[PATCH 1/2] powerpc/pseries: block untrusted device tree changes when locked down

[Nathan Lynch]

The /proc/powerpc/ofdt interface allows the root user to freely alter
the in-kernel device tree, enabling arbitrary physical address writes
via drivers that could bind to malicious device nodes, thus making it
possible to disable lockdown.

Historically this interface has been used on the pseries platform to
facilitate the runtime addition and removal of processor, memory, and
device resources (aka Dynamic Logical Partitioning or DLPAR). Years
ago, the processor and memory use cases were migrated to designs that
happen to be lockdown-friendly: device tree updates are communicated
directly to the kernel from firmware without passing through untrusted
user space. I/O device DLPAR via the "drmgr" command in powerpc-utils
remains the sole legitimate user of /proc/powerpc/ofdt, but it is
already broken in lockdown since it uses /dev/mem to allocate argument
buffers for the rtas syscall. So only illegitimate uses of the
interface should see a behavior change when running on a locked down
kernel.

Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
---
 arch/powerpc/platforms/pseries/reconfig.c | 5 +++++
 include/linux/security.h                  | 1 +
 security/security.c                       | 1 +
 3 files changed, 7 insertions(+)

diff --git a/arch/powerpc/platforms/pseries/reconfig.c b/arch/powerpc/platforms/pseries/reconfig.c
index cad7a0c93117..599bd2c78514 100644
--- a/arch/powerpc/platforms/pseries/reconfig.c
+++ b/arch/powerpc/platforms/pseries/reconfig.c
@@ -10,6 +10,7 @@
 #include <linux/kernel.h>
 #include <linux/notifier.h>
 #include <linux/proc_fs.h>
+#include <linux/security.h>
 #include <linux/slab.h>
 #include <linux/of.h>
 
@@ -361,6 +362,10 @@ static ssize_t ofdt_write(struct file *file, const char __user *buf, size_t coun
 	char *kbuf;
 	char *tmp;
 
+	rv = security_locked_down(LOCKDOWN_DEVICE_TREE);
+	if (rv)
+		return rv;
+
 	kbuf = memdup_user_nul(buf, count);
 	if (IS_ERR(kbuf))
 		return PTR_ERR(kbuf);
diff --git a/include/linux/security.h b/include/linux/security.h
index 7bd0c490703d..1ca8dbacd3cc 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -122,6 +122,7 @@ enum lockdown_reason {
 	LOCKDOWN_XMON_WR,
 	LOCKDOWN_BPF_WRITE_USER,
 	LOCKDOWN_DBG_WRITE_KERNEL,
+	LOCKDOWN_DEVICE_TREE,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_KCORE,
 	LOCKDOWN_KPROBES,
diff --git a/security/security.c b/security/security.c
index 4b95de24bc8d..2863fc31eec6 100644
--- a/security/security.c
+++ b/security/security.c
@@ -60,6 +60,7 @@ const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_XMON_WR] = "xmon write access",
 	[LOCKDOWN_BPF_WRITE_USER] = "use of bpf to write user RAM",
 	[LOCKDOWN_DBG_WRITE_KERNEL] = "use of kgdb/kdb to write kernel RAM",
+	[LOCKDOWN_DEVICE_TREE] = "modifying device tree contents",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",
-- 
2.37.3