linux-hardening - Re: [PATCH] binfmt: replace deprecated strncpy with strscpy

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Date: Thu, 21 Mar 2024 11:23:34 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,  Christian Brauner
 <brauner@...nel.org>,  Jan Kara <jack@...e.cz>,  Kees Cook
 <keescook@...omium.org>,  linux-fsdevel@...r.kernel.org,
  linux-mm@...ck.org,  linux-kernel@...r.kernel.org,
  linux-hardening@...r.kernel.org
Subject: Re: [PATCH] binfmt: replace deprecated strncpy with strscpy_pad

Justin Stitt <justinstitt@...gle.com> writes:

> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
>
> In every other location psinfo->pr_fname is used, it's with strscpy_pad.
> It's clear that this field needs to be NUL-terminated and potentially
> NUL-padded as well.
> binfmt_elf.c +1545:
> |	char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> |	{
> |		task_lock(tsk);
> |		/* Always NUL terminated and zero-padded */
> |		strscpy_pad(buf, tsk->comm, buf_size);
> |		task_unlock(tsk);
> |		return buf;
> |	}
>
> Note that this patch relies on the _new_ 2-argument versions of
> strscpy() and strscpy_pad() introduced in Commit e6584c3964f2f ("string:
> Allow 2-argument strscpy()").

I am perplexed.  Why not use get_task_comm fill_psinfo like binfmt_elf
does?

It seems very silly to copy half the function without locking and then
not copy it's locking as well.

Given that the more highly tested binfmt_elf uses get_task_comm I can't
imagine a reason why binfmt_elf_fdpic can't use it as well.

Eric



> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested only.
>
> Found with: $ rg "strncpy\("
> ---
>  fs/binfmt_elf_fdpic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index 1920ed69279b..0365f14f18fc 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -1359,7 +1359,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
>  	SET_UID(psinfo->pr_uid, from_kuid_munged(cred->user_ns, cred->uid));
>  	SET_GID(psinfo->pr_gid, from_kgid_munged(cred->user_ns, cred->gid));
>  	rcu_read_unlock();
> -	strncpy(psinfo->pr_fname, p->comm, sizeof(psinfo->pr_fname));
> +	strscpy_pad(psinfo->pr_fname, p->comm);
>  
>  	return 0;
>  }
>
> ---
> base-commit: a4145ce1e7bc247fd6f2846e8699473448717b37
> change-id: 20240320-strncpy-fs-binfmt_elf_fdpic-c-828286d76310
>
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>