linux-kernel - [PATCH v2] x86/platform/uv: refactor deprecated strcpy and strncpy

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Message-ID: <20230824-strncpy-arch-x86-platform-uv-uv_nmi-v2-1-e16d9a3ec570@google.com>
Date:   Thu, 24 Aug 2023 18:52:18 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     Steve Wahl <steve.wahl@....com>, Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Yang Yang <yang.yang29@....com.cn>,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH v2] x86/platform/uv: refactor deprecated strcpy and strncpy

Both `strncpy` and `strcpy` are 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` or `strcpy`!

In this case, we can drop both the forced NUL-termination and the `... -1` from:
|       strncpy(arg, val, ACTION_LEN - 1);
as `strscpy` implicitly has this behavior.

Also include slight refactor to code removing possible new-line chars as
per Yang Yang's work at [3]. This reduces code size and complexity by
using more robust and better understood interfaces.

Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://lore.kernel.org/all/202212091545310085328@zte.com.cn/ [3]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@...r.kernel.org
Co-developed-by: Yang Yang <yang.yang29@....com.cn>
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
Changes in v2:
- use `sizeof` on destination string instead of `strlen` (thanks Andy, Kees and Dimitri)
- refactor code to remove potential new-line chars (thanks Yang Yang and Andy)
- Link to v1: https://lore.kernel.org/r/20230822-strncpy-arch-x86-platform-uv-uv_nmi-v1-1-931f2943de0d@google.com
---
Note: build-tested only

Another thing, Yang Yang's patch [3] had some review from Andy regarding
the use of `-1` and `+1` in and around the strnchrnul invocation. I
believe Yang Yang's original implementation is correct but let's also
just use sizeof(arg) instead of ACTION_LEN.

Here's a godbolt link detailing some findings around the new-line
refactor in response to Andy's feedback: https://godbolt.org/z/K8drG3oq5
---
 arch/x86/platform/uv/uv_nmi.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/arch/x86/platform/uv/uv_nmi.c b/arch/x86/platform/uv/uv_nmi.c
index a60af0230e27..913347b2b9ab 100644
--- a/arch/x86/platform/uv/uv_nmi.c
+++ b/arch/x86/platform/uv/uv_nmi.c
@@ -202,21 +202,17 @@ static int param_set_action(const char *val, const struct kernel_param *kp)
 {
 	int i;
 	int n = ARRAY_SIZE(valid_acts);
-	char arg[ACTION_LEN], *p;
+	char arg[ACTION_LEN];
 
 	/* (remove possible '\n') */
-	strncpy(arg, val, ACTION_LEN - 1);
-	arg[ACTION_LEN - 1] = '\0';
-	p = strchr(arg, '\n');
-	if (p)
-		*p = '\0';
+	strscpy(arg, val, strnchrnul(val, sizeof(arg) - 1, '\n') - val + 1);
 
 	for (i = 0; i < n; i++)
 		if (!strcmp(arg, valid_acts[i].action))
 			break;
 
 	if (i < n) {
-		strcpy(uv_nmi_action, arg);
+		strscpy(uv_nmi_action, arg, sizeof(uv_nmi_action));
 		pr_info("UV: New NMI action:%s\n", uv_nmi_action);
 		return 0;
 	}
@@ -959,7 +955,7 @@ static int uv_handle_nmi(unsigned int reason, struct pt_regs *regs)
 
 		/* Unexpected return, revert action to "dump" */
 		if (master)
-			strncpy(uv_nmi_action, "dump", strlen(uv_nmi_action));
+			strscpy(uv_nmi_action, "dump", sizeof(uv_nmi_action));
 	}
 
 	/* Pause as all CPU's enter the NMI handler */

---
base-commit: 706a741595047797872e669b3101429ab8d378ef
change-id: 20230822-strncpy-arch-x86-platform-uv-uv_nmi-474e5295c2c1

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