A natural number is a [i]palindrome[/i] when one obtains the same number when writing its digits in reverse order. For example, $481184$, $131$ and $2$ are palindromes.

Determine all pairs $(m,n)$ of positive integers such that $\underbrace{111\ldots 1}_{m\ {\rm ones}}\times\underbrace{111\ldots 1}_{n\ {\rm ones}}$ is a palindrome.
1. **Understanding the Problem:**
   We need to determine all pairs \((m, n)\) of positive integers such that the product of two numbers, each consisting of \(m\) and \(n\) ones respectively, is a palindrome. 

2. **Formulating the Numbers:**
   Let \(A\) be the number consisting of \(m\) ones and \(B\) be the number consisting of \(n\) ones. These numbers can be written as:
   \[
   A = \underbrace{111\ldots 1}_{m\ \text{ones}} = \frac{10^m - 1}{9}
   \]
   \[
   B = \underbrace{111\ldots 1}_{n\ \text{ones}} = \frac{10^n - 1}{9}
   \]

3. **Product of the Numbers:**
   The product \(N\) of these two numbers is:
   \[
   N = A \times B = \left(\frac{10^m - 1}{9}\right) \times \left(\frac{10^n - 1}{9}\right) = \frac{(10^m - 1)(10^n - 1)}{81}
   \]

4. **Number of Digits in \(N\):**
   The number \(N\) will have \(m + n - 1\) digits. This is because the product of two numbers with \(m\) and \(n\) digits respectively will have at most \(m + n - 1\) digits.

5. **Checking for Palindrome:**
   For \(N\) to be a palindrome, the digits of \(N\) must read the same forwards and backwards. 

6. **Considering Large Values of \(m\) and \(n\):**
   If \(m, n > 9\), then considering the tenth leftmost digit, there would be a carry, thus the number consisting of the first nine digits of \(N\) is bigger than the number formed by the last nine digits of \(N\), in reversed order. Therefore, if \(m, n > 10\), the number \(N\) is not a palindrome.

7. **Checking Small Values of \(m\) and \(n\):**
   We need to check the cases where \(m, n \leq 9\). 

   - For \(m = 1\) and \(n = 1\):
     \[
     N = 1 \times 1 = 1 \quad \text{(Palindrome)}
     \]
   - For \(m = 2\) and \(n = 2\):
     \[
     N = 11 \times 11 = 121 \quad \text{(Palindrome)}
     \]
   - For \(m = 3\) and \(n = 3\):
     \[
     N = 111 \times 111 = 12321 \quad \text{(Palindrome)}
     \]
   - For \(m = 4\) and \(n = 4\):
     \[
     N = 1111 \times 1111 = 1234321 \quad \text{(Palindrome)}
     \]
   - For \(m = 5\) and \(n = 5\):
     \[
     N = 11111 \times 11111 = 123454321 \quad \text{(Palindrome)}
     \]

   This pattern continues for \(m = n\) up to 9.

8. **Conclusion:**
   The pairs \((m, n)\) such that \(N\) is a palindrome are those where \(m = n\) and \(m, n \leq 9\).

The final answer is \( \boxed{ (m, n) } \) such that \(m = n\) and \(m, n \leq 9\).