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Date: Thu, 7 Dec 2023 13:14:37 +0100
From: Joel Granados <j.granados@...sung.com>
To: Thomas Weißschuh <linux@...ssschuh.net>
CC: Luis Chamberlain <mcgrof@...nel.org>, 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 13/18] sysctl: move sysctl type to ctl_table_header

On Wed, Dec 06, 2023 at 06:53:10AM +0100, Thomas Weißschuh wrote:
> On 2023-12-05 14:50:01-0800, Luis Chamberlain wrote:
> > On Tue, Dec 5, 2023 at 2:41 PM Thomas Weißschuh <linux@...ssschuh.net> wrote:
> > >
> > > On 2023-12-05 14:33:38-0800, Luis Chamberlain wrote:
> > > > On Mon, Dec 04, 2023 at 08:52:26AM +0100, Thomas Weißschuh wrote:
> > > > > @@ -231,7 +231,8 @@ static int insert_header(struct ctl_dir *dir, struct ctl_table_header *header)
> > > > >             return -EROFS;
> > > > >
> > > > >     /* Am I creating a permanently empty directory? */
> > > > > -   if (sysctl_is_perm_empty_ctl_header(header)) {
> > > > > +   if (header->ctl_table == sysctl_mount_point ||
> > > > > +       sysctl_is_perm_empty_ctl_header(header)) {
> > > > >             if (!RB_EMPTY_ROOT(&dir->root))
> > > > >                     return -EINVAL;
> > > > >             sysctl_set_perm_empty_ctl_header(dir_h);
> > > >
> > > > While you're at it.
> > >
> > > This hunk is completely gone in v3/the code that you merged.
> > 
> > It is worse in that it is not obvious:
> > 
> > +       if (table == sysctl_mount_point)
> > +               sysctl_set_perm_empty_ctl_header(head);
> > 
> > > Which kind of unsafety do you envision here?
> > 
> > Making the code obvious during patch review hy this is needed /
> > special, and if we special case this, why not remove enum, and make it
> > specific to only that one table. The catch is that it is not
> > immediately obvious that we actually call
> > sysctl_set_perm_empty_ctl_header() in other places, and it begs the
> > question if this can be cleaned up somehow.
> 
> Making it specific won't work because the flag needs to be transferred
> from the leaf table to the table representing the directory.
> 
> What do you think of the aproach taken in the attached patch?
> (On top of current sysctl-next, including my series)
What would this new patch be fixing again? I could not follow ?

Additionally, this might be another reason to set this patch aside :)

> 
> Note: Current sysctl-next ist still based on v6.6.

> From 2fb9887fb2a5024c2620f2d694bc6dcc32afde67 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Thomas=20Wei=C3=9Fschuh?= <linux@...ssschuh.net>
> Date: Wed, 6 Dec 2023 06:17:22 +0100
> Subject: [PATCH] sysctl: simplify handling of permanently empty directories
> 
> ---
>  fs/proc/proc_sysctl.c  | 76 +++++++++++++++++++-----------------------
>  include/linux/sysctl.h | 13 ++------
>  2 files changed, 36 insertions(+), 53 deletions(-)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index c92e9b972ada..c4d6d09b0e68 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 const struct ctl_table sysctl_mount_point[] = {
> -	{ }
> -};
> -
> -/**
> - * 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->type == SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY)
> -#define sysctl_set_perm_empty_ctl_header(hptr)		\
> -	(hptr->type = SYSCTL_TABLE_TYPE_PERMANENTLY_EMPTY)
> -#define sysctl_clear_perm_empty_ctl_header(hptr)		\
> -	(hptr->type = SYSCTL_TABLE_TYPE_DEFAULT)
> -
>  void proc_sys_poll_notify(struct ctl_table_poll *poll)
>  {
>  	if (!poll)
> @@ -199,8 +174,6 @@ static void init_header(struct ctl_table_header *head,
>  	head->set = set;
>  	head->parent = NULL;
>  	head->node = node;
> -	if (table == sysctl_mount_point)
> -		sysctl_set_perm_empty_ctl_header(head);
>  	INIT_HLIST_HEAD(&head->inodes);
>  	if (node) {
>  		const struct ctl_table *entry;
> @@ -228,17 +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);
> @@ -254,8 +219,6 @@ static int insert_header(struct ctl_dir *dir, struct ctl_table_header *header)
>  	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;
> @@ -442,6 +405,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  		struct ctl_table_header *head, const struct ctl_table *table)
>  {
>  	struct ctl_table_root *root = head->root;
> +	struct ctl_dir *ctl_dir;
>  	struct inode *inode;
>  	struct proc_inode *ei;
>  
> @@ -475,7 +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);
>  	}
>  
> @@ -1214,8 +1180,7 @@ static bool get_links(struct ctl_dir *dir,
>  	struct ctl_table_header *tmp_head;
>  	const 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;
>  
>  	/* Are there links available for every entry in table? */
> @@ -1536,6 +1501,33 @@ 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);
> +
> +	if (IS_ERR(dir))
> +		return NULL;
> +
> +	guard(spinlock)(&sysctl_lock);
> +
> +	if (!RB_EMPTY_ROOT(&dir->root)) {
> +		drop_sysctl_table(&dir->header);
> +		return NULL;
> +	}
> +
> +	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 7c96d5abafc7..329e68d484ed 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -177,23 +177,14 @@ struct ctl_table_header {
>  	struct ctl_dir *parent;
>  	struct ctl_node *node;
>  	struct hlist_head inodes; /* head for proc_inode->sysctl_inodes */
> -	/**
> -	 * 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;
>  };
>  
>  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 {
> 
> base-commit: a6fd07f80ab7bd94edb4d56c35e61117ffb9957e
> prerequisite-patch-id: 0000000000000000000000000000000000000000
> prerequisite-patch-id: 13932e9add940cb65c71e04b5efdfcd3622fd27e
> prerequisite-patch-id: 2e4d88f7b8aaa805598f0e87a3ea726825bb4264
> prerequisite-patch-id: 674a680d9cb138cd34cfd0e1a4ec3a5d1c220078
> prerequisite-patch-id: e27c92582aa20b1dfb122c172b336dbaf9d6508a
> prerequisite-patch-id: 9b409a34ab6a4d8d8c5225ba9a72db3116e3c8b3
> prerequisite-patch-id: 86ff15a81d850ebda16bb707491251f4b705e4fd
> prerequisite-patch-id: b7ab65512ac9acfb2dd482b0271b399467afc56d
> prerequisite-patch-id: 0354922fbf2508a89f3e9d9a4e274fc98deb2e93
> prerequisite-patch-id: b71389e82026ffc19cbb717bba1a014ad6cab6da
> prerequisite-patch-id: fbb0201f89bf6c41d0585af867bdeec8d51649b2
> prerequisite-patch-id: e3b4b5b69b4eadf87ed97beb8c03a471e7628cb9
> prerequisite-patch-id: 3fbc9745cf3f28872b3e63f6d1f6e2fd7598be8a
> prerequisite-patch-id: ba2b190c2e54cfb505a282e688c2222712f0acd7
> prerequisite-patch-id: 47e5ca730748bb7bf9248a9e711045d8c1028199
> prerequisite-patch-id: dcd9f87f00290d2f9be83e404f8883eb90c5fb1c
> prerequisite-patch-id: d4629be1a61585ab821da2d2850f246761f72f25
> prerequisite-patch-id: f740190f4b94e57cbf3659f220d94483713341a1
> prerequisite-patch-id: 301c2e530e2af4568267e19247d4a49ac2a9871d
> -- 
> 2.43.0
> 


-- 

Joel Granados

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