[PATCH] Allow restricting permissions in /proc/sys

[Topi Miettinen <toiwoton@gmail.com>]

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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@gmail.com>
---
  fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
  1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d80989b6c344..88c4ca7d2782 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);
@@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry, 
struct iattr *attr)
         struct inode *inode = d_inode(dentry);
         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) {
+               struct ctl_table_header *head = grab_header(inode);
+               struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+               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;
+
+               sysctl_head_finish(head);
+
+               /* 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;
@@ -853,17 +883,8 @@ static int proc_sys_getattr(const struct path 
*path, struct kstat *stat,
                             u32 request_mask, unsigned int query_flags)
  {
         struct inode *inode = d_inode(path->dentry);
-       struct ctl_table_header *head = grab_header(inode);
-       struct ctl_table *table = PROC_I(inode)->sysctl_entry;
-
-       if (IS_ERR(head))
-               return PTR_ERR(head);

         generic_fillattr(inode, stat);
-       if (table)
-               stat->mode = (stat->mode & S_IFMT) | table->mode;
-
-       sysctl_head_finish(head);
         return 0;
  }

-- 
2.24.0.rc1


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From 14ad2d9034ecb43b60f59f6422e597a780c65cd9 Mon Sep 17 00:00:00 2001
From: Topi Miettinen <toiwoton@gmail.com>
Date: Sun, 3 Nov 2019 16:36:43 +0200
Subject: [PATCH] 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@gmail.com>
---
 fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d80989b6c344..88c4ca7d2782 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);
@@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
 	struct inode *inode = d_inode(dentry);
 	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) {
+		struct ctl_table_header *head = grab_header(inode);
+		struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+		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;
+
+		sysctl_head_finish(head);
+
+		/* 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;
@@ -853,17 +883,8 @@ static int proc_sys_getattr(const struct path *path, struct kstat *stat,
 			    u32 request_mask, unsigned int query_flags)
 {
 	struct inode *inode = d_inode(path->dentry);
-	struct ctl_table_header *head = grab_header(inode);
-	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
-
-	if (IS_ERR(head))
-		return PTR_ERR(head);
 
 	generic_fillattr(inode, stat);
-	if (table)
-		stat->mode = (stat->mode & S_IFMT) | table->mode;
-
-	sysctl_head_finish(head);
 	return 0;
 }
 
-- 
2.24.0.rc1