[PATCH] ARC: u-boot args: check that magic number is correct

From: Eugeniy Paltsev
Date: Mon Feb 25 2019 - 12:16:08 EST

In case of devboards we really often disable bootloader and load
Linux image in memory via JTAG. Even if kernel tries to verify
uboot_tag and uboot_arg there is sill a chance that we treat some
garbage in registers as valid u-boot arguments in JTAG case.
E.g. it is enough to have '1' in r0 to treat any value in r2 as
a boot command line.

So check that magic number passed from u-boot is correct and drop
u-boot arguments otherwise. That helps to reduce the possibility
of using garbage as u-boot arguments in JTAG case.

We can safely check U-boot magic value (0x0) in linux passed via
r1 register as U-boot pass it from the beginning. So there is no
backward-compatibility issues.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@xxxxxxxxxxxx>
arch/arc/kernel/head.S | 1 +
arch/arc/kernel/setup.c | 8 ++++++++
2 files changed, 9 insertions(+)

diff --git a/arch/arc/kernel/head.S b/arch/arc/kernel/head.S
index 30e090625916..a72bbda2f7aa 100644
--- a/arch/arc/kernel/head.S
+++ b/arch/arc/kernel/head.S
@@ -106,6 +106,7 @@ ENTRY(stext)
; r2 = pointer to uboot provided cmdline or external DTB in mem
; These are handled later in handle_uboot_args()
st r0, [@uboot_tag]
+ st r1, [@uboot_magic]
st r2, [@uboot_arg]

; setup "current" tsk and optionally cache it in dedicated r25
diff --git a/arch/arc/kernel/setup.c b/arch/arc/kernel/setup.c
index 7b2340996cf8..7b3a7b3b380c 100644
--- a/arch/arc/kernel/setup.c
+++ b/arch/arc/kernel/setup.c
@@ -36,6 +36,7 @@ unsigned int intr_to_DE_cnt;

/* Part of U-boot ABI: see head.S */
int __initdata uboot_tag;
+int __initdata uboot_magic;
char __initdata *uboot_arg;

const struct machine_desc *machine_desc;
@@ -497,6 +498,8 @@ static inline bool uboot_arg_invalid(unsigned long addr)
#define UBOOT_TAG_NONE 0
#define UBOOT_TAG_DTB 2
+/* We always pass 0 as magic from U-boot */

void __init handle_uboot_args(void)
@@ -511,6 +514,11 @@ void __init handle_uboot_args(void)
goto ignore_uboot_args;

+ if (uboot_magic != UBOOT_MAGIC_VALUE) {
+ pr_warn(IGNORE_ARGS "non zero uboot magic\n");
+ goto ignore_uboot_args;
+ }
if (uboot_tag != UBOOT_TAG_NONE &&
uboot_arg_invalid((unsigned long)uboot_arg)) {
pr_warn(IGNORE_ARGS "invalid uboot arg: '%px'\n", uboot_arg);