linux-kernel - [PATCH AUTOSEL 4.9 01/27] ARC: u-boot args: check that magic number is correct

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Message-Id: <20190330005555.28087-1-sashal@kernel.org>
Date:   Fri, 29 Mar 2019 20:55:27 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Sasha Levin <sashal@...nel.org>,
        linux-snps-arc@...ts.infradead.org
Subject: [PATCH AUTOSEL 4.9 01/27] ARC: u-boot args: check that magic number is correct

From: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>

[ Upstream commit edb64bca50cd736c6894cc6081d5263c007ce005 ]

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>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 1f945d0f40da..208bf2c9e7b0 100644
--- a/arch/arc/kernel/head.S
+++ b/arch/arc/kernel/head.S
@@ -107,6 +107,7 @@ 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]
 #endif
 
diff --git a/arch/arc/kernel/setup.c b/arch/arc/kernel/setup.c
index 9119bea503a7..9f96120eee6e 100644
--- a/arch/arc/kernel/setup.c
+++ b/arch/arc/kernel/setup.c
@@ -32,6 +32,7 @@ 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;
@@ -400,6 +401,8 @@ 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)
 {
@@ -415,6 +418,11 @@ 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);
-- 
2.19.1