[PATCH v2] keys: Replace deprecated strncpy in ecryptfs_fill_auth_tok

[Jarkko Sakkinen]

On Fri, Oct 10, 2025 at 03:48:48PM -0700, Kees Cook wrote:
> On Fri, Oct 10, 2025 at 06:13:41PM +0200, Thorsten Blum wrote:
> > strncpy() is deprecated for NUL-terminated destination buffers; use
> > strscpy_pad() instead to retain the zero-padding behavior of strncpy().
> > 
> > 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.
> 
> I would explicitly say that the old code was NUL terminating the buffer
> due to it being "ECRYPTFS_PASSWORD_SIG_SIZE + 1" sized with strncpy
> left to fill ECRYPTFS_PASSWORD_SIG_SIZE. And then you have to answer the
> question, "how was this initialized?" and trace it back to:
> 
>         epayload = kzalloc(sizeof(*epayload) + payload_datalen +
>                            datablob_len + HASH_SIZE + 1, GFP_KERNEL);
> 
> so the final byte was always being zeroed there, but now we're
> explicitly zeroing it (good). So there _is_ a functional change (we're
> writing 1 more byte here now), but it's more robust that way. There is
> no expected _logical_ change, though, yes.

Thanks for the remarks.

Thorsten, would you mind posting +1 with the commit message changes,
and reviewed-by tags (from me and Kees).

> 
> > 
> > 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().
> > 
> > No functional changes.

[just as reminder: removing this sentence was my earlier remark]

> > 
> > Link: https://github.com/KSPP/linux/issues/90
> > Signed-off-by: Thorsten Blum <thorsten.blum at linux.dev>
> 
> With "ECRYPTFS_PASSWORD_SIG_SIZE + 1" and tracing of the destination
> buffer initialization added to the commit log:
> 
> Reviewed-by: Kees Cook <kees at kernel.org>
> 
> -Kees
> 
> -- 
> Kees Cook

BR, Jarkko