[PATCH v2 1/2] kernel/cgroup: use kernfs_create_dir_ns()

[PATCH v2 1/2] kernel/cgroup: use kernfs_create_dir_ns()

[Max Kellermann]

By passing the fsugid to kernfs_create_dir_ns(), we don't need
cgroup_kn_set_ugid() any longer.  That function was added for exactly
this purpose by commit 49957f8e2a43 ("cgroup: newly created dirs and
files should be owned by the creator").

Eliminating this piece of duplicate code means we benefit from future
improvements to kernfs_create_dir_ns(); for example, both are lacking
S_ISGID support currently, which my next patch will add to
kernfs_create_dir_ns().  It cannot (easily) be added to
cgroup_kn_set_ugid() because we can't dereference struct kernfs_iattrs
from there.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Acked-by: Tejun Heo <tj@kernel.org>
--
v1 -> v2: 12-digit commit id
---
 kernel/cgroup/cgroup.c | 31 ++++---------------------------
 1 file changed, 4 insertions(+), 27 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 4b9ff41ca603..a844b421fd83 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4169,20 +4169,6 @@ static struct kernfs_ops cgroup_kf_ops = {
 	.seq_show		= cgroup_seqfile_show,
 };
 
-/* set uid and gid of cgroup dirs and files to that of the creator */
-static int cgroup_kn_set_ugid(struct kernfs_node *kn)
-{
-	struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
-			       .ia_uid = current_fsuid(),
-			       .ia_gid = current_fsgid(), };
-
-	if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
-	    gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
-		return 0;
-
-	return kernfs_setattr(kn, &iattr);
-}
-
 static void cgroup_file_notify_timer(struct timer_list *timer)
 {
 	cgroup_file_notify(container_of(timer, struct cgroup_file,
@@ -4195,25 +4181,18 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
 	char name[CGROUP_FILE_NAME_MAX];
 	struct kernfs_node *kn;
 	struct lock_class_key *key = NULL;
-	int ret;
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	key = &cft->lockdep_key;
 #endif
 	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
 				  cgroup_file_mode(cft),
-				  GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
+				  current_fsuid(), current_fsgid(),
 				  0, cft->kf_ops, cft,
 				  NULL, key);
 	if (IS_ERR(kn))
 		return PTR_ERR(kn);
 
-	ret = cgroup_kn_set_ugid(kn);
-	if (ret) {
-		kernfs_remove(kn);
-		return ret;
-	}
-
 	if (cft->file_offset) {
 		struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
@@ -5616,7 +5595,9 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
 		goto out_cancel_ref;
 
 	/* create the directory */
-	kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
+	kn = kernfs_create_dir_ns(parent->kn, name, mode,
+				  current_fsuid(), current_fsgid(),
+				  cgrp, NULL);
 	if (IS_ERR(kn)) {
 		ret = PTR_ERR(kn);
 		goto out_stat_exit;
@@ -5761,10 +5742,6 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 	 */
 	kernfs_get(cgrp->kn);
 
-	ret = cgroup_kn_set_ugid(cgrp->kn);
-	if (ret)
-		goto out_destroy;
-
 	ret = css_populate_dir(&cgrp->self);
 	if (ret)
 		goto out_destroy;
-- 
2.39.2

[PATCH v2 2/2] fs/kernfs/dir: obey S_ISGID

[Max Kellermann]

Handling of S_ISGID is usually done by inode_init_owner() in all other
filesystems, but kernfs doesn't use that function.  In kernfs, struct
kernfs_node is the primary data structure, and struct inode is only
created from it on demand.  Therefore, inode_init_owner() can't be
used and we need to imitate its behavior.

S_ISGID support is useful for the cgroup filesystem; it allows
subtrees managed by an unprivileged process to retain a certain owner
gid, which then enables sharing access to the subtree with another
unprivileged process.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Acked-by: Tejun Heo <tj@kernel.org>
--
v1 -> v2: minor coding style fix (comment)
---
 fs/kernfs/dir.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 8b2bd65d70e7..62d39ecf0a46 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -676,6 +676,18 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
 {
 	struct kernfs_node *kn;
 
+	if (parent->mode & S_ISGID) {
+		/* this code block imitates inode_init_owner() for
+		 * kernfs
+		 */
+
+		if (parent->iattr)
+			gid = parent->iattr->ia_gid;
+
+		if (flags & KERNFS_DIR)
+			mode |= S_ISGID;
+	}
+
 	kn = __kernfs_new_node(kernfs_root(parent), parent,
 			       name, mode, uid, gid, flags);
 	if (kn) {
-- 
2.39.2

Re: [PATCH v2 1/2] kernel/cgroup: use kernfs_create_dir_ns()

[Michal Koutný]

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On Fri, Dec 08, 2023 at 10:33:09AM +0100, Max Kellermann <max.kellermann@ionos.com> wrote:
>  kernel/cgroup/cgroup.c | 31 ++++---------------------------
>  1 file changed, 4 insertions(+), 27 deletions(-)

Nice,
Reviewed-by: Michal Koutný <mkoutny@suse.com>


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Re: [PATCH v2 1/2] kernel/cgroup: use kernfs_create_dir_ns()

[Bagas Sanjaya]

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On Fri, Dec 08, 2023 at 10:33:09AM +0100, Max Kellermann wrote:
> By passing the fsugid to kernfs_create_dir_ns(), we don't need
> cgroup_kn_set_ugid() any longer.  That function was added for exactly
> this purpose by commit 49957f8e2a43 ("cgroup: newly created dirs and
> files should be owned by the creator").
> 
> Eliminating this piece of duplicate code means we benefit from future
> improvements to kernfs_create_dir_ns(); for example, both are lacking
> S_ISGID support currently, which my next patch will add to
> kernfs_create_dir_ns().  It cannot (easily) be added to
> cgroup_kn_set_ugid() because we can't dereference struct kernfs_iattrs
> from there.
> 
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> --
> v1 -> v2: 12-digit commit id
> ---
>  kernel/cgroup/cgroup.c | 31 ++++---------------------------
>  1 file changed, 4 insertions(+), 27 deletions(-)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 4b9ff41ca603..a844b421fd83 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -4169,20 +4169,6 @@ static struct kernfs_ops cgroup_kf_ops = {
>  	.seq_show		= cgroup_seqfile_show,
>  };
>  
> -/* set uid and gid of cgroup dirs and files to that of the creator */
> -static int cgroup_kn_set_ugid(struct kernfs_node *kn)
> -{
> -	struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
> -			       .ia_uid = current_fsuid(),
> -			       .ia_gid = current_fsgid(), };
> -
> -	if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
> -	    gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
> -		return 0;
> -
> -	return kernfs_setattr(kn, &iattr);
> -}
> -
>  static void cgroup_file_notify_timer(struct timer_list *timer)
>  {
>  	cgroup_file_notify(container_of(timer, struct cgroup_file,
> @@ -4195,25 +4181,18 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
>  	char name[CGROUP_FILE_NAME_MAX];
>  	struct kernfs_node *kn;
>  	struct lock_class_key *key = NULL;
> -	int ret;
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	key = &cft->lockdep_key;
>  #endif
>  	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
>  				  cgroup_file_mode(cft),
> -				  GLOBAL_ROOT_UID, GLOBAL_ROOT_GID,
> +				  current_fsuid(), current_fsgid(),
>  				  0, cft->kf_ops, cft,
>  				  NULL, key);
>  	if (IS_ERR(kn))
>  		return PTR_ERR(kn);
>  
> -	ret = cgroup_kn_set_ugid(kn);
> -	if (ret) {
> -		kernfs_remove(kn);
> -		return ret;
> -	}
> -
>  	if (cft->file_offset) {
>  		struct cgroup_file *cfile = (void *)css + cft->file_offset;
>  
> @@ -5616,7 +5595,9 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
>  		goto out_cancel_ref;
>  
>  	/* create the directory */
> -	kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
> +	kn = kernfs_create_dir_ns(parent->kn, name, mode,
> +				  current_fsuid(), current_fsgid(),
> +				  cgrp, NULL);
>  	if (IS_ERR(kn)) {
>  		ret = PTR_ERR(kn);
>  		goto out_stat_exit;
> @@ -5761,10 +5742,6 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
>  	 */
>  	kernfs_get(cgrp->kn);
>  
> -	ret = cgroup_kn_set_ugid(cgrp->kn);
> -	if (ret)
> -		goto out_destroy;
> -
>  	ret = css_populate_dir(&cgrp->self);
>  	if (ret)
>  		goto out_destroy;

No noticeable regressions with this patch applied.

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>

-- 
An old man doll... just what I always wanted! - Clara

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Re: [PATCH v2 2/2] fs/kernfs/dir: obey S_ISGID

[Bagas Sanjaya]

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On Fri, Dec 08, 2023 at 10:33:10AM +0100, Max Kellermann wrote:
> Handling of S_ISGID is usually done by inode_init_owner() in all other
> filesystems, but kernfs doesn't use that function.  In kernfs, struct
> kernfs_node is the primary data structure, and struct inode is only
> created from it on demand.  Therefore, inode_init_owner() can't be
> used and we need to imitate its behavior.
> 
> S_ISGID support is useful for the cgroup filesystem; it allows
> subtrees managed by an unprivileged process to retain a certain owner
> gid, which then enables sharing access to the subtree with another
> unprivileged process.
> 
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> --
> v1 -> v2: minor coding style fix (comment)
> ---
>  fs/kernfs/dir.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 8b2bd65d70e7..62d39ecf0a46 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -676,6 +676,18 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
>  {
>  	struct kernfs_node *kn;
>  
> +	if (parent->mode & S_ISGID) {
> +		/* this code block imitates inode_init_owner() for
> +		 * kernfs
> +		 */
> +
> +		if (parent->iattr)
> +			gid = parent->iattr->ia_gid;
> +
> +		if (flags & KERNFS_DIR)
> +			mode |= S_ISGID;
> +	}
> +
>  	kn = __kernfs_new_node(kernfs_root(parent), parent,
>  			       name, mode, uid, gid, flags);
>  	if (kn) {

No noticeable regressions with this patch applied.

Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>

-- 
An old man doll... just what I always wanted! - Clara

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Re: [PATCH v2 1/2] kernel/cgroup: use kernfs_create_dir_ns()

[Tejun Heo]

On Fri, Dec 08, 2023 at 10:33:09AM +0100, Max Kellermann wrote:
> By passing the fsugid to kernfs_create_dir_ns(), we don't need
> cgroup_kn_set_ugid() any longer.  That function was added for exactly
> this purpose by commit 49957f8e2a43 ("cgroup: newly created dirs and
> files should be owned by the creator").
> 
> Eliminating this piece of duplicate code means we benefit from future
> improvements to kernfs_create_dir_ns(); for example, both are lacking
> S_ISGID support currently, which my next patch will add to
> kernfs_create_dir_ns().  It cannot (easily) be added to
> cgroup_kn_set_ugid() because we can't dereference struct kernfs_iattrs
> from there.
> 
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> Acked-by: Tejun Heo <tj@kernel.org>

Applied to cgroup/for-6.8. Greg, can you please take the second patch?

Thanks.

-- 
tejun

Re: [PATCH v2 1/2] kernel/cgroup: use kernfs_create_dir_ns()

[Greg Kroah-Hartman]

On Fri, Dec 22, 2023 at 06:29:43AM +0900, Tejun Heo wrote:
> On Fri, Dec 08, 2023 at 10:33:09AM +0100, Max Kellermann wrote:
> > By passing the fsugid to kernfs_create_dir_ns(), we don't need
> > cgroup_kn_set_ugid() any longer.  That function was added for exactly
> > this purpose by commit 49957f8e2a43 ("cgroup: newly created dirs and
> > files should be owned by the creator").
> > 
> > Eliminating this piece of duplicate code means we benefit from future
> > improvements to kernfs_create_dir_ns(); for example, both are lacking
> > S_ISGID support currently, which my next patch will add to
> > kernfs_create_dir_ns().  It cannot (easily) be added to
> > cgroup_kn_set_ugid() because we can't dereference struct kernfs_iattrs
> > from there.
> > 
> > Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> > Acked-by: Tejun Heo <tj@kernel.org>
> 
> Applied to cgroup/for-6.8. Greg, can you please take the second patch?

Both are already in my tree, thanks.

greg k-h