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authorNicolai Stange <nicstange@gmail.com>2016-03-22 14:11:13 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-04-12 14:14:21 -0700
commit9fd4dcece43a53e5a9e65a973df5693702ee6401 (patch)
tree70fe5fc79019aec56775a74d71c26094e211abd5 /lib
parent3a3a5fece6f28c14d3d05c74fb7696412e53a067 (diff)
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 'lib')
-rw-r--r--lib/Kconfig.debug1
1 files changed, 1 insertions, 0 deletions
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1e9a607534ca..ddb0e8337aae 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