debugfs: Add access restriction option

Since debugfs include sensitive information it need to be treated
carefully. But it also has many very useful debug functions for userspace.
With this option we can have same configuration for system with
need of debugfs and a way to turn it off. This gives a extra protection
for exposure on systems where user-space services with system
access are attacked.

It is controlled by a configurable default value that can be override
with a kernel command line parameter. (debugfs=)

It can be on or off, but also internally on but not seen from user-space.
This no-mount mode do not register a debugfs as filesystem, but client can
register their parts in the internal structures. This data can be readed
with a debugger or saved with a crashkernel. When it is off clients
get EPERM error when accessing the functions for registering their
components.

Signed-off-by: Peter Enderborg <peter.enderborg@sony.com>
Link: https://lore.kernel.org/r/20200716071511.26864-3-peter.enderborg@sony.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fb95fad81c79a065e728202a34a3dabac5653cb4..6766a308ad9645e96337e4ff8cdabe01c7151fda 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -827,6 +827,21 @@
                        useful to also enable the page_owner functionality.
                        on: enable the feature
 
+       debugfs=        [KNL] This parameter enables what is exposed to userspace
+                       and debugfs internal clients.
+                       Format: { on, no-mount, off }
+                       on:     All functions are enabled.
+                       no-mount:
+                               Filesystem is not registered but kernel clients can
+                               access APIs and a crashkernel can be used to read
+                               its content. There is nothing to mount.
+                       off:    Filesystem is not registered and clients
+                               get a -EPERM as result when trying to register files
+                               or directories within debugfs.
+                               This is equivalent of the runtime functionality if
+                               debugfs was not enabled in the kernel at all.
+                       Default value is set in build-time with a kernel configuration.
+
        debugpat        [X86] Enable PAT debugging
 
        decnet.addr=    [HW,NET]

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index b7f2e971ecbc83f4f824dc6bf74c461a214c7a8d..2fcf66473436bb672259a0ef4171c8728bfc942b 100644 (file)
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -35,6 +35,7 @@
 static struct vfsmount *debugfs_mount;
 static int debugfs_mount_count;
 static bool debugfs_registered;
+static unsigned int debugfs_allow = DEFAULT_DEBUGFS_ALLOW_BITS;
 
 /*
  * Don't allow access attributes to be changed whilst the kernel is locked down
@@ -266,6 +267,9 @@ static struct dentry *debug_mount(struct file_system_type *fs_type,
                        int flags, const char *dev_name,
                        void *data)
 {
+       if (!(debugfs_allow & DEBUGFS_ALLOW_API))
+               return ERR_PTR(-EPERM);
+
        return mount_single(fs_type, flags, data, debug_fill_super);
 }
 
@@ -311,6 +315,9 @@ static struct dentry *start_creating(const char *name, struct dentry *parent)
        struct dentry *dentry;
        int error;
 
+       if (!(debugfs_allow & DEBUGFS_ALLOW_API))
+               return ERR_PTR(-EPERM);
+
        pr_debug("creating file '%s'\n", name);
 
        if (IS_ERR(parent))
@@ -385,6 +392,11 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
        if (IS_ERR(dentry))
                return dentry;
 
+       if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
+               failed_creating(dentry);
+               return ERR_PTR(-EPERM);
+       }
+
        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode)) {
                pr_err("out of free dentries, can not create file '%s'\n",
@@ -541,6 +553,11 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
        if (IS_ERR(dentry))
                return dentry;
 
+       if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
+               failed_creating(dentry);
+               return ERR_PTR(-EPERM);
+       }
+
        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode)) {
                pr_err("out of free dentries, can not create directory '%s'\n",
@@ -583,6 +600,11 @@ struct dentry *debugfs_create_automount(const char *name,
        if (IS_ERR(dentry))
                return dentry;
 
+       if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
+               failed_creating(dentry);
+               return ERR_PTR(-EPERM);
+       }
+
        inode = debugfs_get_inode(dentry->d_sb);
        if (unlikely(!inode)) {
                pr_err("out of free dentries, can not create automount '%s'\n",
@@ -786,10 +808,27 @@ bool debugfs_initialized(void)
 }
 EXPORT_SYMBOL_GPL(debugfs_initialized);
 
+static int __init debugfs_kernel(char *str)
+{
+       if (str) {
+               if (!strcmp(str, "on"))
+                       debugfs_allow = DEBUGFS_ALLOW_API | DEBUGFS_ALLOW_MOUNT;
+               else if (!strcmp(str, "no-mount"))
+                       debugfs_allow = DEBUGFS_ALLOW_API;
+               else if (!strcmp(str, "off"))
+                       debugfs_allow = 0;
+       }
+
+       return 0;
+}
+early_param("debugfs", debugfs_kernel);
 static int __init debugfs_init(void)
 {
        int retval;
 
+       if (!(debugfs_allow & DEBUGFS_ALLOW_MOUNT))
+               return -EPERM;
+
        retval = sysfs_create_mount_point(kernel_kobj, "debug");
        if (retval)
                return retval;

diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h
index 034e6973cead30a79918091f7e985453b4b06ae8..92af8ae313134654e52bcb9f90b22280a6ae775e 100644 (file)
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -29,4 +29,18 @@ struct debugfs_fsdata {
  */
 #define DEBUGFS_FSDATA_IS_REAL_FOPS_BIT BIT(0)
 
+/* Access BITS */
+#define DEBUGFS_ALLOW_API      BIT(0)
+#define DEBUGFS_ALLOW_MOUNT    BIT(1)
+
+#ifdef CONFIG_DEBUG_FS_ALLOW_ALL
+#define DEFAULT_DEBUGFS_ALLOW_BITS (DEBUGFS_ALLOW_MOUNT | DEBUGFS_ALLOW_API)
+#endif
+#ifdef CONFIG_DEBUG_FS_DISALLOW_MOUNT
+#define DEFAULT_DEBUGFS_ALLOW_BITS (DEBUGFS_ALLOW_API)
+#endif
+#ifdef CONFIG_DEBUG_FS_ALLOW_NONE
+#define DEFAULT_DEBUGFS_ALLOW_BITS (0)
+#endif
+
 #endif /* _DEBUGFS_INTERNAL_H_ */

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2bff6315224d9fdb929e01c3739f0c08fabdcb25..256e4ce65f1ab69b79b70328a2a5090c91eedf3a 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -476,6 +476,38 @@ config DEBUG_FS
 
          If unsure, say N.
 
+choice
+       prompt "Debugfs default access"
+       depends on DEBUG_FS
+       default DEBUG_FS_ALLOW_ALL
+       help
+         This selects the default access restrictions for debugfs.
+         It can be overridden with kernel command line option
+         debugfs=[on,no-mount,off]. The restrictions apply for API access
+         and filesystem registration.
+
+config DEBUG_FS_ALLOW_ALL
+       bool "Access normal"
+       help
+         No restrictions apply. Both API and filesystem registration
+         is on. This is the normal default operation.
+
+config DEBUG_FS_DISALLOW_MOUNT
+       bool "Do not register debugfs as filesystem"
+       help
+         The API is open but filesystem is not loaded. Clients can still do
+         their work and read with debug tools that do not need
+         debugfs filesystem.
+
+config DEBUG_FS_ALLOW_NONE
+       bool "No access"
+       help
+         Access is off. Clients get -PERM when trying to create nodes in
+         debugfs tree and debugfs is not registered as a filesystem.
+         Client can then back-off or continue without debugfs access.
+
+endchoice
+
 source "lib/Kconfig.kgdb"
 
 source "lib/Kconfig.ubsan"