stackleak: remove redundant check

In __stackleak_erase() we check that the `erase_low` value derived from
`current->lowest_stack` is above the lowest legitimate stack pointer
value, but this is already enforced by stackleak_track_stack() when
recording the lowest stack value.

Remove the redundant check.

There should be no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220427173128.2603085-4-mark.rutland@arm.com

kernel/stackleak.c

@@ -78,10 +78,6 @@ static __always_inline void __stackleak_erase(void)
 	unsigned int poison_count = 0;
 	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
 
-	/* Check that 'lowest_stack' value is sane */
-	if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
-		kstack_ptr = boundary;
-
 	/* Search for the poison value in the kernel stack */
 	while (kstack_ptr > boundary && poison_count <= depth) {
 		if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)