Message-ID: <ed51f7dd-50a2-fbf5-7ea8-4bab6d48279e@gmail.com>
Date:   Mon, 4 Nov 2019 14:07:29 +0200
From:   Topi Miettinen <toiwoton@...il.com>
To:     Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
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@...il.com>
---
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


View attachment "0001-proc-Allow-restricting-permissions-in-proc-sys.patch" of type "text/x-diff" (4046 bytes)

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