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Date: Tue, 12 Dec 2023 23:47:28 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: Thomas Weißschuh <linux@...ssschuh.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Joel Granados <j.granados@...sung.com>,
	Kees Cook <keescook@...omium.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Iurii Zaikin <yzaikin@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 00/18] sysctl: constify sysctl ctl_tables

On Mon, Dec 11, 2023 at 12:25:10PM +0100, Thomas Weißschuh wrote:
> Before sending it I'd like to get feedback on the internal rework of the
> is_empty detection from you and/or Luis.
> 
> https://git.sr.ht/~t-8ch/linux/commit/ea27507070f3c47be6febebe451bbb88f6ea707e
> or the attached patch.

Please send as a new patch as RFC and please ensure on the To field is
first "Eric W. Biederman" <ebiederm@...ssion.com> with a bit more
elaborate commit log as suggested from my review below. If there are
any hidden things me and Joel could probably miss I'm sure Eric will
be easily able to spot it.

> From ea27507070f3c47be6febebe451bbb88f6ea707e Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Thomas=20Wei=C3=9Fschuh?= <linux@...ssschuh.net>
> Date: Sun, 3 Dec 2023 21:56:46 +0100
> Subject: [PATCH] sysctl: move permanently empty flag to ctl_dir
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Simplify the logic by always keeping the permanently_empty flag on the
> ctl_dir.
> The previous logic kept the flag in the leaf ctl_table and from there
> transferred it to the ctl_table from the directory.
> 
> This also removes the need to have a mutable ctl_table and will allow
> the constification of those structs.

Please elaborate a bit more on this here in your next RFC.

> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
>  fs/proc/proc_sysctl.c  | 74 +++++++++++++++++++-----------------------
>  include/linux/sysctl.h | 13 ++------
>  2 files changed, 36 insertions(+), 51 deletions(-)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 35c97ad54f34..33f41af58e9b 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -17,6 +17,7 @@
>  #include <linux/bpf-cgroup.h>
>  #include <linux/mount.h>
>  #include <linux/kmemleak.h>
> +#include <linux/cleanup.h>

>  #include "internal.h"
>  
>  #define list_for_each_table_entry(entry, header)	\
> @@ -29,32 +30,6 @@ static const struct inode_operations proc_sys_inode_operations;
>  static const struct file_operations proc_sys_dir_file_operations;
>  static const struct inode_operations proc_sys_dir_operations;
>  
> -/* Support for permanently empty directories */
> -static struct ctl_table sysctl_mount_point[] = {
> -	{.type = SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY }
> -};
> -
> -/**
> - * register_sysctl_mount_point() - registers a sysctl mount point
> - * @path: path for the mount point
> - *
> - * Used to create a permanently empty directory to serve as mount point.
> - * There are some subtle but important permission checks this allows in the
> - * case of unprivileged mounts.
> - */
> -struct ctl_table_header *register_sysctl_mount_point(const char *path)
> -{
> -	return register_sysctl(path, sysctl_mount_point);
> -}
> -EXPORT_SYMBOL(register_sysctl_mount_point);
> -
> -#define sysctl_is_perm_empty_ctl_header(hptr)		\
> -	(hptr->ctl_table[0].type == SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY)
> -#define sysctl_set_perm_empty_ctl_header(hptr)		\
> -	(hptr->ctl_table[0].type = SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY)
> -#define sysctl_clear_perm_empty_ctl_header(hptr)	\
> -	(hptr->ctl_table[0].type = SYSCTL_TABLE_TYPE_DEFAULT)
> -
>  void proc_sys_poll_notify(struct ctl_table_poll *poll)
>  {
>  	if (!poll)
> @@ -226,17 +201,9 @@ static int insert_header(struct ctl_dir *dir, struct ctl_table_header *header)
>  
>  
>  	/* Is this a permanently empty directory? */
> -	if (sysctl_is_perm_empty_ctl_header(dir_h))
> +	if (dir->permanently_empty)
>  		return -EROFS;
>  
> -	/* Am I creating a permanently empty directory? */
> -	if (header->ctl_table_size > 0 &&
> -	    sysctl_is_perm_empty_ctl_header(header)) {
> -		if (!RB_EMPTY_ROOT(&dir->root))
> -			return -EINVAL;
> -		sysctl_set_perm_empty_ctl_header(dir_h);
> -	}
> -
>  	dir_h->nreg++;
>  	header->parent = dir;
>  	err = insert_links(header);
> @@ -252,8 +219,6 @@ fail:
>  	erase_header(header);
>  	put_links(header);
>  fail_links:
> -	if (header->ctl_table == sysctl_mount_point)
> -		sysctl_clear_perm_empty_ctl_header(dir_h);
>  	header->parent = NULL;
>  	drop_sysctl_table(dir_h);
>  	return err;
> @@ -440,6 +405,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  		struct ctl_table_header *head, struct ctl_table *table)
>  {
>  	struct ctl_table_root *root = head->root;
> +	struct ctl_dir *ctl_dir;
>  	struct inode *inode;
>  	struct proc_inode *ei;
>  
> @@ -473,7 +439,9 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  		inode->i_mode |= S_IFDIR;
>  		inode->i_op = &proc_sys_dir_operations;
>  		inode->i_fop = &proc_sys_dir_file_operations;
> -		if (sysctl_is_perm_empty_ctl_header(head))
> +
> +		ctl_dir = container_of(head, struct ctl_dir, header);
> +		if (ctl_dir->permanently_empty)
>  			make_empty_dir_inode(inode);
>  	}
>  
> @@ -1211,8 +1179,7 @@ static bool get_links(struct ctl_dir *dir,
>  	struct ctl_table_header *tmp_head;
>  	struct ctl_table *entry, *link;
>  
> -	if (header->ctl_table_size == 0 ||
> -	    sysctl_is_perm_empty_ctl_header(header))
> +	if (header->ctl_table_size == 0 || dir->permanently_empty)
>  		return true;

Why are we now checking the target directory whereas before it was the
header?

>  	/* Are there links available for every entry in table? */
> @@ -1533,6 +1500,33 @@ void unregister_sysctl_table(struct ctl_table_header * header)
>  }
>  EXPORT_SYMBOL(unregister_sysctl_table);
>  
> +/**
> + * register_sysctl_mount_point() - registers a sysctl mount point
> + * @path: path for the mount point
> + *
> + * Used to create a permanently empty directory to serve as mount point.
> + * There are some subtle but important permission checks this allows in the
> + * case of unprivileged mounts.
> + */
> +struct ctl_table_header *register_sysctl_mount_point(const char *path)
> +{
> +	struct ctl_dir *dir = sysctl_mkdir_p(&sysctl_table_root.default_set.dir, path);

Even though __register_sysctl_table() did more than we needed it is not
obvious how the sysctl_table_root.default_set.dir.header.nreg++ is not
needed but I see get_subdir() also bumps it as it is called from
sysctl_mkdir_p(), so we were already bumping it at least once. I
suppose that is safe.

Wouldn't we be able to re-register the same sysctl mount point more
than once with this though?

> +
> +	if (IS_ERR(dir))
> +		return NULL;
> +
> +	guard(spinlock)(&sysctl_lock);

For those that did not get the memo (I guess the docs could be
improved):

https://lwn.net/Articles/940117/

> +
> +	if (!RB_EMPTY_ROOT(&dir->root)) {
> +		drop_sysctl_table(&dir->header);
> +		return NULL;
> +	}

This seems to silently allow races in that the above allows
an existing sysctl mount point to already exist and here we
bail on an assumption we know we can't have.

> +
> +	dir->permanently_empty = true;
> +	return &dir->header;
> +}
> +EXPORT_SYMBOL(register_sysctl_mount_point);
> +
>  void setup_sysctl_set(struct ctl_table_set *set,
>  	struct ctl_table_root *root,
>  	int (*is_seen)(struct ctl_table_set *))
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index ada36ef8cecb..57cb0060d7d7 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -137,17 +137,6 @@ struct ctl_table {
>  	void *data;
>  	int maxlen;
>  	umode_t mode;
> -	/**
> -	 * enum type - Enumeration to differentiate between ctl target types
> -	 * @SYSCTL_TABLE_TYPE_DEFAULT: ctl target with no special considerations
> -	 * @SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY: Used to identify a permanently
> -	 *                                       empty directory target to serve
> -	 *                                       as mount point.
> -	 */
> -	enum {
> -		SYSCTL_TABLE_TYPE_DEFAULT,
> -		SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY
> -	} type;
>  	proc_handler *proc_handler;	/* Callback for text formatting */
>  	struct ctl_table_poll *poll;
>  	void *extra1;
> @@ -194,6 +183,8 @@ struct ctl_dir {
>  	/* Header must be at the start of ctl_dir */
>  	struct ctl_table_header header;
>  	struct rb_root root;
> +	/* Permanently empty directory target to serve as mount point. */
> +	bool permanently_empty;
>  };
>  
>  struct ctl_table_set {

It's a pretty aggressive cleanup, specially with the new hipster guard()
call but I'd love Eric's eyeballs on a proper v2.

  Luis

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