[PATCH] proc: Allow restricting permissions in /proc/sys

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Several items in /proc/sys need not be accessible to unprivileged
tasks. Let the system administrator change the permissions, but only
to more restrictive modes than what the sysctl tables allow.

Signed-off-by: Topi Miettinen <toiwoton@xxxxxxxxx>
---
v2: actually keep track of changed permissions instead of relying on inode cache 
---
 fs/proc/proc_sysctl.c  | 42 ++++++++++++++++++++++++++++++++++++++----
 include/linux/sysctl.h |  1 +
 2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d80989b6c344..1f75382c49fd 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int mask) 
        if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
                return -EACCES;

+       error = generic_permission(inode, mask);
+       if (error)
+               return error;
+
        head = grab_header(inode);
        if (IS_ERR(head))
                return PTR_ERR(head);
@@ -835,17 +839,46 @@ static int proc_sys_permission(struct inode *inode, int mask) 
 static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
 {
        struct inode *inode = d_inode(dentry);
+       struct ctl_table_header *head = grab_header(inode);
+       struct ctl_table *table = PROC_I(inode)->sysctl_entry;
        int error;

-       if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
+       if (attr->ia_valid & (ATTR_UID | ATTR_GID))
                return -EPERM;

+       if (attr->ia_valid & ATTR_MODE) {
+               umode_t max_mode = 0777; /* Only these bits may change */
+
+               if (IS_ERR(head))
+                       return PTR_ERR(head);
+
+               if (!table) /* global root - r-xr-xr-x */
+                       max_mode &= ~0222;
+               else /*
+                     * Don't allow permissions to become less
+                     * restrictive than the sysctl table entry
+                     */
+                       max_mode &= table->mode;
+
+               /* Execute bits only allowed for directories */
+               if (!S_ISDIR(inode->i_mode))
+                       max_mode &= ~0111;
+
+               if (attr->ia_mode & ~S_IFMT & ~max_mode)
+                       return -EPERM;
+       }
+
        error = setattr_prepare(dentry, attr);
        if (error)
                return error;

        setattr_copy(inode, attr);
        mark_inode_dirty(inode);
+
+       if (table)
+               table->current_mode = inode->i_mode;
+       sysctl_head_finish(head);
+
        return 0;
 }
@@ -861,7 +894,7 @@ static int proc_sys_getattr(const struct path *path, struct kstat *stat, 

        generic_fillattr(inode, stat);
        if (table)
-               stat->mode = (stat->mode & S_IFMT) | table->mode;
+               stat->mode = (stat->mode & S_IFMT) | table->current_mode;

        sysctl_head_finish(head);
        return 0;
@@ -981,7 +1014,7 @@ static struct ctl_dir *new_dir(struct ctl_table_set *set, 
        memcpy(new_name, name, namelen);
        new_name[namelen] = '\0';
        table[0].procname = new_name;
-       table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
+       table[0].current_mode = table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
        init_header(&new->header, set->dir.header.root, set, node, table);

        return new;
@@ -1155,6 +1188,7 @@ static int sysctl_check_table(const char *path, struct ctl_table *table) 
                if ((table->mode & (S_IRUGO|S_IWUGO)) != table->mode)
                        err |= sysctl_err(path, table, "bogus .mode 0%o",
                                table->mode);
+               table->current_mode = table->mode;
        }
        return err;
 }
@@ -1192,7 +1226,7 @@ static struct ctl_table_header *new_links(struct ctl_dir *dir, struct ctl_table 
                int len = strlen(entry->procname) + 1;
                memcpy(link_name, entry->procname, len);
                link->procname = link_name;
-               link->mode = S_IFLNK|S_IRWXUGO;
+               link->current_mode = link->mode = S_IFLNK|S_IRWXUGO;
                link->data = link_root;
                link_name += len;
        }
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 6df477329b76..7c519c35bf9c 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -126,6 +126,7 @@ struct ctl_table
        void *data;
        int maxlen;
        umode_t mode;
+       umode_t current_mode;
        struct ctl_table *child;        /* Deprecated */
        proc_handler *proc_handler;     /* Callback for text formatting */
        struct ctl_table_poll *poll;
--
2.24.0.rc1


>From 3cde64e0aa2734c335355ee6d0d9f12c1f1e8a87 Mon Sep 17 00:00:00 2001
From: Topi Miettinen <toiwoton@xxxxxxxxx>
Date: Sun, 3 Nov 2019 16:36:43 +0200
Subject: [PATCH] proc: Allow restricting permissions in /proc/sys

Several items in /proc/sys need not be accessible to unprivileged
tasks. Let the system administrator change the permissions, but only
to more restrictive modes than what the sysctl tables allow.

Signed-off-by: Topi Miettinen <toiwoton@xxxxxxxxx>
---
 fs/proc/proc_sysctl.c  | 42 ++++++++++++++++++++++++++++++++++++++----
 include/linux/sysctl.h |  1 +
 2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d80989b6c344..1f75382c49fd 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int mask)
 	if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
 		return -EACCES;
 
+	error = generic_permission(inode, mask);
+	if (error)
+		return error;
+
 	head = grab_header(inode);
 	if (IS_ERR(head))
 		return PTR_ERR(head);
@@ -835,17 +839,46 @@ static int proc_sys_permission(struct inode *inode, int mask)
 static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
 {
 	struct inode *inode = d_inode(dentry);
+	struct ctl_table_header *head = grab_header(inode);
+	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
 	int error;
 
-	if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
+	if (attr->ia_valid & (ATTR_UID | ATTR_GID))
 		return -EPERM;
 
+	if (attr->ia_valid & ATTR_MODE) {
+		umode_t max_mode = 0777; /* Only these bits may change */
+
+		if (IS_ERR(head))
+			return PTR_ERR(head);
+
+		if (!table) /* global root - r-xr-xr-x */
+			max_mode &= ~0222;
+		else /*
+		      * Don't allow permissions to become less
+		      * restrictive than the sysctl table entry
+		      */
+			max_mode &= table->mode;
+
+		/* Execute bits only allowed for directories */
+		if (!S_ISDIR(inode->i_mode))
+			max_mode &= ~0111;
+
+		if (attr->ia_mode & ~S_IFMT & ~max_mode)
+			return -EPERM;
+	}
+
 	error = setattr_prepare(dentry, attr);
 	if (error)
 		return error;
 
 	setattr_copy(inode, attr);
 	mark_inode_dirty(inode);
+
+	if (table)
+		table->current_mode = inode->i_mode;
+	sysctl_head_finish(head);
+
 	return 0;
 }
 
@@ -861,7 +894,7 @@ static int proc_sys_getattr(const struct path *path, struct kstat *stat,
 
 	generic_fillattr(inode, stat);
 	if (table)
-		stat->mode = (stat->mode & S_IFMT) | table->mode;
+		stat->mode = (stat->mode & S_IFMT) | table->current_mode;
 
 	sysctl_head_finish(head);
 	return 0;
@@ -981,7 +1014,7 @@ static struct ctl_dir *new_dir(struct ctl_table_set *set,
 	memcpy(new_name, name, namelen);
 	new_name[namelen] = '\0';
 	table[0].procname = new_name;
-	table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
+	table[0].current_mode = table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
 	init_header(&new->header, set->dir.header.root, set, node, table);
 
 	return new;
@@ -1155,6 +1188,7 @@ static int sysctl_check_table(const char *path, struct ctl_table *table)
 		if ((table->mode & (S_IRUGO|S_IWUGO)) != table->mode)
 			err |= sysctl_err(path, table, "bogus .mode 0%o",
 				table->mode);
+		table->current_mode = table->mode;
 	}
 	return err;
 }
@@ -1192,7 +1226,7 @@ static struct ctl_table_header *new_links(struct ctl_dir *dir, struct ctl_table
 		int len = strlen(entry->procname) + 1;
 		memcpy(link_name, entry->procname, len);
 		link->procname = link_name;
-		link->mode = S_IFLNK|S_IRWXUGO;
+		link->current_mode = link->mode = S_IFLNK|S_IRWXUGO;
 		link->data = link_root;
 		link_name += len;
 	}
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 6df477329b76..7c519c35bf9c 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -126,6 +126,7 @@ struct ctl_table
 	void *data;
 	int maxlen;
 	umode_t mode;
+	umode_t current_mode;
 	struct ctl_table *child;	/* Deprecated */
 	proc_handler *proc_handler;	/* Callback for text formatting */
 	struct ctl_table_poll *poll;
-- 
2.24.0.rc1
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