[PATCH V36 27/29] tracefs: Restrict tracefs when the kernel is locked down

[Matthew Garrett]

Tracefs may release more information about the kernel than desirable, so
restrict it when the kernel is locked down in confidentiality mode by
preventing open().

Signed-off-by: Matthew Garrett <mj...@google.com>
Cc: Steven Rostedt <rost...@goodmis.org>
---
 fs/tracefs/inode.c           | 38 +++++++++++++++++++++++++++++++++++-
 include/linux/security.h     |  1 +
 security/lockdown/lockdown.c |  1 +
 3 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index eeeae0475da9..8a20137e1d8f 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -20,6 +20,7 @@
 #include <linux/parser.h>
 #include <linux/magic.h>
 #include <linux/slab.h>
+#include <linux/security.h>
 
 #define TRACEFS_DEFAULT_MODE   0700
 
@@ -27,6 +28,23 @@ static struct vfsmount *tracefs_mount;
 static int tracefs_mount_count;
 static bool tracefs_registered;
 
+static int default_open_file(struct inode *inode, struct file *filp)
+{
+       struct dentry *dentry = filp->f_path.dentry;
+       struct file_operations *real_fops;
+       int ret;
+
+       if (!dentry)
+               return -EINVAL;
+
+       ret = security_locked_down(LOCKDOWN_TRACEFS);
+       if (ret)
+               return ret;
+
+       real_fops = dentry->d_fsdata;
+       return real_fops->open(inode, filp);
+}
+
 static ssize_t default_read_file(struct file *file, char __user *buf,
                                 size_t count, loff_t *ppos)
 {
@@ -221,6 +239,12 @@ static int tracefs_apply_options(struct super_block *sb)
        return 0;
 }
 
+static void tracefs_destroy_inode(struct inode *inode)
+{
+       if (S_ISREG(inode->i_mode))
+               kfree(inode->i_fop);
+}
+
 static int tracefs_remount(struct super_block *sb, int *flags, char *data)
 {
        int err;
@@ -256,6 +280,7 @@ static int tracefs_show_options(struct seq_file *m, struct 
dentry *root)
 
 static const struct super_operations tracefs_super_operations = {
        .statfs         = simple_statfs,
+       .destroy_inode  = tracefs_destroy_inode,
        .remount_fs     = tracefs_remount,
        .show_options   = tracefs_show_options,
 };
@@ -387,6 +412,7 @@ struct dentry *tracefs_create_file(const char *name, 
umode_t mode,
                                   struct dentry *parent, void *data,
                                   const struct file_operations *fops)
 {
+       struct file_operations *proxy_fops;
        struct dentry *dentry;
        struct inode *inode;
 
@@ -402,8 +428,18 @@ struct dentry *tracefs_create_file(const char *name, 
umode_t mode,
        if (unlikely(!inode))
                return failed_creating(dentry);
 
+       proxy_fops = kzalloc(sizeof(struct file_operations), GFP_KERNEL);
+       if (!proxy_fops)
+               return failed_creating(dentry);
+
+       if (!fops)
+               fops = &tracefs_file_operations;
+
+       dentry->d_fsdata = (void *)fops;
+       memcpy(proxy_fops, fops, sizeof(*proxy_fops));
+       proxy_fops->open = default_open_file;
        inode->i_mode = mode;
-       inode->i_fop = fops ? fops : &tracefs_file_operations;
+       inode->i_fop = proxy_fops;
        inode->i_private = data;
        d_instantiate(dentry, inode);
        fsnotify_create(dentry->d_parent->d_inode, dentry);
diff --git a/include/linux/security.h b/include/linux/security.h
index d92323b44a3f..807dc0d24982 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -121,6 +121,7 @@ enum lockdown_reason {
        LOCKDOWN_KPROBES,
        LOCKDOWN_BPF_READ,
        LOCKDOWN_PERF,
+       LOCKDOWN_TRACEFS,
        LOCKDOWN_CONFIDENTIALITY_MAX,
 };
 
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 88064ce1c844..173191562047 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -36,6 +36,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] 
= {
        [LOCKDOWN_KPROBES] = "use of kprobes",
        [LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
        [LOCKDOWN_PERF] = "unsafe use of perf",
+       [LOCKDOWN_TRACEFS] = "use of tracefs",
        [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };
 
-- 
2.22.0.510.g264f2c817a-goog