Re: [PATCH v3 1/3] LSM: add security_execve_abort() hook

[Paul Moore <paul@paul-moore.com>]

On Feb  6, 2024 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> 
> A regression caused by commit 978ffcbf00d8 ("execve: open the executable
> file before doing anything else") has been fixed by commit 4759ff71f23e
> ("exec: Check __FMODE_EXEC instead of in_execve for LSMs") and commit
> 3eab830189d9 ("uselib: remove use of __FMODE_EXEC"). While fixing this
> regression, Linus commented that we want to remove current->in_execve flag.
> 
> The current->in_execve flag was introduced by commit f9ce1f1cda8b ("Add
> in_execve flag into task_struct.") when TOMOYO LSM was merged, and the
> reason was explained in commit f7433243770c ("LSM adapter functions.").
> 
> In short, TOMOYO's design is not compatible with COW credential model
> introduced in Linux 2.6.29, and the current->in_execve flag was added for
> emulating security_bprm_free() hook which has been removed by introduction
> of COW credential model.

If you wanted to mention the relevant commit where security_bprm_free()
was removed, it was a6f76f23d297 ("CRED: Make execve() take advantage
of copy-on-write credentials").

> security_task_alloc()/security_task_free() hooks have been removed by
> commit f1752eec6145 ("CRED: Detach the credentials from task_struct"),
> and these hooks have been revived by commit 1a2a4d06e1e9 ("security:
> create task_free security callback") and commit e4e55b47ed9a ("LSM: Revive
> security_task_alloc() hook and per "struct task_struct" security blob.").
> 
> But security_bprm_free() hook did not revive until now. Now that Linus
> wants TOMOYO to stop carrying state across two independent execve() calls,
> and TOMOYO can stop carrying state if a hook for restoring previous state
> upon failed execve() call were provided, this patch revives the hook.
> 
> Since security_bprm_committing_creds() and security_bprm_committed_creds()
> hooks are called when an execve() request succeeded, we don't need to call
> security_bprm_free() hook when an execve() request succeeded. Therefore,
> this patch adds security_execve_abort() hook which is called only when an
> execve() request failed after successful prepare_bprm_creds() call.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Acked-by: Serge E. Hallyn <serge@hallyn.com>
> ---
>  fs/exec.c                     |  1 +
>  include/linux/lsm_hook_defs.h |  1 +
>  include/linux/security.h      |  5 +++++
>  security/security.c           | 11 +++++++++++
>  4 files changed, 18 insertions(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index af4fbb61cd53..d6d35a06fd08 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1521,6 +1521,7 @@ static void free_bprm(struct linux_binprm *bprm)
>  	if (bprm->cred) {
>  		mutex_unlock(&current->signal->cred_guard_mutex);
>  		abort_creds(bprm->cred);
> +		security_execve_abort();
>  	}
>  	do_close_execat(bprm->file);
>  	if (bprm->executable)
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 76458b6d53da..fd100ab71a33 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -54,6 +54,7 @@ LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, const struct f
>  LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
>  LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, const struct linux_binprm *bprm)
>  LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, const struct linux_binprm *bprm)
> +LSM_HOOK(void, LSM_RET_VOID, execve_abort, void)
>  LSM_HOOK(int, 0, fs_context_submount, struct fs_context *fc, struct super_block *reference)
>  LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
>  	 struct fs_context *src_sc)
> diff --git a/include/linux/security.h b/include/linux/security.h
> index d0eb20f90b26..31532b30c4f0 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -299,6 +299,7 @@ int security_bprm_creds_from_file(struct linux_binprm *bprm, const struct file *
>  int security_bprm_check(struct linux_binprm *bprm);
>  void security_bprm_committing_creds(const struct linux_binprm *bprm);
>  void security_bprm_committed_creds(const struct linux_binprm *bprm);
> +void security_execve_abort(void);
>  int security_fs_context_submount(struct fs_context *fc, struct super_block *reference);
>  int security_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc);
>  int security_fs_context_parse_param(struct fs_context *fc, struct fs_parameter *param);
> @@ -648,6 +649,10 @@ static inline void security_bprm_committed_creds(const struct linux_binprm *bprm
>  {
>  }
>  
> +static inline void security_execve_abort(void)
> +{
> +}
> +
>  static inline int security_fs_context_submount(struct fs_context *fc,
>  					   struct super_block *reference)
>  {
> diff --git a/security/security.c b/security/security.c
> index 3aaad75c9ce8..10adc4d3c5e0 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1223,6 +1223,17 @@ void security_bprm_committed_creds(const struct linux_binprm *bprm)
>  	call_void_hook(bprm_committed_creds, bprm);
>  }
>  
> +/**
> + * security_execve_abort() - Notify that exec() has failed
> + *
> + * This hook is for undoing changes which cannot be discarded by
> + * abort_creds().
> + */
> +void security_execve_abort(void)
> +{
> +	call_void_hook(execve_abort);
> +}

I don't have a problem with reinstating something like
security_bprm_free(), but I don't like the name security_execve_abort(),
especially given that it is being called from alloc_bprm() as well as
all of the execve code.  At the risk of bikeshedding this, I'd be much
happier if this hook were renamed to security_bprm_free() and the
hook's description explained that this hook is called when a linux_bprm
instance is being destroyed, after the bprm creds have been released,
and is intended to cleanup any internal LSM state associated with the
linux_bprm instance.

Are you okay with that?

--
paul-moore.com