kasan: skip HW tagging for all kernel thread stacks

HW-tag KASAN never checks kernel stacks because stack pointers carry the
match-all tag, so setting/poisoning tags is pure overhead.

- Add __GFP_SKIP_KASAN to THREADINFO_GFP so every stack allocator that
  uses it skips tagging (fork path plus arch users)
- Add __GFP_SKIP_KASAN to GFP_VMAP_STACK for the fork-specific vmap
  stacks.
- When reusing cached vmap stacks, skip kasan_unpoison_range() if HW tags
  are enabled.

Software KASAN is unchanged; this only affects tag-based KASAN.

Link: https://lore.kernel.org/20260429102704.680174-3-dev.jain@arm.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ben Segall <bsegall@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 8ac38beae360..ec6a120291e5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -204,7 +204,7 @@ static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
  * accounting is performed by the code assigning/releasing stacks to tasks.
  * We need a zeroed memory without __GFP_ACCOUNT.
  */
-#define GFP_VMAP_STACK (GFP_KERNEL | __GFP_ZERO)
+#define GFP_VMAP_STACK (GFP_KERNEL | __GFP_ZERO | __GFP_SKIP_KASAN)
 
 struct vm_stack {
 	struct rcu_head rcu;
@@ -342,7 +342,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		}
 
 		/* Reset stack metadata. */
-		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
+		if (!kasan_hw_tags_enabled())
+			kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
 
 		stack = kasan_reset_tag(vm_area->addr);