[PATCH 6.1.y] mm/kfence: fix KASAN hardware tag faults during late enablement

[Sasha Levin <sashal@kernel.org>]

From: Alexander Potapenko <glider@google.com>

[ Upstream commit d155aab90fffa00f93cea1f107aef0a3d548b2ff ]

When KASAN hardware tags are enabled, re-enabling KFENCE late (via
/sys/module/kfence/parameters/sample_interval) causes KASAN faults.

This happens because the KFENCE pool and metadata are allocated via the
page allocator, which tags the memory, while KFENCE continues to access it
using untagged pointers during initialization.

Use __GFP_SKIP_KASAN for late KFENCE pool and metadata allocations to
ensure the memory remains untagged, consistent with early allocations from
memblock.  To support this, add __GFP_SKIP_KASAN to the allowlist in
__alloc_contig_verify_gfp_mask().

Link: https://lkml.kernel.org/r/20260220144940.2779209-1-glider@google.com
Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
Signed-off-by: Alexander Potapenko <glider@google.com>
Suggested-by: Ernesto Martinez Garcia <ernesto.martinezgarcia@tugraz.at>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ expand __GFP_SKIP_KASAN + nr_pages_pool => nr_pages ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/kfence/core.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index edf6deb382b67..bdb864384f2af 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -897,7 +897,8 @@ static int kfence_init_late(void)
 #ifdef CONFIG_CONTIG_ALLOC
 	struct page *pages;
 
-	pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node, NULL);
+	pages = alloc_contig_pages(nr_pages, GFP_KERNEL | __GFP_SKIP_KASAN_UNPOISON |
+				   __GFP_SKIP_KASAN_POISON, first_online_node, NULL);
 	if (!pages)
 		return -ENOMEM;
 	__kfence_pool = page_to_virt(pages);
@@ -906,7 +907,9 @@ static int kfence_init_late(void)
 		pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
 		return -EINVAL;
 	}
-	__kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL);
+	__kfence_pool = alloc_pages_exact(KFENCE_POOL_SIZE, GFP_KERNEL |
+					  __GFP_SKIP_KASAN_UNPOISON |
+					  __GFP_SKIP_KASAN_POISON);
 	if (!__kfence_pool)
 		return -ENOMEM;
 #endif
-- 
2.51.0