strncpy() is deprecated for NUL-terminated destination buffers; use
strscpy_pad() instead to retain the NUL-padding behavior of strncpy().

The destination buffer is initialized using kzalloc() with a 'signature'
size of ECRYPTFS_PASSWORD_SIG_SIZE + 1. strncpy() then copies up to
ECRYPTFS_PASSWORD_SIG_SIZE bytes from 'key_desc', NUL-padding any
remaining bytes if needed, but expects the last byte to be zero.

strscpy_pad() also copies the source string to 'signature', and NUL-pads
the destination buffer if needed, but ensures it's always NUL-terminated
without relying on it being zero-initialized.

strscpy_pad() automatically determines the size of the fixed-length
destination buffer via sizeof() when the optional size argument is
omitted, making an explicit size unnecessary.

In encrypted_init(), the source string 'key_desc' is validated by
valid_ecryptfs_desc() before calling ecryptfs_fill_auth_tok(), and is
therefore NUL-terminated and satisfies the __must_be_cstr() requirement
of strscpy_pad().

Link: https://github.com/KSPP/linux/issues/90
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
Changes in v3:
- Improve commit message
- Link to v2: https://lore.kernel.org/lkml/20251010161340.458707-2-thorsten.blum@linux.dev/

Changes in v2:
- Improve commit message as suggested by Jarkko and Kees
- Link to v1: https://lore.kernel.org/lkml/20251009180316.394708-3-thorsten.blum@linux.dev/
---
 security/keys/encrypted-keys/ecryptfs_format.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/security/keys/encrypted-keys/ecryptfs_format.c b/security/keys/encrypted-keys/ecryptfs_format.c
index 8fdd76105ce3..2fc6f3a66135 100644
--- a/security/keys/encrypted-keys/ecryptfs_format.c
+++ b/security/keys/encrypted-keys/ecryptfs_format.c
@@ -54,8 +54,7 @@ int ecryptfs_fill_auth_tok(struct ecryptfs_auth_tok *auth_tok,
 	auth_tok->version = (((uint16_t)(major << 8) & 0xFF00)
 			     | ((uint16_t)minor & 0x00FF));
 	auth_tok->token_type = ECRYPTFS_PASSWORD;
-	strncpy((char *)auth_tok->token.password.signature, key_desc,
-		ECRYPTFS_PASSWORD_SIG_SIZE);
+	strscpy_pad(auth_tok->token.password.signature, key_desc);
 	auth_tok->token.password.session_key_encryption_key_bytes =
 		ECRYPTFS_MAX_KEY_BYTES;
 	/*
-- 
2.51.0

Dear Thorsten,


Thank you for the patch.

Am 13.10.25 um 17:26 schrieb Thorsten Blum:
> strncpy() is deprecated for NUL-terminated destination buffers; use
> strscpy_pad() instead to retain the NUL-padding behavior of strncpy().
> 
> The destination buffer is initialized using kzalloc() with a 'signature'
> size of ECRYPTFS_PASSWORD_SIG_SIZE + 1. strncpy() then copies up to
> ECRYPTFS_PASSWORD_SIG_SIZE bytes from 'key_desc', NUL-padding any
> remaining bytes if needed, but expects the last byte to be zero.
> 
> strscpy_pad() also copies the source string to 'signature', and NUL-pads
> the destination buffer if needed, but ensures it's always NUL-terminated
> without relying on it being zero-initialized.
> 
> strscpy_pad() automatically determines the size of the fixed-length
> destination buffer via sizeof() when the optional size argument is
> omitted, making an explicit size unnecessary.
> 
> In encrypted_init(), the source string 'key_desc' is validated by
> valid_ecryptfs_desc() before calling ecryptfs_fill_auth_tok(), and is
> therefore NUL-terminated and satisfies the __must_be_cstr() requirement
> of strscpy_pad().
> 
> Link: https://github.com/KSPP/linux/issues/90
> Reviewed-by: Kees Cook <kees@kernel.org>
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> ---
> Changes in v3:
> - Improve commit message
> - Link to v2: https://lore.kernel.org/lkml/20251010161340.458707-2-thorsten.blum@linux.dev/
> 
> Changes in v2:
> - Improve commit message as suggested by Jarkko and Kees
> - Link to v1: https://lore.kernel.org/lkml/20251009180316.394708-3-thorsten.blum@linux.dev/
> ---
>   security/keys/encrypted-keys/ecryptfs_format.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/security/keys/encrypted-keys/ecryptfs_format.c b/security/keys/encrypted-keys/ecryptfs_format.c
> index 8fdd76105ce3..2fc6f3a66135 100644
> --- a/security/keys/encrypted-keys/ecryptfs_format.c
> +++ b/security/keys/encrypted-keys/ecryptfs_format.c
> @@ -54,8 +54,7 @@ int ecryptfs_fill_auth_tok(struct ecryptfs_auth_tok *auth_tok,
>   	auth_tok->version = (((uint16_t)(major << 8) & 0xFF00)
>   			     | ((uint16_t)minor & 0x00FF));
>   	auth_tok->token_type = ECRYPTFS_PASSWORD;
> -	strncpy((char *)auth_tok->token.password.signature, key_desc,
> -		ECRYPTFS_PASSWORD_SIG_SIZE);
> +	strscpy_pad(auth_tok->token.password.signature, key_desc);
>   	auth_tok->token.password.session_key_encryption_key_bytes =
>   		ECRYPTFS_MAX_KEY_BYTES;
>   	/*

Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>


Kind regards,

Paul

On Mon, Oct 13, 2025 at 06:11:38PM +0200, Paul Menzel wrote:
> Dear Thorsten,
> 
> 
> Thank you for the patch.
> 
> Am 13.10.25 um 17:26 schrieb Thorsten Blum:
> > strncpy() is deprecated for NUL-terminated destination buffers; use
> > strscpy_pad() instead to retain the NUL-padding behavior of strncpy().
> > 
> > The destination buffer is initialized using kzalloc() with a 'signature'
> > size of ECRYPTFS_PASSWORD_SIG_SIZE + 1. strncpy() then copies up to
> > ECRYPTFS_PASSWORD_SIG_SIZE bytes from 'key_desc', NUL-padding any
> > remaining bytes if needed, but expects the last byte to be zero.
> > 
> > strscpy_pad() also copies the source string to 'signature', and NUL-pads
> > the destination buffer if needed, but ensures it's always NUL-terminated
> > without relying on it being zero-initialized.
> > 
> > strscpy_pad() automatically determines the size of the fixed-length
> > destination buffer via sizeof() when the optional size argument is
> > omitted, making an explicit size unnecessary.
> > 
> > In encrypted_init(), the source string 'key_desc' is validated by
> > valid_ecryptfs_desc() before calling ecryptfs_fill_auth_tok(), and is
> > therefore NUL-terminated and satisfies the __must_be_cstr() requirement
> > of strscpy_pad().
> > 
> > Link: https://github.com/KSPP/linux/issues/90
> > Reviewed-by: Kees Cook <kees@kernel.org>
> > Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> > Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> > ---
> > Changes in v3:
> > - Improve commit message
> > - Link to v2: https://lore.kernel.org/lkml/20251010161340.458707-2-thorsten.blum@linux.dev/
> > 
> > Changes in v2:
> > - Improve commit message as suggested by Jarkko and Kees
> > - Link to v1: https://lore.kernel.org/lkml/20251009180316.394708-3-thorsten.blum@linux.dev/
> > ---
> >   security/keys/encrypted-keys/ecryptfs_format.c | 3 +--
> >   1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/security/keys/encrypted-keys/ecryptfs_format.c b/security/keys/encrypted-keys/ecryptfs_format.c
> > index 8fdd76105ce3..2fc6f3a66135 100644
> > --- a/security/keys/encrypted-keys/ecryptfs_format.c
> > +++ b/security/keys/encrypted-keys/ecryptfs_format.c
> > @@ -54,8 +54,7 @@ int ecryptfs_fill_auth_tok(struct ecryptfs_auth_tok *auth_tok,
> >   	auth_tok->version = (((uint16_t)(major << 8) & 0xFF00)
> >   			     | ((uint16_t)minor & 0x00FF));
> >   	auth_tok->token_type = ECRYPTFS_PASSWORD;
> > -	strncpy((char *)auth_tok->token.password.signature, key_desc,
> > -		ECRYPTFS_PASSWORD_SIG_SIZE);
> > +	strscpy_pad(auth_tok->token.password.signature, key_desc);
> >   	auth_tok->token.password.session_key_encryption_key_bytes =
> >   		ECRYPTFS_MAX_KEY_BYTES;
> >   	/*
> 
> Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>

Thanks for the review Paul.

David: and also this is in my tree now.

> 
> 
> Kind regards,
> 
> Paul

BR, Jarkko