fs/select: Fix memory corruption in compat_get_fd_set()

Commit 464d6242 ("select: switch compat_{get,put}_fd_set() to
compat_{get,put}_bitmap()") changed the calculation on how many bytes
need to be zeroed when userspace handed over a NULL pointer for a fdset
array in the select syscall.

The calculation was changed in compat_get_fd_set() wrongly from
	memset(fdset, 0, ((nr + 1) & ~1)*sizeof(compat_ulong_t));
to
	memset(fdset, 0, ALIGN(nr, BITS_PER_LONG));

The ALIGN(nr, BITS_PER_LONG) calculates the number of _bits_ which need
to be zeroed in the target fdset array (rounded up to the next full bits
for an unsigned long).

But the memset() call expects the number of _bytes_ to be zeroed.

This leads to clearing more memory than wanted (on the stack area or
even at kmalloc()ed memory areas) and to random kernel crashes as we
have seen them on the parisc platform.

The correct change should have been

	memset(fdset, 0, (ALIGN(nr, BITS_PER_LONG) / BITS_PER_LONG) * BYTES_PER_LONG);

which is the same as can be archieved with a call to

	zero_fd_set(nr, fdset).

Fixes: 464d6242 ("select: switch compat_{get,put}_fd_set() to compat_{get,put}_bitmap()"
Acked-by: : Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/select.c

@@ -1164,11 +1164,7 @@ int compat_get_fd_set(unsigned long nr, compat_ulong_t __user *ufdset,
 	if (ufdset) {
 		return compat_get_bitmap(fdset, ufdset, nr);
 	} else {
-		/* Tricky, must clear full unsigned long in the
-		 * kernel fdset at the end, ALIGN makes sure that
-		 * actually happens.
-		 */
-		memset(fdset, 0, ALIGN(nr, BITS_PER_LONG));
+		zero_fd_set(nr, fdset);
 		return 0;
 	}
 }