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

[Damien Le Moal]

On 1/20/23 19:23, Peter Zijlstra wrote:
> 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))

Why not one if with a && ?

>> +			return -EINVAL;
>> +	}
>> +
>>  	if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {
>>  		if (!IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) ||
>>  		    !IS_ENABLED(CONFIG_SECCOMP))
>> -- 
>> 2.39.0
>>

-- 
Damien Le Moal
Western Digital Research