LinuxLists.cc - [PATCH for-6.1-fixes] memcg: Fix possible use-after-free in memcg_write_event

2022-12-08 04:26:26

Subject: [PATCH for-6.1-fixes] memcg: Fix possible use-after-free in memcg_write_event_control()

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 347c4a874710 ("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 <[email protected]>
Fixes: 347c4a874710 ("memcg: remove cgroup_event->cft")
Cc: [email protected] # v3.14+
Reported-by: Jann Horn <[email protected]>
Cc: Linus Torvalds <[email protected]>
---
include/linux/cgroup.h | 1 +
kernel/cgroup/cgroup-internal.h | 1 -
mm/memcontrol.c | 15 +++++++++++++--
3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 528bd44b59e2..2b7d077de7ef 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -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;

diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index fd4020835ec6..367b0a42ada9 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -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) \
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a1a35c12635e..266a1ab05434 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -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))

2022-12-08 04:43:36

by Roman Gushchin

[permalink] [raw]

Subject: Re: [PATCH for-6.1-fixes] memcg: Fix possible use-after-free in memcg_write_event_control()

On Wed, Dec 07, 2022 at 04:53:15PM -1000, Tejun Heo wrote:
> 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 347c4a874710 ("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 <[email protected]>
> Fixes: 347c4a874710 ("memcg: remove cgroup_event->cft")
> Cc: [email protected] # v3.14+
> Reported-by: Jann Horn <[email protected]>
> Cc: Linus Torvalds <[email protected]>

Acked-by: Roman Gushchin <[email protected]>

Thanks!

2022-12-08 15:35:16

by Johannes Weiner

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