[PATCH v3] um: refactor deprecated strncpy to memcpy

[Justin Stitt]

Use `memcpy` since `console_buf` is not expected to be NUL-terminated
and it more accurately describes what is happening with the buffers
`console_buf` and `string` as per Kees' analysis [1].

Also mark char buffer as `__nonstring` as per Kees' suggestion [2].

This change now makes it more clear what this code does and that
`console_buf` is not expected to be NUL-terminated.

Link: https://lore.kernel.org/all/202308081708.D5ADC80F@keescook/ [1]
Link: https://github.com/KSPP/linux/issues/90 [2]
Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Cc: linux-hardening@xxxxxxxxxxxxxxx
Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v3:
- Swap `strtomem` to `memcpy` (thanks Kees)
- Link to v2: https://lore.kernel.org/r/20230808-arch-um-v2-1-5fbe76eaf853@xxxxxxxxxx

Changes in v2:
- Keep usage of ARRAY_SIZE (thanks Bill)
- Remove unnecessary import (thanks Kees and Bill)
- Link to v1: https://lore.kernel.org/r/20230807-arch-um-v1-1-86dbbfb59709@xxxxxxxxxx
---
Notes:
I only build tested this patch.
---
 arch/um/drivers/mconsole_kern.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index 5026e7b9adfe..ff4bda95b9c7 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -554,7 +554,7 @@ struct mconsole_output {
 
 static DEFINE_SPINLOCK(client_lock);
 static LIST_HEAD(clients);
-static char console_buf[MCONSOLE_MAX_DATA];
+static char console_buf[MCONSOLE_MAX_DATA] __nonstring;
 
 static void console_write(struct console *console, const char *string,
 			  unsigned int len)
@@ -567,7 +567,7 @@ static void console_write(struct console *console, const char *string,
 
 	while (len > 0) {
 		n = min((size_t) len, ARRAY_SIZE(console_buf));
-		strncpy(console_buf, string, n);
+		memcpy(console_buf, string, n);
 		string += n;
 		len -= n;
 

---
base-commit: c1a515d3c0270628df8ae5f5118ba859b85464a2
change-id: 20230807-arch-um-3ef24413427e

Best regards,
--
Justin Stitt <justinstitt@xxxxxxxxxx>