[PATCH] x86/mm/mem_encrypt: Disable all instrumentation for SME early boot code

[Hook, Gary]

Enablement of AMD's Secure Memory Encryption feature is determined
very early in the boot cycle. Part of this procedure involves scanning
the command line for the paramater 'mem_encrypt'.

To determine intended state, the function sme_enable() uses library
functions cmdline_find_option() and strncmp().  Their use occurs early
enough such that we can't assume that any instrumentation subsystem is
initialized. For example, making calls to a KASAN-instrumented
function before KASAN is set up will likely result in the use of
uninitialized memory and a boot failure.

Avoid instrumenting these dependent functions by:

1) Making a local, static, renamed copy of strncpy() for use solely in
mem_encrypt_identity.c. In this file we are able to vet its few uses
and avoid exposing the rest of the kernel to a ubiquitously used but
un-instrumented function.

2) Disable instrumention of arch/x86/lib/cmdline.c based on the
assumption that the needed function (cmdline_find_option()) is vetted
through its use to date, and contains no lurking flaws that have not
yet been found through instrumentation such as KASAN.

Fixes: aca20d546214  ("x86/mm: Add support to make use of Secure Memory Encryption")
Reported-by: Li RongQing <lirongqing@xxxxxxxxx>
Signed-off-by: Gary R Hook <gary.hook@xxxxxxx>
---
 arch/x86/lib/Makefile              |   13 +++++++++++++
 arch/x86/mm/mem_encrypt_identity.c |   26 ++++++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 140e61843a07..38182a64fa81 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -6,6 +6,19 @@
 # Produces uninteresting flaky coverage.
 KCOV_INSTRUMENT_delay.o	:= n
 
+# SME early boot code checks the cmdline, so don't instrument
+KCOV_INSTRUMENT_cmdline.o              := n
+
+KASAN_SANITIZE_cmdline.o               := n
+
+ifdef CONFIG_FUNCTION_TRACER
+CFLAGS_REMOVE_cmdline.o                        = -pg
+endif
+
+# No stack protector
+nostackp := $(call cc-option, -fno-stack-protector)
+CFLAGS_cmdline.o               := $(nostackp)
+
 inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
 inat_tables_maps = $(srctree)/arch/x86/lib/x86-opcode-map.txt
 quiet_cmd_inat_tables = GEN     $@
diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index 4aa9b1480866..0a68d7ccb371 100644
--- a/arch/x86/mm/mem_encrypt_identity.c
+++ b/arch/x86/mm/mem_encrypt_identity.c
@@ -77,6 +77,28 @@ static char sme_cmdline_arg[] __initdata = "mem_encrypt";
 static char sme_cmdline_on[]  __initdata = "on";
 static char sme_cmdline_off[] __initdata = "off";
 
+/*
+ * Local copy to avoid instrumentation
+ * Copied from lib/string.c and renamed to be unique.
+ * This file is early boot code, and we assume that instrumentation
+ * subsystems (e.g. KASAN) are not yet initialized.
+ */
+static int sme_strncmp(const char *cs, const char *ct, size_t count)
+{
+	unsigned char c1, c2;
+
+	while (count) {
+		c1 = *cs++;
+		c2 = *ct++;
+		if (c1 != c2)
+			return c1 < c2 ? -1 : 1;
+		if (!c1)
+			break;
+		count--;
+	}
+	return 0;
+}
+
 static void __init sme_clear_pgd(struct sme_populate_pgd_data *ppd)
 {
 	unsigned long pgd_start, pgd_end, pgd_size;
@@ -557,9 +579,9 @@ void __init sme_enable(struct boot_params *bp)
 
 	cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer));
 
-	if (!strncmp(buffer, cmdline_on, sizeof(buffer)))
+	if (!sme_strncmp(buffer, cmdline_on, sizeof(buffer)))
 		sme_me_mask = me_mask;
-	else if (!strncmp(buffer, cmdline_off, sizeof(buffer)))
+	else if (!sme_strncmp(buffer, cmdline_off, sizeof(buffer)))
 		sme_me_mask = 0;
 	else
 		sme_me_mask = active_by_default ? me_mask : 0;