idr: fix idr_alloc() returning an ID out of range

If you use an IDR with a non-zero base, and specify a range that lies
entirely below the base, 'max - base' becomes very large and
idr_get_free() can return an ID that lies outside of the requested range.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 6ce711f27500 ("idr: Make 1-based IDRs more efficient")
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Reported-by: Jan Sokolowski <[e-mail]>
Reported-by: Koen Koning <[e-mail]>
Reported-by: Peter Senna Tschudin <[e-mail]>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6449
Reviewed-by: Christian König <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/idr.c

···
 
 	if (WARN_ON_ONCE(!(idr->idr_rt.xa_flags & ROOT_IS_IDR)))
 		idr->idr_rt.xa_flags |= IDR_RT_MARKER;
+	if (max < base)
+		return -ENOSPC;
 
 	id = (id < base) ? 0 : id - base;
 	radix_tree_iter_init(&iter, id);

tools/testing/radix-tree/idr-test.c

···
 	idr_destroy(&idr);
 }
 
+void idr_alloc2_test(void)
+{
+	int id;
+	struct idr idr = IDR_INIT_BASE(idr, 1);
+
+	id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL);
+	assert(id == -ENOSPC);
+
+	id = idr_alloc(&idr, idr_alloc2_test, 1, 2, GFP_KERNEL);
+	assert(id == 1);
+
+	id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL);
+	assert(id == -ENOSPC);
+
+	id = idr_alloc(&idr, idr_alloc2_test, 0, 2, GFP_KERNEL);
+	assert(id == -ENOSPC);
+
+	idr_destroy(&idr);
+}
+
 void idr_replace_test(void)
 {
 	DEFINE_IDR(idr);
···
 
 	idr_replace_test();
 	idr_alloc_test();
+	idr_alloc2_test();
 	idr_null_test();
 	idr_nowait_test();
 	idr_get_next_test(0);