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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>
This commit is contained in:
Peter Enderborg 2020-07-16 09:15:11 +02:00 committed by Greg Kroah-Hartman
parent 072e133d55
commit a24c6f7bc9
4 changed files with 100 additions and 0 deletions
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15
Documentation/admin-guide/kernel-parameters.txt
View File

@ -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]

39
fs/debugfs/inode.c
View File

@ -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;

14
fs/debugfs/internal.h
View File

@ -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_ */

32
lib/Kconfig.debug
View File

@ -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"