Supporting VMAP_STACK with KASAN_VMALLOC is straightforward:

 - clear the shadow region of vmapped stacks when swapping them in
 - tweak Kconfig to allow VMAP_STACK to be turned on with KASAN

Reviewed-by: Dmitry Vyukov <dvyu...@google.com>
Signed-off-by: Daniel Axtens <d...@axtens.net>
---
 arch/Kconfig  | 9 +++++----
 kernel/fork.c | 4 ++++
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git arch/Kconfig arch/Kconfig
index 5f8a5d84dbbe..2d914990402f 100644
--- arch/Kconfig
+++ arch/Kconfig
@@ -843,16 +843,17 @@ config HAVE_ARCH_VMAP_STACK
 config VMAP_STACK
        default y
        bool "Use a virtually-mapped stack"
-       depends on HAVE_ARCH_VMAP_STACK && !KASAN
+       depends on HAVE_ARCH_VMAP_STACK
+       depends on !KASAN || KASAN_VMALLOC
        ---help---
          Enable this if you want the use virtually-mapped kernel stacks
          with guard pages.  This causes kernel stack overflows to be
          caught immediately rather than causing difficult-to-diagnose
          corruption.
 
-         This is presently incompatible with KASAN because KASAN expects
-         the stack to map directly to the KASAN shadow map using a formula
-         that is incorrect if the stack is in vmalloc space.
+         To use this with KASAN, the architecture must support backing
+         virtual mappings with real shadow memory, and KASAN_VMALLOC must
+         be enabled.
 
 config ARCH_OPTIONAL_KERNEL_RWX
        def_bool n
diff --git kernel/fork.c kernel/fork.c
index bcdf53125210..484ca6b0ae6c 100644
--- kernel/fork.c
+++ kernel/fork.c
@@ -94,6 +94,7 @@
 #include <linux/livepatch.h>
 #include <linux/thread_info.h>
 #include <linux/stackleak.h>
+#include <linux/kasan.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -224,6 +225,9 @@ static unsigned long *alloc_thread_stack_node(struct 
task_struct *tsk, int node)
                if (!s)
                        continue;
 
+               /* Clear the KASAN shadow of the stack. */
+               kasan_unpoison_shadow(s->addr, THREAD_SIZE);
+
                /* Clear stale pointers from reused stack. */
                memset(s->addr, 0, THREAD_SIZE);
 
-- 
2.20.1

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