linux-kernel - [PATCH 2/2] powerpc/rtas: block error injection when locked down

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Message-Id: <20220922193817.106041-3-nathanl@linux.ibm.com>
Date:   Thu, 22 Sep 2022 14:38:17 -0500
From:   Nathan Lynch <nathanl@...ux.ibm.com>
To:     linuxppc-dev@...ts.ozlabs.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     jmorris@...ei.org, mpe@...erman.id.au, paul@...l-moore.com,
        serge@...lyn.com, ajd@...ux.ibm.com, gcwilson@...ux.ibm.com,
        nayna@...ux.ibm.com
Subject: [PATCH 2/2] powerpc/rtas: block error injection when locked down

The error injection facility on pseries VMs allows corruption of
arbitrary guest memory, potentially enabling a sufficiently privileged
user to disable lockdown or perform other modifications of the running
kernel via the rtas syscall.

Block the PAPR error injection facility from being opened or called
when locked down.

Signed-off-by: Nathan Lynch <nathanl@...ux.ibm.com>
---
 arch/powerpc/kernel/rtas.c | 25 ++++++++++++++++++++++++-
 include/linux/security.h   |  1 +
 security/security.c        |  1 +
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index 693133972294..c2540d393f1c 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -23,6 +23,7 @@
 #include <linux/memblock.h>
 #include <linux/slab.h>
 #include <linux/reboot.h>
+#include <linux/security.h>
 #include <linux/syscalls.h>
 #include <linux/of.h>
 #include <linux/of_fdt.h>
@@ -464,6 +465,9 @@ void rtas_call_unlocked(struct rtas_args *args, int token, int nargs, int nret,
 	va_end(list);
 }
 
+static int ibm_open_errinjct_token;
+static int ibm_errinjct_token;
+
 int rtas_call(int token, int nargs, int nret, int *outputs, ...)
 {
 	va_list list;
@@ -476,6 +480,16 @@ int rtas_call(int token, int nargs, int nret, int *outputs, ...)
 	if (!rtas.entry || token == RTAS_UNKNOWN_SERVICE)
 		return -1;
 
+	if (token == ibm_open_errinjct_token || token == ibm_errinjct_token) {
+		/*
+		 * It would be nicer to not discard the error value
+		 * from security_locked_down(), but callers expect an
+		 * RTAS status, not an errno.
+		 */
+		if (security_locked_down(LOCKDOWN_RTAS_ERROR_INJECTION))
+			return -1;
+	}
+
 	if ((mfmsr() & (MSR_IR|MSR_DR)) != (MSR_IR|MSR_DR)) {
 		WARN_ON_ONCE(1);
 		return -1;
@@ -1227,6 +1241,14 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
 	if (block_rtas_call(token, nargs, &args))
 		return -EINVAL;
 
+	if (token == ibm_open_errinjct_token || token == ibm_errinjct_token) {
+		int err;
+
+		err = security_locked_down(LOCKDOWN_RTAS_ERROR_INJECTION);
+		if (err)
+			return err;
+	}
+
 	/* Need to handle ibm,suspend_me call specially */
 	if (token == rtas_token("ibm,suspend-me")) {
 
@@ -1325,7 +1347,8 @@ void __init rtas_initialize(void)
 #ifdef CONFIG_RTAS_ERROR_LOGGING
 	rtas_last_error_token = rtas_token("rtas-last-error");
 #endif
-
+	ibm_open_errinjct_token = rtas_token("ibm,open-errinjct");
+	ibm_errinjct_token = rtas_token("ibm,errinjct");
 	rtas_syscall_filter_init();
 }
 
diff --git a/include/linux/security.h b/include/linux/security.h
index 1ca8dbacd3cc..b5d5138ae66a 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -123,6 +123,7 @@ enum lockdown_reason {
 	LOCKDOWN_BPF_WRITE_USER,
 	LOCKDOWN_DBG_WRITE_KERNEL,
 	LOCKDOWN_DEVICE_TREE,
+	LOCKDOWN_RTAS_ERROR_INJECTION,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_KCORE,
 	LOCKDOWN_KPROBES,
diff --git a/security/security.c b/security/security.c
index 2863fc31eec6..6518b239ada2 100644
--- a/security/security.c
+++ b/security/security.c
@@ -61,6 +61,7 @@ const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[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_RTAS_ERROR_INJECTION] = "RTAS error injection",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",
-- 
2.37.3