linux-hardening - [PATCH] RISC-V: cpu: refactor deprecated strncpy

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Date:   Tue, 01 Aug 2023 21:14:56 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-hardening@...r.kernel.org,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] RISC-V: cpu: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on its destination buffer argument which is
_not_ the case for `strncpy`!

The `sv_type` buffer is declared with a size of 16 which is then
followed by some `strncpy` calls to populate the buffer with one of:
"sv32", "sv57", "sv48", "sv39" or "none". Hard-coding the max length as 5 is
error-prone and involves counting the number of characters (and
hopefully not forgetting to count the NUL-byte) in the raw string.

Using a pre-determined max length in combination with `strscpy` provides
a cleaner, less error-prone as well as a less ambiguous implementation.
`strscpy` guarantees that it's destination buffer is NUL-terminated even
if it's source argument exceeds the max length as defined by the third
argument.

To be clear, there is no bug (i think?) in the current implementation
but the current hard-coded values in combination with using a deprecated
interface make this a worthwhile change, IMO.

[1]: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
[2]: manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html

Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
 arch/riscv/kernel/cpu.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index a2fc952318e9..1c576e4ec171 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -17,6 +17,8 @@
 #include <asm/smp.h>
 #include <asm/pgtable.h>

+#define SV_TYPE_MAX_LENGTH 16
+
 /*
  * Returns the hart ID of the given device tree node, or -ENODEV if the node
  * isn't an enabled and valid RISC-V hart node.
@@ -271,21 +273,21 @@ static void print_isa(struct seq_file *f, const char *isa)

 static void print_mmu(struct seq_file *f)
 {
-	char sv_type[16];
+	char sv_type[SV_TYPE_MAX_LENGTH];

 #ifdef CONFIG_MMU
 #if defined(CONFIG_32BIT)
-	strncpy(sv_type, "sv32", 5);
+	strscpy(sv_type, "sv32", SV_TYPE_MAX_LENGTH);
 #elif defined(CONFIG_64BIT)
 	if (pgtable_l5_enabled)
-		strncpy(sv_type, "sv57", 5);
+		strscpy(sv_type, "sv57", SV_TYPE_MAX_LENGTH);
 	else if (pgtable_l4_enabled)
-		strncpy(sv_type, "sv48", 5);
+		strscpy(sv_type, "sv48", SV_TYPE_MAX_LENGTH);
 	else
-		strncpy(sv_type, "sv39", 5);
+		strscpy(sv_type, "sv39", SV_TYPE_MAX_LENGTH);
 #endif
 #else
-	strncpy(sv_type, "none", 5);
+	strscpy(sv_type, "none", SV_TYPE_MAX_LENGTH);
 #endif /* CONFIG_MMU */
 	seq_printf(f, "mmu\t\t: %s\n", sv_type);
 }

---
base-commit: 5d0c230f1de8c7515b6567d9afba1f196fb4e2f4
change-id: 20230801-arch-riscv-kernel-14a048cc6467

Best regards,
--
Justin Stitt <justinstitt@...gle.com>