Re: [REVIEW][PATCH] mm: Add a user_ns owner to mm_struct and fix ptrace_may_access

[Jann Horn <jann-XZ1E9jl8jIdeoWH0uzbU5w@public.gmane.org>]

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On Mon, Oct 17, 2016 at 11:39:49AM -0500, Eric W. Biederman wrote:
> 
> During exec dumpable is cleared if the file that is being executed is
> not readable by the user executing the file.  A bug in
> ptrace_may_access allows reading the file if the executable happens to
> enter into a subordinate user namespace (aka clone(CLONE_NEWUSER),
> unshare(CLONE_NEWUSER), or setns(fd, CLONE_NEWUSER).
> 
> This problem is fixed with only necessary userspace breakage by adding
> a user namespace owner to mm_struct, captured at the time of exec,
> so it is clear in which user namespace CAP_SYS_PTRACE must be present
> in to be able to safely give read permission to the executable.
> 
> The function ptrace_may_access is modified to verify that the ptracer
> has CAP_SYS_ADMIN in task->mm->user_ns instead of task->cred->user_ns.
> This ensures that if the task changes it's cred into a subordinate
> user namespace it does not become ptraceable.

This looks good! Basically applies the same rules that already apply to
EUID/... changes to namespace changes, and anyone entering a user
namespace can now safely drop UIDs and GIDs to namespace root.

This integrates better in the existing security concept than my old
patch "ptrace: being capable wrt a process requires mapped uids/gids",
and it has less issues in cases where e.g. the extra privileges of an
entering process are the filesystem root or so.

FWIW, if you want, you can add "Reviewed-by: Jann Horn <jann-XZ1E9jl8jIdeoWH0uzbU5w@public.gmane.org>".

> Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Fixes: 8409cca70561 ("userns: allow ptrace from non-init user namespaces")
> Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
> ---
> 
> It turns out that dumpable needs to be fixed to be user namespace
> aware to fix this issue.  When this patch is ready I plan to place it in
> my userns tree and send it to Linus, hopefully for -rc2.
> 
>  include/linux/mm_types.h |  1 +
>  kernel/fork.c            |  9 ++++++---
>  kernel/ptrace.c          | 17 ++++++-----------
>  mm/init-mm.c             |  2 ++
>  4 files changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 4a8acedf4b7d..08d947fc4c59 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -473,6 +473,7 @@ struct mm_struct {
>  	 */
>  	struct task_struct __rcu *owner;
>  #endif
> +	struct user_namespace *user_ns;
>  
>  	/* store ref to file /proc/<pid>/exe symlink points to */
>  	struct file __rcu *exe_file;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 623259fc794d..fd85c68c2791 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -742,7 +742,8 @@ static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
>  #endif
>  }
>  
> -static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
> +static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> +	struct user_namespace *user_ns)
>  {
>  	mm->mmap = NULL;
>  	mm->mm_rb = RB_ROOT;
> @@ -782,6 +783,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
>  	if (init_new_context(p, mm))
>  		goto fail_nocontext;
>  
> +	mm->user_ns = get_user_ns(user_ns);
>  	return mm;
>  
>  fail_nocontext:
> @@ -827,7 +829,7 @@ struct mm_struct *mm_alloc(void)
>  		return NULL;
>  
>  	memset(mm, 0, sizeof(*mm));
> -	return mm_init(mm, current);
> +	return mm_init(mm, current, current_user_ns());
>  }
>  
>  /*
> @@ -842,6 +844,7 @@ void __mmdrop(struct mm_struct *mm)
>  	destroy_context(mm);
>  	mmu_notifier_mm_destroy(mm);
>  	check_mm(mm);
> +	put_user_ns(mm->user_ns);
>  	free_mm(mm);
>  }
>  EXPORT_SYMBOL_GPL(__mmdrop);
> @@ -1123,7 +1126,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk)
>  
>  	memcpy(mm, oldmm, sizeof(*mm));
>  
> -	if (!mm_init(mm, tsk))
> +	if (!mm_init(mm, tsk, mm->user_ns))
>  		goto fail_nomem;
>  
>  	err = dup_mmap(mm, oldmm);
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 2a99027312a6..f2d1b9afb3f8 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -220,7 +220,7 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
>  static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  {
>  	const struct cred *cred = current_cred(), *tcred;
> -	int dumpable = 0;
> +	struct mm_struct *mm;
>  	kuid_t caller_uid;
>  	kgid_t caller_gid;
>  
> @@ -271,16 +271,11 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	return -EPERM;
>  ok:
>  	rcu_read_unlock();
> -	smp_rmb();
> -	if (task->mm)
> -		dumpable = get_dumpable(task->mm);
> -	rcu_read_lock();
> -	if (dumpable != SUID_DUMP_USER &&
> -	    !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
> -		rcu_read_unlock();
> -		return -EPERM;
> -	}
> -	rcu_read_unlock();
> +	mm = task->mm;
> +	if (!mm ||
> +	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
> +	     !ptrace_has_cap(mm->user_ns, mode)))
> +	    return -EPERM;
>  
>  	return security_ptrace_access_check(task, mode);
>  }
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index a56a851908d2..975e49f00f34 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -6,6 +6,7 @@
>  #include <linux/cpumask.h>
>  
>  #include <linux/atomic.h>
> +#include <linux/user_namespace.h>
>  #include <asm/pgtable.h>
>  #include <asm/mmu.h>
>  
> @@ -21,5 +22,6 @@ struct mm_struct init_mm = {
>  	.mmap_sem	= __RWSEM_INITIALIZER(init_mm.mmap_sem),
>  	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
>  	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
> +	.user_ns	= &init_user_ns,
>  	INIT_MM_CONTEXT(init_mm)
>  };
> -- 
> 2.8.3

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