Re: [PATCH v2 3/5] ptrace: make PTRACE_SEIZE set ptrace options specified in 'data' parameter

[Oleg Nesterov <oleg@redhat.com>]

On 02/10, Denys Vlasenko wrote:
>
> This can be used to close a few corner cases in strace where we get
> unwanted racy behavior after attach, but before we have a chance
> to set options (the notorious post-execve SIGTRAP comes to mind),
> and removes the need to track "did we set opts for this task" state
> in strace internals.
> 
> While we are at it:
> 
> Make it possible to extend SEIZE in the future with more functionality
> by passing non-zero 'addr' parameter.
> To that end, error out if 'addr' is non-zero.
> PTRACE_ATTACH did not (and still does not) have such check,
> and users (strace) do pass garbage there... let's avoid repeating
> this mistake with SEIZE.
> 
> Set all task->ptrace bits in one operation - before this change,
> we were adding PT_SEIZED and PT_PTRACE_CAP with task->ptrace |= BIT ops.
> This was probably ok (not a bug), but let's be on a safer side.
> 
> Changes in v2: update ptrace_attach() call in compat_sys_ptrace() too.
> 
> Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
> Acked-by: Tejun Heo <tj@kernel.org>

Reviewed-by: Oleg Nesterov <oleg@redhat.com>

> ---
>  kernel/ptrace.c |   31 +++++++++++++++++++++----------
>  1 files changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 9acd07a..4661c5b 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -231,6 +231,7 @@ bool ptrace_may_access(struct task_struct *task, unsigned int mode)
>  }
>  
>  static int ptrace_attach(struct task_struct *task, long request,
> +			 unsigned long addr,
>  			 unsigned long flags)
>  {
>  	bool seize = (request == PTRACE_SEIZE);
> @@ -238,19 +239,29 @@ static int ptrace_attach(struct task_struct *task, long request,
>  
>  	/*
>  	 * SEIZE will enable new ptrace behaviors which will be implemented
> -	 * gradually.  SEIZE_DEVEL is used to prevent applications
> +	 * gradually.  SEIZE_DEVEL bit is used to prevent applications
>  	 * expecting full SEIZE behaviors trapping on kernel commits which
>  	 * are still in the process of implementing them.
>  	 *
>  	 * Only test programs for new ptrace behaviors being implemented
>  	 * should set SEIZE_DEVEL.  If unset, SEIZE will fail with -EIO.
>  	 *
> -	 * Once SEIZE behaviors are completely implemented, this flag and
> -	 * the following test will be removed.
> +	 * Once SEIZE behaviors are completely implemented, this flag
> +	 * will be removed.
>  	 */
>  	retval = -EIO;
> -	if (seize && !(flags & PTRACE_SEIZE_DEVEL))
> -		goto out;
> +	if (seize) {
> +		if (addr != 0)
> +			goto out;
> +		if (!(flags & PTRACE_SEIZE_DEVEL))
> +			goto out;
> +		flags &= ~(unsigned long)PTRACE_SEIZE_DEVEL;
> +		if (flags & ~(unsigned long)PTRACE_O_MASK)
> +			goto out;
> +		flags = PT_PTRACED | PT_SEIZED | (flags << PT_OPT_FLAG_SHIFT);
> +	} else {
> +		flags = PT_PTRACED;
> +	}
>  
>  	audit_ptrace(task);
>  
> @@ -282,11 +293,11 @@ static int ptrace_attach(struct task_struct *task, long request,
>  	if (task->ptrace)
>  		goto unlock_tasklist;
>  
> -	task->ptrace = PT_PTRACED;
>  	if (seize)
> -		task->ptrace |= PT_SEIZED;
> +		flags |= PT_SEIZED;
>  	if (ns_capable(task_user_ns(task), CAP_SYS_PTRACE))
> -		task->ptrace |= PT_PTRACE_CAP;
> +		flags |= PT_PTRACE_CAP;
> +	task->ptrace = flags;
>  
>  	__ptrace_link(task, current);
>  
> @@ -879,7 +890,7 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
>  	}
>  
>  	if (request == PTRACE_ATTACH || request == PTRACE_SEIZE) {
> -		ret = ptrace_attach(child, request, data);
> +		ret = ptrace_attach(child, request, addr, data);
>  		/*
>  		 * Some architectures need to do book-keeping after
>  		 * a ptrace attach.
> @@ -1022,7 +1033,7 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
>  	}
>  
>  	if (request == PTRACE_ATTACH || request == PTRACE_SEIZE) {
> -		ret = ptrace_attach(child, request, data);
> +		ret = ptrace_attach(child, request, addr, data);
>  		/*
>  		 * Some architectures need to do book-keeping after
>  		 * a ptrace attach.
> -- 
> 1.7.7.6
>