Re: [PATCH RFC v3 3/4] ptrace: add ptracer_access_allowed()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/20/26 23:48, Christian Brauner (Amutable) wrote:
> Add a helper that encapsulates all of the logic for checking ptrace
> access and remove open-coded versions in follow-up patches.
> 
> Reviewed-by: Jann Horn <jannh@google.com>
> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
> ---
>  include/linux/ptrace.h |  1 +
>  kernel/ptrace.c        | 27 +++++++++++++++++++++++++++
>  2 files changed, 28 insertions(+)
> 
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index 90507d4afcd6..ef314f7a9ecc 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -17,6 +17,7 @@ struct syscall_info {
>  	struct seccomp_data	data;
>  };
>  
> +bool ptracer_access_allowed(struct task_struct *tsk);
>  extern int ptrace_access_vm(struct task_struct *tsk, unsigned long addr,
>  			    void *buf, int len, unsigned int gup_flags);
>  
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 07398c9c8fe3..0e1f80f73a7f 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -13,6 +13,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/mm.h>
>  #include <linux/sched/coredump.h>
> +#include <linux/sched/exec_state.h>
>  #include <linux/sched/task.h>
>  #include <linux/errno.h>
>  #include <linux/mm.h>
> @@ -36,6 +37,32 @@
>  
>  #include <asm/syscall.h>	/* for syscall_get_* */
>  
> +/**
> + * ptracer_access_allowed - may current peek/poke @tsk's address space?
> + * @tsk: tracee
> + *
> + * Per-access check used by ptrace_access_vm() and architecture-specific
> + * tag/register accessors.  Returns true iff current is the registered
> + * ptracer of @tsk and either @tsk is owner-dumpable or current holds
> + * CAP_SYS_PTRACE in @tsk's exec namespace.  Lighter than
> + * __ptrace_may_access(): it re-validates only dumpability and
> + * capability on every access, without re-running LSM hooks or
> + * cred_cap_issubset() checks performed at attach time.
> + */
> +bool ptracer_access_allowed(struct task_struct *tsk)
> +{
> +	const struct task_exec_state *es;
> +
> +	if (!tsk->ptrace)
> +		return false;
> +	if (current != tsk->parent)
> +		return false;
> +	guard(rcu)();
> +	es = task_exec_state_rcu(tsk);
> +	return READ_ONCE(es->dumpable) == TASK_DUMPABLE_OWNER ||
> +	       ptracer_capable(tsk, es->user_ns);
> +}
> +
>  /*
>   * Access another process' address space via ptrace.
>   * Source/target buffer must be kernel space,
> 

Besides the new RCU + old MM handling, this matches what we do in
ptrace_access_vm().

Reviewed-by: David Hildenbrand (arm) <david@kernel.org>

-- 
Cheers,

David