[PATCH 6.12 440/567] idr: fix idr_alloc() returning an ID out of range

Greg Kroah-Hartman
2026-01-06T17:38:36Z

From: Matthew Wilcox (Oracle) <willy@infradead.org>

commit c6e8e595a0798ad67da0f7bebaf69c31ef70dfff upstream.

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/20251128161853.3200058-1-willy@infradead.org
Fixes: 6ce711f27500 ("idr: Make 1-based IDRs more efficient")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Jan Sokolowski <jan.sokolowski@intel.com>
Reported-by: Koen Koning <koen.koning@intel.com>
Reported-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6449
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
lib/idr.c | 2 ++
tools/testing/radix-tree/idr-test.c | 21 +++++++++++++++++++++
2 files changed, 23 insertions(+)

--- a/lib/idr.c
+++ b/lib/idr.c
@@ -40,6 +40,8 @@ int idr_alloc_u32(struct idr *idr, void

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);
--- a/tools/testing/radix-tree/idr-test.c
+++ b/tools/testing/radix-tree/idr-test.c
@@ -57,6 +57,26 @@ void idr_alloc_test(void)
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);
@@ -409,6 +429,7 @@ void idr_checks(void)

idr_replace_test();
idr_alloc_test();
+ idr_alloc2_test();
idr_null_test();
idr_nowait_test();
idr_get_next_test(0);