Subject: Re: [RFC][PATCH v3] Unprivileged: Disable raising of privileges

From: "Serge E. Hallyn" <[email-masked]>
Date: Thu, 31 Dec 2009 09:25:23 -0600

Quoting Eric W. Biederman ([email-masked]):
> 
> If we can know that a process will never raise
> it's priveleges we can enable a lot of features
> without privilege (such as unsharing namespaces
> and unprivileged mounts) that otherwise would be unsafe,
> because they could break assumptions of existing
> suid executables.
> 
> To allow this to be used as a sand boxing feature
> also disable ptracing other executables without
> this new restriction.
> 
> For the moment I have used a per thread flag because
> we are out of per process flags.
> 
> To ensure all descendants get this flag I rely on
> the default copying of procss structures.
> 
> Added bprm->nosuid to make remove the need to add
> duplicate error prone checks.  This ensures that
> the disabling of suid executables is exactly the
> same as MNT_NOSUID.
> 
> Signed-off-by: Eric W. Biederman <[email-masked]>
> ---
>  arch/x86/include/asm/thread_info.h |    2 ++
>  fs/exec.c                          |    6 ++++--
>  include/linux/binfmts.h            |    1 +
>  include/linux/prctl.h              |    3 +++
>  kernel/ptrace.c                    |    4 ++++
>  kernel/sys.c                       |   21 +++++++++++++++++++++
>  security/commoncap.c               |    3 +--
>  security/selinux/hooks.c           |    2 +-
>  8 files changed, 37 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 375c917..e716203 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -82,6 +82,7 @@ struct thread_info {
>  #define TIF_SYSCALL_EMU		6	/* syscall emulation active */
>  #define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
>  #define TIF_SECCOMP		8	/* secure computing */
> +#define TIF_NOSUID		9	/* suid exec permanently disabled */
>  #define TIF_MCE_NOTIFY		10	/* notify userspace of an MCE */
>  #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
>  #define TIF_NOTSC		16	/* TSC is not accessible in userland */
> @@ -107,6 +108,7 @@ struct thread_info {
>  #define _TIF_SYSCALL_EMU	(1 << TIF_SYSCALL_EMU)
>  #define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
>  #define _TIF_SECCOMP		(1 << TIF_SECCOMP)
> +#define _TIF_NOSUID		(1 << TIF_NOSUID)
>  #define _TIF_MCE_NOTIFY		(1 << TIF_MCE_NOTIFY)
>  #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
>  #define _TIF_NOTSC		(1 << TIF_NOTSC)
> diff --git a/fs/exec.c b/fs/exec.c
> index 632b02e..5cba5ac 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1131,8 +1131,10 @@ int prepare_binprm(struct linux_binprm *bprm)
>  	/* clear any previous set[ug]id data from a previous binary */
>  	bprm->cred->euid = current_euid();
>  	bprm->cred->egid = current_egid();
> -
> -	if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)) {
> +	bprm->nosuid =
> +		(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) ||
> +		test_tsk_thread_flag(current, TIF_NOSUID);
> +	if (bprm->nosuid) {
>  		/* Set-uid? */
>  		if (mode & S_ISUID) {
>  			bprm->per_clear |= PER_CLEAR_ON_SETID;
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index cd4349b..c3b5a30 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -44,6 +44,7 @@ struct linux_binprm{
>  #ifdef __alpha__
>  	unsigned int taso:1;
>  #endif
> +	unsigned int nosuid:1;	/* True if suid bits are ignored */
>  	unsigned int recursion_depth;
>  	struct file * file;
>  	struct cred *cred;	/* new credentials */
> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
> index a3baeb2..8adc517 100644
> --- a/include/linux/prctl.h
> +++ b/include/linux/prctl.h
> @@ -102,4 +102,7 @@
> 
>  #define PR_MCE_KILL_GET 34
> 
> +#define PR_SET_NOSUID	35
> +#define PR_GET_NOSUID	36
> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 23bd09c..b91040c 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -152,6 +152,10 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	if (!dumpable && !capable(CAP_SYS_PTRACE))
>  		return -EPERM;
> 
> +	if (test_tsk_thread_flag(current, TIF_NOSUID) &&
> +	    !test_tsk_thread_flag(task, TIF_NOSUID))
> +		return -EPERM;
> +
>  	return security_ptrace_access_check(task, mode);
>  }
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 26a6b73..8731f2a 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1578,6 +1578,27 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  			else
>  				error = PR_MCE_KILL_DEFAULT;
>  			break;
> +		case PR_SET_NOSUID:
> +		{
> +			const struct cred *cred = current->cred;
> +			error = -EINVAL;
> +			/* Don't support cases that could be unsafe */
> +			if (	(cred->uid != cred->suid) ||
> +				(cred->uid != cred->euid) ||
> +				(cred->uid != cred->fsuid) ||
> +				(cred->gid != cred->sgid) ||
> +				(cred->gid != cred->egid) ||
> +				(cred->gid != cred->fsgid) ||
> +				!cap_isclear(cred->cap_permitted) ||
> +				(atomic_read(&current->signal->count) != 1))
> +				break;
> +			error = 0;
> +			set_tsk_thread_flag(current, TIF_NOSUID);
> +			break;
> +		}
> +		case PR_GET_NOSUID:
> +			error = !!test_tsk_thread_flag(current, TIF_NOSUID);
> +			break;
>  		default:
>  			error = -EINVAL;
>  			break;
> diff --git a/security/commoncap.c b/security/commoncap.c
> index f800fdb..34500e3 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -389,7 +389,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective)
>  	if (!file_caps_enabled)
>  		return 0;
> 
> -	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
> +	if (bprm->nosuid)
>  		return 0;

I'm sorry, this may actually not be sufficient.

Could you try the following test on a kernel with this patch? :

1. become root
2. do prctl(PR_SET_NOSUID);
3. run bash, and examine your capabilities in /proc/self/status

I think the code in security/commoncap.c:457-458 will re-raise your
capabilities.

> 
>  	dentry = dget(bprm->file->f_dentry);
> @@ -868,7 +868,6 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  		else
>  			new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
>  		goto changed;
> -
>  	default:
>  		/* No functionality available - continue with default */
>  		error = -ENOSYS;
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 7a374c2..bd77a2b 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2147,7 +2147,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
>  	COMMON_AUDIT_DATA_INIT(&ad, FS);
>  	ad.u.fs.path = bprm->file->f_path;
> 
> -	if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> +	if (bprm->nosuid)
>  		new_tsec->sid = old_tsec->sid;
> 
>  	if (new_tsec->sid == old_tsec->sid) {
> -- 
> 1.6.5.2.143.g8cc62
> 
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