[PATCH] kfence: fix is_kfence_address() for addresses below KFENCE_POOL_SIZE

[Marco Elver]

Originally the addr != NULL check was meant to take care of the case
where __kfence_pool == NULL (KFENCE is disabled). However, this does not
work for addresses where addr > 0 && addr < KFENCE_POOL_SIZE.

This can be the case on NULL-deref where addr > 0 && addr < PAGE_SIZE or
any other faulting access with addr < KFENCE_POOL_SIZE. While the kernel
would likely crash, the stack traces and report might be confusing due
to double faults upon KFENCE's attempt to unprotect such an address.

Fix it by just checking that __kfence_pool != NULL instead.

Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
Reported-by: Kuan-Ying Lee <Kuan-Ying.Lee@xxxxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>    [5.12+]
---
 include/linux/kfence.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/kfence.h b/include/linux/kfence.h
index a70d1ea03532..3fe6dd8a18c1 100644
--- a/include/linux/kfence.h
+++ b/include/linux/kfence.h
@@ -51,10 +51,11 @@ extern atomic_t kfence_allocation_gate;
 static __always_inline bool is_kfence_address(const void *addr)
 {
 	/*
-	 * The non-NULL check is required in case the __kfence_pool pointer was
-	 * never initialized; keep it in the slow-path after the range-check.
+	 * The __kfence_pool != NULL check is required to deal with the case
+	 * where __kfence_pool == NULL && addr < KFENCE_POOL_SIZE. Keep it in
+	 * the slow-path after the range-check!
 	 */
-	return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && addr);
+	return unlikely((unsigned long)((char *)addr - __kfence_pool) < KFENCE_POOL_SIZE && __kfence_pool);
 }
 
 /**
-- 
2.33.0.rc1.237.g0d66db33f3-goog