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memcg: Fix possible use-after-free in memcg_write_event_control()

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memcg_write_event_control() accesses the dentry->d_name of the specified
control fd to route the write call.  As a cgroup interface file can't be
renamed, it's safe to access d_name as long as the specified file is a
regular cgroup file.  Also, as these cgroup interface files can't be
removed before the directory, it's safe to access the parent too.

Prior to 347c4a8747 ("memcg: remove cgroup_event->cft"), there was a
call to __file_cft() which verified that the specified file is a regular
cgroupfs file before further accesses.  The cftype pointer returned from
__file_cft() was no longer necessary and the commit inadvertently
dropped the file type check with it allowing any file to slip through.
With the invarients broken, the d_name and parent accesses can now race
against renames and removals of arbitrary files and cause
use-after-free's.

Fix the bug by resurrecting the file type check in __file_cft().  Now
that cgroupfs is implemented through kernfs, checking the file
operations needs to go through a layer of indirection.  Instead, let's
check the superblock and dentry type.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 347c4a8747 ("memcg: remove cgroup_event->cft")
Cc: stable@kernel.org # v3.14+
Reported-by: Jann Horn <jannh@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Tejun Heo 2022-12-07 16:53:15 -10:00 committed by Linus Torvalds
parent 479174d402
commit fbf8321238
3 changed files with 14 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/cgroup.h
View File

@ -68,6 +68,7 @@ struct css_task_iter {
struct list_head iters_node; /* css_set->task_iters */
};
extern struct file_system_type cgroup_fs_type;
extern struct cgroup_root cgrp_dfl_root;
extern struct css_set init_css_set;

1
kernel/cgroup/cgroup-internal.h
View File

@ -167,7 +167,6 @@ struct cgroup_mgctx {
extern spinlock_t css_set_lock;
extern struct cgroup_subsys *cgroup_subsys[];
extern struct list_head cgroup_roots;
extern struct file_system_type cgroup_fs_type;
/* iterate across the hierarchies */
#define for_each_root(root) \

15
mm/memcontrol.c
View File

@ -4832,6 +4832,7 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
unsigned int efd, cfd;
struct fd efile;
struct fd cfile;
struct dentry *cdentry;
const char *name;
char *endp;
int ret;
@ -4885,6 +4886,16 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
if (ret < 0)
goto out_put_cfile;
/*
* The control file must be a regular cgroup1 file. As a regular cgroup
* file can't be renamed, it's safe to access its name afterwards.
*/
cdentry = cfile.file->f_path.dentry;
if (cdentry->d_sb->s_type != &cgroup_fs_type || !d_is_reg(cdentry)) {
ret = -EINVAL;
goto out_put_cfile;
}
/*
* Determine the event callbacks and set them in @event. This used
* to be done via struct cftype but cgroup core no longer knows
@ -4893,7 +4904,7 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
*
* DO NOT ADD NEW FILES.
*/
name = cfile.file->f_path.dentry->d_name.name;
name = cdentry->d_name.name;
if (!strcmp(name, "memory.usage_in_bytes")) {
event->register_event = mem_cgroup_usage_register_event;
@ -4917,7 +4928,7 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
* automatically removed on cgroup destruction but the removal is
* asynchronous, so take an extra ref on @css.
*/
cfile_css = css_tryget_online_from_dir(cfile.file->f_path.dentry->d_parent,
cfile_css = css_tryget_online_from_dir(cdentry->d_parent,
&memory_cgrp_subsys);
ret = -EINVAL;
if (IS_ERR(cfile_css))