Re: [PATCH v2] KEYS: encrypted: Replace deprecated strcpy and improve get_derived_key

[David Laight <david.laight.linux@gmail.com>]

On Thu, 13 Nov 2025 22:55:45 +0100
Thorsten Blum <thorsten.blum@linux.dev> wrote:

> strcpy() is deprecated; use the safer strscpy() and use its return
> value, the number of bytes copied, instead of calling strlen() on the
> destination buffer again. String truncation can be ignored since
> 'derived_buf' is guaranteed to be large enough.
> 
> Link: https://github.com/KSPP/linux/issues/88
> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> ---
> Changes in v2:
> - Revert some changes to include the trailing '\0' even if key_type == 0
>   since this would change the bytes passed to sha256() (Eric Biggers)
> - Use strscpy() and its return value instead
> - Link to v1: https://lore.kernel.org/lkml/20251113135831.98587-1-thorsten.blum@linux.dev/
> ---
>  security/keys/encrypted-keys/encrypted.c | 21 ++++++++-------------
>  1 file changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
> index 15841466b5d4..94408909f1dd 100644
> --- a/security/keys/encrypted-keys/encrypted.c
> +++ b/security/keys/encrypted-keys/encrypted.c
> @@ -12,6 +12,7 @@
>   */
>  
>  #include <linux/uaccess.h>
> +#include <linux/minmax.h>
>  #include <linux/module.h>
>  #include <linux/init.h>
>  #include <linux/slab.h>
> @@ -330,23 +331,17 @@ static int get_derived_key(u8 *derived_key, enum derived_key_type key_type,
>  			   const u8 *master_key, size_t master_keylen)
>  {
>  	u8 *derived_buf;
> -	unsigned int derived_buf_len;
> -
> -	derived_buf_len = strlen("AUTH_KEY") + 1 + master_keylen;
> -	if (derived_buf_len < HASH_SIZE)
> -		derived_buf_len = HASH_SIZE;
> +	size_t derived_buf_len;
> +	const char *key_name;
> +	ssize_t len;
>  
> +	derived_buf_len = max(strlen("AUTH_KEY") + 1 + master_keylen, HASH_SIZE);
>  	derived_buf = kzalloc(derived_buf_len, GFP_KERNEL);
>  	if (!derived_buf)
>  		return -ENOMEM;
> -
> -	if (key_type)
> -		strcpy(derived_buf, "AUTH_KEY");
> -	else
> -		strcpy(derived_buf, "ENC_KEY");
> -
> -	memcpy(derived_buf + strlen(derived_buf) + 1, master_key,
> -	       master_keylen);
> +	key_name = key_type ? "AUTH_KEY" : "ENC_KEY";
> +	len = strscpy(derived_buf, key_name, derived_buf_len);
> +	memcpy(derived_buf + len + 1, master_key, master_keylen);
>  	sha256(derived_buf, derived_buf_len, derived_key);

I'm not sure this is an improvement, but has this code ever been correct?
The buffer passed to sha256 is either:
	"AUTH_KEY"'\0'master_key
or
	"ENC_KEY"'\0'master_key
For short master_key the buffer is HASH_SIZE bytes and padded with zeros (ok).
However for long master_key the length is calculated using "AUTH_KEY" so
there is an additional trailing '\0' in the "ENC_KEY" case.

	David



>  	kfree_sensitive(derived_buf);
>  	return 0;