Re: [PATCH v4 3/3] prctl: Allow ptrace capable processes to change /proc/self/exe

[Serge E. Hallyn]

On Wed, Jul 01, 2020 at 10:55:37AM +0200, Christian Brauner wrote:
> On Wed, Jul 01, 2020 at 08:49:06AM +0200, Adrian Reber wrote:
> > From: Nicolas Viennot <Nicolas.Viennot@xxxxxxxxxxxx>
> > 
> > Previously, the current process could only change the /proc/self/exe
> > link with local CAP_SYS_ADMIN.
> > This commit relaxes this restriction by permitting such change with
> > CAP_CHECKPOINT_RESTORE, and the ability to use ptrace.
> > 
> > With access to ptrace facilities, a process can do the following: fork a
> > child, execve() the target executable, and have the child use ptrace()
> > to replace the memory content of the current process. This technique
> > makes it possible to masquerade an arbitrary program as any executable,
> > even setuid ones.
> > 
> > Signed-off-by: Nicolas Viennot <Nicolas.Viennot@xxxxxxxxxxxx>
> > Signed-off-by: Adrian Reber <areber@xxxxxxxxxx>
> > ---
> >  include/linux/lsm_hook_defs.h |  1 +
> >  include/linux/security.h      |  6 ++++++
> >  kernel/sys.c                  | 12 ++++--------
> >  security/commoncap.c          | 26 ++++++++++++++++++++++++++
> >  security/security.c           |  5 +++++
> >  security/selinux/hooks.c      | 14 ++++++++++++++
> >  6 files changed, 56 insertions(+), 8 deletions(-)
> > 
> > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> > index 0098852bb56a..90e51d5e093b 100644
> > --- a/include/linux/lsm_hook_defs.h
> > +++ b/include/linux/lsm_hook_defs.h
> > @@ -211,6 +211,7 @@ LSM_HOOK(int, 0, task_kill, struct task_struct *p, struct kernel_siginfo *info,
> >  	 int sig, const struct cred *cred)
> >  LSM_HOOK(int, -ENOSYS, task_prctl, int option, unsigned long arg2,
> >  	 unsigned long arg3, unsigned long arg4, unsigned long arg5)
> > +LSM_HOOK(int, 0, prctl_set_mm_exe_file, struct file *exe_file)
> >  LSM_HOOK(void, LSM_RET_VOID, task_to_inode, struct task_struct *p,
> >  	 struct inode *inode)
> >  LSM_HOOK(int, 0, ipc_permission, struct kern_ipc_perm *ipcp, short flag)
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index 2797e7f6418e..0f594eb7e766 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -412,6 +412,7 @@ int security_task_kill(struct task_struct *p, struct kernel_siginfo *info,
> >  			int sig, const struct cred *cred);
> >  int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> >  			unsigned long arg4, unsigned long arg5);
> > +int security_prctl_set_mm_exe_file(struct file *exe_file);
> >  void security_task_to_inode(struct task_struct *p, struct inode *inode);
> >  int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
> >  void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid);
> > @@ -1124,6 +1125,11 @@ static inline int security_task_prctl(int option, unsigned long arg2,
> >  	return cap_task_prctl(option, arg2, arg3, arg4, arg5);
> >  }
> >  
> > +static inline int security_prctl_set_mm_exe_file(struct file *exe_file)
> > +{
> > +	return cap_prctl_set_mm_exe_file(exe_file);
> > +}
> > +
> >  static inline void security_task_to_inode(struct task_struct *p, struct inode *inode)
> >  { }
> >  
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 00a96746e28a..bb53e8408c63 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -1851,6 +1851,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> >  	if (err)
> >  		goto exit;
> >  
> > +	err = security_prctl_set_mm_exe_file(exe.file);
> > +	if (err)
> > +		goto exit;
> > +
> >  	/*
> >  	 * Forbid mm->exe_file change if old file still mapped.
> >  	 */
> > @@ -2006,14 +2010,6 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
> >  	}
> >  
> >  	if (prctl_map.exe_fd != (u32)-1) {
> > -		/*
> > -		 * Make sure the caller has the rights to
> > -		 * change /proc/pid/exe link: only local sys admin should
> > -		 * be allowed to.
> > -		 */
> > -		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
> > -			return -EINVAL;
> > -
> >  		error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
> >  		if (error)
> >  			return error;
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index 59bf3c1674c8..663d00fe2ecc 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -1291,6 +1291,31 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> >  	}
> >  }
> >  
> > +/**
> > + * cap_prctl_set_mm_exe_file - Determine whether /proc/self/exe can be changed
> > + * by the current process.
> > + * @exe_file: The new exe file
> > + * Returns 0 if permission is granted, -ve if denied.
> > + *
> > + * The current process is permitted to change its /proc/self/exe link via two policies:
> > + * 1) The current user can do checkpoint/restore. At the time of this writing,
> > + *    this means CAP_SYS_ADMIN or CAP_CHECKPOINT_RESTORE capable.
> > + * 2) The current user can use ptrace.
> > + *
> > + * With access to ptrace facilities, a process can do the following:
> > + * fork a child, execve() the target executable, and have the child use
> > + * ptrace() to replace the memory content of the current process.
> > + * This technique makes it possible to masquerade an arbitrary program as the
> > + * target executable, even if it is setuid.
> > + */
> > +int cap_prctl_set_mm_exe_file(struct file *exe_file)
> > +{
> > +	if (checkpoint_restore_ns_capable(current_user_ns()))
> > +		return 0;
> > +
> > +	return security_ptrace_access_check(current, PTRACE_MODE_ATTACH_REALCREDS);
> > +}
> 
> What is the reason for having this be a new security hook? Doesn't look
> like it needs to be unless I'm missing something. This just seems more
> complex than it needs to be.

Yeah, agreed, and that in turn risks actually making it less safe.  Is
there a good reason not to do what Christian suggests?

> I might be wrong here but if you look at the callsites for
> security_ptrace_access_check() right now, you'll see that it's only
> called from kernel/ptrace.c in __ptrace_may_access() and that function
> checks right at the top:
> 
> 	/* Don't let security modules deny introspection */
> 	if (same_thread_group(task, current))
> 		return 0;
> 
> since you're passing in same_thread_group(current, current) you're
> passing this check and never even hitting
> security_ptrace_access_check(). So the contract seems to be (as is
> obvious from the comment) that a task can't be denied ptrace
> introspection. But if you're using security_ptrace_access_check(current)
> here and _if_ there would be any lsm that would deny ptrace
> introspection to current you'd suddenly introduce a callsite where
> ptrace introspection is denied. That seems wrong. So either you meant to
> do something else here or you really just want:
> 
> checkpoint_restore_ns_capable(current_user_ns())
> 
> and none of the rest. But I might be missing the big picture in this
> patch.
> 
> > +	if (checkpoint_restore_ns_capable(current_user_ns()))
> > +		return 0;
> > +
> >  /**
> >   * cap_vm_enough_memory - Determine whether a new virtual mapping is permitted
> >   * @mm: The VM space in which the new mapping is to be made
> > @@ -1356,6 +1381,7 @@ static struct security_hook_list capability_hooks[] __lsm_ro_after_init = {
> >  	LSM_HOOK_INIT(mmap_file, cap_mmap_file),
> >  	LSM_HOOK_INIT(task_fix_setuid, cap_task_fix_setuid),
> >  	LSM_HOOK_INIT(task_prctl, cap_task_prctl),
> > +	LSM_HOOK_INIT(prctl_set_mm_exe_file, cap_prctl_set_mm_exe_file),
> >  	LSM_HOOK_INIT(task_setscheduler, cap_task_setscheduler),
> >  	LSM_HOOK_INIT(task_setioprio, cap_task_setioprio),
> >  	LSM_HOOK_INIT(task_setnice, cap_task_setnice),
> > diff --git a/security/security.c b/security/security.c
> > index 2bb912496232..13a1ed32f9e3 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1790,6 +1790,11 @@ int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> >  	return rc;
> >  }
> >  
> > +int security_prctl_set_mm_exe_file(struct file *exe_file)
> > +{
> > +	return call_int_hook(prctl_set_mm_exe_file, 0, exe_file);
> > +}
> > +
> >  void security_task_to_inode(struct task_struct *p, struct inode *inode)
> >  {
> >  	call_void_hook(task_to_inode, p, inode);
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index ca901025802a..fca5581392b8 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -4156,6 +4156,19 @@ static int selinux_task_kill(struct task_struct *p, struct kernel_siginfo *info,
> >  			    secid, task_sid(p), SECCLASS_PROCESS, perm, NULL);
> >  }
> >  
> > +static int selinux_prctl_set_mm_exe_file(struct file *exe_file)
> > +{
> > +	u32 sid = current_sid();
> > +
> > +	struct common_audit_data ad = {
> > +		.type = LSM_AUDIT_DATA_FILE,
> > +		.u.file = exe_file,
> > +	};
> > +
> > +	return avc_has_perm(&selinux_state, sid, sid,
> > +			    SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
> > +}
> > +
> >  static void selinux_task_to_inode(struct task_struct *p,
> >  				  struct inode *inode)
> >  {
> > @@ -7057,6 +7070,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
> >  	LSM_HOOK_INIT(task_getscheduler, selinux_task_getscheduler),
> >  	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
> >  	LSM_HOOK_INIT(task_kill, selinux_task_kill),
> > +	LSM_HOOK_INIT(prctl_set_mm_exe_file, selinux_prctl_set_mm_exe_file),
> >  	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
> >  
> >  	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
> > -- 
> > 2.26.2
> >