Re: [PATCH v3 1/3] ptrace,syscall_user_dispatch: Implement Syscall User Dispatch Suspension

[Gregory Price]

On Fri, Jan 20, 2023 at 04:22:51PM +0100, Oleg Nesterov wrote:
> Hi Gregory,
> 
> I'll try to read this series next Monday, I need to recall what does
> syscall-user-dispatch actually do ;)
> 
> just one question for now,
> 
> On 01/20, Gregory Price wrote:
> >
> > --- a/kernel/ptrace.c
> > +++ b/kernel/ptrace.c
> > @@ -370,6 +370,10 @@ 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) &&
> > +	    (!IS_ENABLED(CONFIG_CHECKPOINT_RESTART)))
> > +			return -EINVAL;
> 
> Hmm? git grep CHECKPOINT_RESTART shows nothing.
> 
> Oleg.
>

TIL the mailing lists don't like responses from proxy addresses.
Resending response to it goes out to everyone


Good catch, I always mixup RESTART/RESTORE.  This should be RESTORE
Adjusted patch below, will send a v4 tomorrow so as not to spam the
lists.  Attached an updated patch for the time being.



(brief syscall user dispatch overview)

syscall-user-dispatch is relatively simple, the goal is to implement
syscall interposition for foreign syscalls (windows, non-posix,
whatever).  Since the ABI of these syscalls can't be trusted to be
anything like linux, syscall dispatch produces a SIGSYS before anything
else can do things like check register values.

How to use

1) User registers a SIGSYS signal handler
2) User does
   prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON,
	       <address>, <length>, char* selector)

3) All 'syscall' instructions *outside* the virtual address range
   (address, address+length) now produce a SIGSYS on the thread that
	 executed the syscall.

   <selector> can be set to SYSCALL_DISPATCH_FILTER_ALLOW or 
	 SYSCALL_DISPATCH_FILTER_BLOCK to enable/disable this signal
	 production from userland without having to make kernel calls.

docs: https://docs.kernel.org/admin-guide/syscall-user-dispatch.html


Updated patch


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..b5ec75164805 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))
+               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..a348b68d07a2 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -370,6 +370,10 @@ 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) &&
+           (!IS_ENABLED(CONFIG_CHECKPOINT_RESTORE)))
+                       return -EINVAL;
+
        if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {
                if (!IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) ||
                    !IS_ENABLED(CONFIG_SECCOMP))