Re: [PATCH v2 6/7] ptrace: introduce PTRACE_SET_SYSCALL_INFO request

["Dmitry V. Levin" <ldv@strace.io>]

On Thu, Jan 16, 2025 at 06:04:03PM +0200, Dmitry V. Levin wrote:
> On Thu, Jan 16, 2025 at 04:21:38PM +0100, Oleg Nesterov wrote:
> > On 01/13, Dmitry V. Levin wrote:
> > >
> > > +static int
> > > +ptrace_set_syscall_info(struct task_struct *child, unsigned long user_size,
> > > +			void __user *datavp)
> > > +{
> > > +	struct pt_regs *regs = task_pt_regs(child);
> > > +	struct ptrace_syscall_info info;
> > > +	int error;
> > > +
> > > +	BUILD_BUG_ON(sizeof(struct ptrace_syscall_info) < PTRACE_SYSCALL_INFO_SIZE_VER0);
> > > +
> > > +	if (user_size < PTRACE_SYSCALL_INFO_SIZE_VER0 || user_size > PAGE_SIZE)
> > > +		return -EINVAL;
> > > +
> > > +	error = copy_struct_from_user(&info, sizeof(info), datavp, user_size);
> > 
> > OK, I certainly can't understand why copy_struct_from_user/check_zeroed_user
> > is useful, at least in this case. In particular, this won't allow to run the
> > new code (which uses the "extended" ptrace_syscall_info) on the older kernels?
> > 
> > Can't we just use user_size as a version number?
> > 
> > We can also turn info->reserved into info->version filled by
> > ptrace_get_syscall_info().
> > 
> > ptrace_set_syscall_info() can check that info->version matches user_size.
> 
> The idea is to use "op" to specify the operation, and "flags" to specify
> future extensions to the operation.  For example, we could later add
> PTRACE_SYSCALL_INFO_SECCOMP_SKIP operation to specify an exit-like
> data for seccomp stops, or some flag to set instruction_pointer or
> stack_pointer.  I don't think any of these would require a version field,
> though.
> 
> That is, the zero check implied by copy_struct_from_user() is not really
> needed here since the compatibility is tracked by "op" and "flags":
> if "op" and "flags" do not instruct the kernel to use these unknown
> extra bits, the kernel is not obliged to check them either.
> For the same reason I don't think the kernel is obliged to read more
> than sizeof(info) from userspace.
> 
> What would you recommend using instead of copy_struct_from_user in this
> case?

Something like this?

        if (user_size < PTRACE_SYSCALL_INFO_SIZE_VER0 || user_size > PAGE_SIZE)
                return -EINVAL;

        if (copy_from_user(&info, datavp, min(sizeof(info), user_size)))
                return -EFAULT;

        if (user_size < sizeof(info))
                memset((void *)&info + user_size, 0, sizeof(info) - user_size);

-- 
ldv