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

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commit 4a7ba45b1a435e7097ca0f79a847d0949d0eb088 upstream.

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.

Link: https://lkml.kernel.org/r/Y5FRm/[email protected]
Fixes: 347c4a8747 ("memcg: remove cgroup_event->cft")
Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Jann Horn <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: <[email protected]>	[3.14+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Tejun Heo 2022-12-07 16:53:15 -10:00 • committed by Greg Kroah-Hartman
commit 35963b3182
3 changed files with 14 additions and 3 deletions
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15
mm/memcontrol.c
View file

@ -4709,6 +4709,7 @@ 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;
@ -4759,6 +4760,16 @@ 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
@ -4767,7 +4778,7 @@ 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;
@ -4791,7 +4802,7 @@ 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))