Re: [PATCH] user_ns: use correct check for single-threadedness

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> From: Ricky Zhou <rickyz@xxxxxxxxxxxx>
>
> Checking mm_users > 1 does not mean a process is multithreaded. For
> example, reading /proc/PID/maps temporarily increments mm_users, allowing
> other processes to (accidentally) interfere with unshare() calls.
>
> This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
> returning EINVAL if another processes happened to be simultaneously
> reading the maps file.
>
> Signed-off-by: Ricky Zhou <rickyz@xxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

This looks like a good fix.  Any chance you can drudge up the commit where
this hack came in so that Greg & Company know how far to back port this?

> ---
>  kernel/fork.c           | 4 ++--
>  kernel/user_namespace.c | 3 ++-
>  2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index dbd9b8d7b7cc..7d138f152dcd 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -75,6 +75,7 @@
>  #include <linux/aio.h>
>  #include <linux/compiler.h>
>  #include <linux/sysctl.h>
> +#include <linux/sched.h>
>  
>  #include <asm/pgtable.h>
>  #include <asm/pgalloc.h>
> @@ -1876,8 +1877,7 @@ static int check_unshare_flags(unsigned long unshare_flags)
>  	 * needs to unshare vm.
>  	 */
>  	if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
> -		/* FIXME: get_task_mm() increments ->mm_users */
> -		if (atomic_read(&current->mm->mm_users) > 1)
> +		if (!current_is_single_threaded())
>  			return -EINVAL;
>  	}
>  
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 4109f8320684..7e0e021e4304 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -22,6 +22,7 @@
>  #include <linux/ctype.h>
>  #include <linux/projid.h>
>  #include <linux/fs_struct.h>
> +#include <linux/sched.h>
>  
>  static struct kmem_cache *user_ns_cachep __read_mostly;
>  static DEFINE_MUTEX(userns_state_mutex);
> @@ -977,7 +978,7 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
>  		return -EINVAL;
>  
>  	/* Threaded processes may not enter a different user namespace */
> -	if (atomic_read(&current->mm->mm_users) > 1)
> +	if (!current_is_single_threaded())
>  		return -EINVAL;
>  
>  	if (current->fs->users != 1)
> -- 
> 1.9.1
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