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Message-ID: <20091231152523.GA12969@us.ibm.com>
Date: Thu, 31 Dec 2009 09:25:23 -0600
From: "Serge E. Hallyn" <serue@...ibm.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: "Andrew G. Morgan" <morgan@...nel.org>,
Bryan Donlan <bdonlan@...il.com>,
Alan Cox <alan@...rguk.ukuu.org.uk>,
Benny Amorsen <benny+usenet@...rsen.dk>,
Michael Stone <michael@...top.org>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
linux-security-module@...r.kernel.org,
Andi Kleen <andi@...stfloor.org>, David Lang <david@...g.hm>,
Oliver Hartkopp <socketcan@...tkopp.net>,
Herbert Xu <herbert@...dor.apana.org.au>,
Valdis Kletnieks <Valdis.Kletnieks@...edu>,
Evgeniy Polyakov <zbr@...emap.net>,
"C. Scott Ananian" <cscott@...ott.net>,
James Morris <jmorris@...ei.org>,
Bernie Innocenti <bernie@...ewiz.org>,
Mark Seaborn <mrs@...hic-beasts.com>,
Randy Dunlap <randy.dunlap@...cle.com>,
Américo Wang <xiyou.wangcong@...il.com>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
Samir Bellabes <sam@...ack.fr>,
Casey Schaufler <casey@...aufler-ca.com>,
Pavel Machek <pavel@....cz>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC][PATCH v3] Unprivileged: Disable raising of privileges
Quoting Eric W. Biederman (ebiederm@...ssion.com):
>
> 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 <ebiederm@...ssion.com>
> ---
> 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(¤t->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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