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

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Message-Id: <20190225171601.5004-1-Eugeniy.Paltsev@synopsys.com>
Date:   Mon, 25 Feb 2019 20:16:01 +0300
From:   Eugeniy Paltsev <eugeniy.paltsev@...opsys.com>
To:     linux-snps-arc@...ts.infradead.org,
        Vineet Gupta <vineet.gupta1@...opsys.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        Eugeniy Paltsev <eugeniy.paltsev@...opsys.com>
Subject: [PATCH] ARC: u-boot args: check that magic number is correct

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@...opsys.com>
---
 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_CMDLINE	1
 #define UBOOT_TAG_DTB		2
+/* We always pass 0 as magic from U-boot */
+#define UBOOT_MAGIC_VALUE	0
 
 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);
-- 
2.14.5