Re: [CFT][PATCH] ptrace: Properly initialize ptracer_cred on fork

[Takashi Iwai <tiwai@suse.de>]

On Mon, 22 May 2017 23:04:48 +0200,
Eric W. Biederman wrote:
> 
> 
> When I introduced ptracer_cred I failed to consider the weirdness of
> fork where the task_struct copies the old value by default.  This
> winds up leaving ptracer_cred set even when a process forks and
> the child process does not wind up being ptraced.
> 
> Because ptracer_cred is not set on non-ptraced processes whose
> parents were ptraced this has broken the ability of the enlightenment
> window manager to start setuid children.
> 
> Fix this by properly initializing ptracer_cred in ptrace_init_task
> 
> This must be done with a little bit of care to preserve the current value
> of ptracer_cred when ptrace carries through fork.  Re-reading the
> ptracer_cred from the ptracing process at this point is inconsistent
> with how PT_PTRACE_CAP has been maintained all of these years.
> 
> Fixes: 64b875f7ac8a ("ptrace: Capture the ptracer's creds not PT_PTRACE_CAP")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
> 
> If I could get some folks to test and verify this fixes the
> enlightenment issue I would really appreciate it.

This seems giving a compile warning and it becomes error in the
following:

In file included from ./include/linux/mutex.h:13:0,
                 from ./include/linux/kernfs.h:13,
                 from ./include/linux/sysfs.h:15,
                 from ./include/linux/kobject.h:21,
                 from ./include/linux/device.h:17,
                 from drivers/gpu/drm/i915/gvt/kvmgt.c:32:
./include/linux/ptrace.h: In function ‘ptrace_init_task’:
./arch/x86/include/asm/current.h:17:17: error: passing argument 3 of ‘__ptrace_link’ discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
 #define current get_current()
                 ^
./include/linux/ptrace.h:210:41: note: in expansion of macro ‘current’
   __ptrace_link(child, current->parent, current->ptracer_cred);
                                         ^~~~~~~
In file included from ./arch/x86/include/asm/stacktrace.h:10:0,
                 from ./arch/x86/include/asm/perf_event.h:246,
                 from ./include/linux/perf_event.h:24,
                 from ./arch/x86/include/asm/kvm_host.h:24,
                 from ./include/linux/kvm_host.h:37,
                 from drivers/gpu/drm/i915/gvt/kvmgt.c:41:
./include/linux/ptrace.h:56:13: note: expected ‘struct cred *’ but argument is of type ‘const struct cred *’
 extern void __ptrace_link(struct task_struct *child,
             ^~~~~~~~~~~~~
cc1: all warnings being treated as errors


thanks,

Takashi

> 
>  include/linux/ptrace.h |  7 +++++--
>  kernel/ptrace.c        | 20 +++++++++++++-------
>  2 files changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index 422bc2e4cb6a..23c5716f7bd2 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -54,7 +54,8 @@ extern int ptrace_request(struct task_struct *child, long request,
>  			  unsigned long addr, unsigned long data);
>  extern void ptrace_notify(int exit_code);
>  extern void __ptrace_link(struct task_struct *child,
> -			  struct task_struct *new_parent);
> +			  struct task_struct *new_parent,
> +			  struct cred *ptracer_cred);
>  extern void __ptrace_unlink(struct task_struct *child);
>  extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
>  #define PTRACE_MODE_READ	0x01
> @@ -206,7 +207,7 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
>  
>  	if (unlikely(ptrace) && current->ptrace) {
>  		child->ptrace = current->ptrace;
> -		__ptrace_link(child, current->parent);
> +		__ptrace_link(child, current->parent, current->ptracer_cred);
>  
>  		if (child->ptrace & PT_SEIZED)
>  			task_set_jobctl_pending(child, JOBCTL_TRAP_STOP);
> @@ -215,6 +216,8 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
>  
>  		set_tsk_thread_flag(child, TIF_SIGPENDING);
>  	}
> +	else
> +		child->ptracer_cred = NULL;
>  }
>  
>  /**
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 266ddcc1d8bb..79cbe00fe787 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -60,19 +60,25 @@ int ptrace_access_vm(struct task_struct *tsk, unsigned long addr,
>  }
>  
>  
> +void __ptrace_link(struct task_struct *child, struct task_struct *new_parent,
> +		    struct cred *ptracer_cred)
> +{
> +	BUG_ON(!list_empty(&child->ptrace_entry));
> +	list_add(&child->ptrace_entry, &new_parent->ptraced);
> +	child->parent = new_parent;
> +	child->ptracer_cred = get_cred(ptracer_cred);
> +}
> +
>  /*
>   * ptrace a task: make the debugger its new parent and
>   * move it to the ptrace list.
>   *
>   * Must be called with the tasklist lock write-held.
>   */
> -void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
> +static void ptrace_link(struct task_struct *child, struct task_struct *new_parent)
>  {
> -	BUG_ON(!list_empty(&child->ptrace_entry));
> -	list_add(&child->ptrace_entry, &new_parent->ptraced);
> -	child->parent = new_parent;
>  	rcu_read_lock();
> -	child->ptracer_cred = get_cred(__task_cred(new_parent));
> +	__ptrace_link(child, new_parent, __task_cred(new_parent));
>  	rcu_read_unlock();
>  }
>  
> @@ -386,7 +392,7 @@ static int ptrace_attach(struct task_struct *task, long request,
>  		flags |= PT_SEIZED;
>  	task->ptrace = flags;
>  
> -	__ptrace_link(task, current);
> +	ptrace_link(task, current);
>  
>  	/* SEIZE doesn't trap tracee on attach */
>  	if (!seize)
> @@ -459,7 +465,7 @@ static int ptrace_traceme(void)
>  		 */
>  		if (!ret && !(current->real_parent->flags & PF_EXITING)) {
>  			current->ptrace = PT_PTRACED;
> -			__ptrace_link(current, current->real_parent);
> +			ptrace_link(current, current->real_parent);
>  		}
>  	}
>  	write_unlock_irq(&tasklist_lock);
> -- 
> 2.10.1
>