2023-08-09 19:03:45

by Justin Stitt

Subject: [PATCH v3] um: refactor deprecated strncpy to memcpy

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: [email protected]
Suggested-by: Kees Cook <[email protected]>
Signed-off-by: Justin Stitt <[email protected]>
---
Changes in v3:
- Swap `strtomem` to `memcpy` (thanks Kees)
- Link to v2: https://lore.kernel.org/r/[email protected]

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/[email protected]
---
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 <[email protected]>

2023-08-10 20:16:35

by Kees Cook

Subject: Re: [PATCH v3] um: refactor deprecated strncpy to memcpy

On Wed, Aug 09, 2023 at 06:19:32PM +0000, Justin Stitt wrote:
> 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: [email protected]
> Suggested-by: Kees Cook <[email protected]>
> Signed-off-by: Justin Stitt <[email protected]>

Thanks for digging through this!

Reviewed-by: Kees Cook <[email protected]>

--
Kees Cook