered;
@@ -341,8 +346,12 @@ 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_open_proxy_file_operations
+ : &debugfs_noop_file_operations;
+ dentry->d_fsdata = (void *)fops;
+
d_instantiate(dentry, inode);
fsnotify_create(d_inode(dentry->d_parent), dentry);
return end_creating(dentry);
@@ -570,6 +579,7 @@ void debugfs_remove(struct dentry *dentry)
inode_unlock(d_inode(parent));
if (!ret)
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
+ synchronize_srcu(&debugfs_srcu);
}
EXPORT_SYMBOL_GPL(debugfs_remove);
@@ -647,6 +657,7 @@ void debugfs_remove_recursive(struct dentry *dentry)
if (!__debugfs_remove(child, parent))
simple_release_fs(&debugfs_mount, &debugfs_mount_count);
inode_unlock(d_inode(parent));
+ synchronize_srcu(&debugfs_srcu);
}
EXPORT_SYMBOL_GPL(debugfs_remove_recursive);