Re: [PATCH 1/3] ptrace,syscall_user_dispatch: Implement Syscall User Dispatch Suspension

[Peter Zijlstra]

On Wed, Jan 18, 2023 at 03:10:53PM -0500, Gregory Price wrote:
> Adds PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH to ptrace options, and
> modify Syscall User Dispatch to suspend interception when enabled.
> 
> This is modeled after the SUSPEND_SECCOMP feature, which suspends
> SECCOMP interposition.  Without doing this, software like CRIU will
> inject system calls into a process and be intercepted by Syscall
> User Dispatch, either causing a crash (due to blocked signals) or
> the delivery of those signals to a ptracer (not the intended behavior).
> 
> Since Syscall User Dispatch is not a privileged feature, a check
> for permissions is not required, however attempting to set this
> option when CONFIG_CHECKPOINT_RESTORE it not supported should be
> disallowed, as its intended use is checkpoint/resume.
> 
> Signed-off-by: Gregory Price <gregory.price@xxxxxxxxxxxx>

One small nit -- see below, otherwise:

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
>  include/linux/ptrace.h               | 2 ++
>  include/uapi/linux/ptrace.h          | 6 +++++-
>  kernel/entry/syscall_user_dispatch.c | 5 +++++
>  kernel/ptrace.c                      | 5 +++++
>  4 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index eaaef3ffec22..461ae5c99d57 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -45,6 +45,8 @@ extern int ptrace_access_vm(struct task_struct *tsk, unsigned long addr,
>  
>  #define PT_EXITKILL		(PTRACE_O_EXITKILL << PT_OPT_FLAG_SHIFT)
>  #define PT_SUSPEND_SECCOMP	(PTRACE_O_SUSPEND_SECCOMP << PT_OPT_FLAG_SHIFT)
> +#define PT_SUSPEND_SYSCALL_USER_DISPATCH \
> +	(PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH << PT_OPT_FLAG_SHIFT)
>  
>  extern long arch_ptrace(struct task_struct *child, long request,
>  			unsigned long addr, unsigned long data);
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index 195ae64a8c87..ba9e3f19a22c 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -146,9 +146,13 @@ struct ptrace_rseq_configuration {
>  /* eventless options */
>  #define PTRACE_O_EXITKILL		(1 << 20)
>  #define PTRACE_O_SUSPEND_SECCOMP	(1 << 21)
> +#define PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH	(1 << 22)
>  
>  #define PTRACE_O_MASK		(\
> -	0x000000ff | PTRACE_O_EXITKILL | PTRACE_O_SUSPEND_SECCOMP)
> +	0x000000ff | \
> +	PTRACE_O_EXITKILL | \
> +	PTRACE_O_SUSPEND_SECCOMP | \
> +	PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH)
>  
>  #include <asm/ptrace.h>
>  
> diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
> index 0b6379adff6b..7607f4598dd8 100644
> --- a/kernel/entry/syscall_user_dispatch.c
> +++ b/kernel/entry/syscall_user_dispatch.c
> @@ -8,6 +8,7 @@
>  #include <linux/uaccess.h>
>  #include <linux/signal.h>
>  #include <linux/elf.h>
> +#include <linux/ptrace.h>
>  
>  #include <linux/sched/signal.h>
>  #include <linux/sched/task_stack.h>
> @@ -36,6 +37,10 @@ bool syscall_user_dispatch(struct pt_regs *regs)
>  	struct syscall_user_dispatch *sd = &current->syscall_dispatch;
>  	char state;
>  
> +	if (IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) &&
> +		unlikely(current->ptrace & PT_SUSPEND_SYSCALL_USER_DISPATCH))

Align with the '(' pleaase.

> +		return false;
> +
>  	if (likely(instruction_pointer(regs) - sd->offset < sd->len))
>  		return false;
>  
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 54482193e1ed..a6ad815bd4be 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -370,6 +370,11 @@ static int check_ptrace_options(unsigned long data)
>  	if (data & ~(unsigned long)PTRACE_O_MASK)
>  		return -EINVAL;
>  
> +	if (unlikely(data & PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH)) {
> +		if (!IS_ENABLED(CONFIG_CHECKPOINT_RESTART))
> +			return -EINVAL;
> +	}
> +
>  	if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {
>  		if (!IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) ||
>  		    !IS_ENABLED(CONFIG_SECCOMP))
> -- 
> 2.39.0
>