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

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:

quoted

On 01/13, Dmitry V. Levin wrote:

quoted

+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