ptrace: Fix ->ptracer_cred handling for PTRACE_TRACEME

Fix two issues:

When called for PTRACE_TRACEME, ptrace_link() would obtain an RCU
reference to the parent's objective credentials, then give that pointer
to get_cred().  However, the object lifetime rules for things like
struct cred do not permit unconditionally turning an RCU reference into
a stable reference.

PTRACE_TRACEME records the parent's credentials as if the parent was
acting as the subject, but that's not the case.  If a malicious
unprivileged child uses PTRACE_TRACEME and the parent is privileged, and
at a later point, the parent process becomes attacker-controlled
(because it drops privileges and calls execve()), the attacker ends up
with control over two processes with a privileged ptrace relationship,
which can be abused to ptrace a suid binary and obtain root privileges.

Fix both of these by always recording the credentials of the process
that is requesting the creation of the ptrace relationship:
current_cred() can't change under us, and current is the proper subject
for access control.

This change is theoretically userspace-visible, but I am not aware of
any code that it will actually break.

Fixes: 64b875f7ac8a ("ptrace: Capture the ptracer's creds not PT_PTRACE_CAP")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
int compat_ptrace_request(struct task_struct *child, compat_long_t request,
			  compat_ulong_t addr, compat_ulong_t data)
{
	compat_ulong_t __user *datap = compat_ptr(data);
	compat_ulong_t word;
	kernel_siginfo_t siginfo;
	int ret;

	switch (request) {
	case PTRACE_PEEKTEXT:
	case PTRACE_PEEKDATA:
		ret = ptrace_access_vm(child, addr, &word, sizeof(word),
				FOLL_FORCE);
		if (ret != sizeof(word))
			ret = -EIO;
		else
			ret = put_user(word, datap);
		break;

	case PTRACE_POKETEXT:
	case PTRACE_POKEDATA:
		ret = ptrace_access_vm(child, addr, &data, sizeof(data),
				FOLL_FORCE | FOLL_WRITE);
		ret = (ret != sizeof(data) ? -EIO : 0);
		break;

	case PTRACE_GETEVENTMSG:
		ret = put_user((compat_ulong_t) child->ptrace_message, datap);
		break;

	case PTRACE_GETSIGINFO:
		ret = ptrace_getsiginfo(child, &siginfo);
		if (!ret)
			ret = copy_siginfo_to_user32(
				(struct compat_siginfo __user *) datap,
				&siginfo);
		break;

	case PTRACE_SETSIGINFO:
		ret = copy_siginfo_from_user32(
			&siginfo, (struct compat_siginfo __user *) datap);
		if (!ret)
			ret = ptrace_setsiginfo(child, &siginfo);
		break;
#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
	case PTRACE_GETREGSET:
	case PTRACE_SETREGSET:
	{
		struct iovec kiov;
		struct compat_iovec __user *uiov =
			(struct compat_iovec __user *) datap;
		compat_uptr_t ptr;
		compat_size_t len;

		if (!access_ok(uiov, sizeof(*uiov)))
			return -EFAULT;

		if (__get_user(ptr, &uiov->iov_base) ||
		    __get_user(len, &uiov->iov_len))
			return -EFAULT;

		kiov.iov_base = compat_ptr(ptr);
		kiov.iov_len = len;

		ret = ptrace_regset(child, request, addr, &kiov);
		if (!ret)
			ret = __put_user(kiov.iov_len, &uiov->iov_len);
		break;
	}
#endif

	default:
		ret = ptrace_request(child, request, addr, data);
	}

	return ret;
}

int compat_ptrace_request(struct task_struct *child, compat_long_t request,
			  compat_ulong_t addr, compat_ulong_t data)
{
	compat_ulong_t __user *datap = compat_ptr(data);
	compat_ulong_t word;
	kernel_siginfo_t siginfo;
	int ret;

	switch (request) {
	case PTRACE_PEEKTEXT:
	case PTRACE_PEEKDATA:
		ret = ptrace_access_vm(child, addr, &word, sizeof(word),
				FOLL_FORCE);
		if (ret != sizeof(word))
			ret = -EIO;
		else
			ret = put_user(word, datap);
		break;

	case PTRACE_POKETEXT:
	case PTRACE_POKEDATA:
		ret = ptrace_access_vm(child, addr, &data, sizeof(data),
				FOLL_FORCE | FOLL_WRITE);
		ret = (ret != sizeof(data) ? -EIO : 0);
		break;

	case PTRACE_GETEVENTMSG:
		ret = put_user((compat_ulong_t) child->ptrace_message, datap);
		break;

	case PTRACE_GETSIGINFO:
		ret = ptrace_getsiginfo(child, &siginfo);
		if (!ret)
			ret = copy_siginfo_to_user32(
				(struct compat_siginfo __user *) datap,
				&siginfo);
		break;

	case PTRACE_SETSIGINFO:
		ret = copy_siginfo_from_user32(
			&siginfo, (struct compat_siginfo __user *) datap);
		if (!ret)
			ret = ptrace_setsiginfo(child, &siginfo);
		break;
#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
	case PTRACE_GETREGSET:
	case PTRACE_SETREGSET:
	{
		struct iovec kiov;
		struct compat_iovec __user *uiov =
			(struct compat_iovec __user *) datap;
		compat_uptr_t ptr;
		compat_size_t len;

		if (!access_ok(uiov, sizeof(*uiov)))
			return -EFAULT;

		if (__get_user(ptr, &uiov->iov_base) ||
		    __get_user(len, &uiov->iov_len))
			return -EFAULT;

		kiov.iov_base = compat_ptr(ptr);
		kiov.iov_len = len;

		ret = ptrace_regset(child, request, addr, &kiov);
		if (!ret)
			ret = __put_user(kiov.iov_len, &uiov->iov_len);
		break;
	}
#endif

	default:
		ret = ptrace_request(child, request, addr, data);
	}

	return ret;
}
