[PATCH v2 2/3] kasan: skip HW tagging for all kernel thread stacks

[Muhammad Usama Anjum <usama.anjum@arm.com>]

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.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
---
Changes since v1:
- Specify _GFP_SKIP_KASAN in THREADINFO_GFP and GFP_VMAP_STACK to use
  it everywhere and cover the missed locations
- Update description
---
 include/linux/thread_info.h | 2 +-
 kernel/fork.c               | 5 +++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 051e429026904..307b8390fc670 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -92,7 +92,7 @@ static inline long set_restart_fn(struct restart_block *restart,
 #define THREAD_ALIGN	THREAD_SIZE
 #endif
 
-#define THREADINFO_GFP		(GFP_KERNEL_ACCOUNT | __GFP_ZERO)
+#define THREADINFO_GFP		(GFP_KERNEL_ACCOUNT | __GFP_ZERO | __GFP_SKIP_KASAN)
 
 /*
  * flag set/clear/test wrappers
diff --git a/kernel/fork.c b/kernel/fork.c
index bb0c2613a5604..4bc7a03662109 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -207,7 +207,7 @@ 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;
@@ -345,7 +345,8 @@ 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);
 
-- 
2.47.3