Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Cc: Marco Elver <elver@xxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Cc: kasan-dev@xxxxxxxxxxxxxxxx Cc: Ilya Leoshkevich <iii@xxxxxxxxxxxxx> Cc: Nicholas Miehlbradt <nicholas@xxxxxxxxxxxxx> Hi folks, (adding KMSAN reviewers and IBM people who are currently porting KMSAN to other architectures, plus Paul for his opinion on refactoring RCU) this patch broke x86 KMSAN in a subtle way. For every memory access in the code instrumented by KMSAN we call kmsan_get_metadata() to obtain the metadata for the memory being accessed. For virtual memory the metadata pointers are stored in the corresponding `struct page`, therefore we need to call virt_to_page() to get them. According to the comment in arch/x86/include/asm/page.h, virt_to_page(kaddr) returns a valid pointer iff virt_addr_valid(kaddr) is true, so KMSAN needs to call virt_addr_valid() as well. To avoid recursion, kmsan_get_metadata() must not call instrumented code, therefore ./arch/x86/include/asm/kmsan.h forks parts of arch/x86/mm/physaddr.c to check whether a virtual address is valid or not. But the introduction of rcu_read_lock() to pfn_valid() added instrumented RCU API calls to virt_to_page_or_null(), which is called by kmsan_get_metadata(), so there is an infinite recursion now. I do not think it is correct to stop that recursion by doing kmsan_enter_runtime()/kmsan_exit_runtime() in kmsan_get_metadata(): that would prevent instrumented functions called from within the runtime from tracking the shadow values, which might introduce false positives. I am currently looking into inlining __rcu_read_lock()/__rcu_read_unlock(), into KMSAN code to prevent it from being instrumented, but that might require factoring out parts of kernel/rcu/tree_plugin.h into a non-private header. Do you think this is feasible? Another option is to cut some edges in the code calling virt_to_page(). First, my observation is that virt_addr_valid() is quite rare in the kernel code, i.e. not all cases of calling virt_to_page() are covered with it. Second, every memory access to KMSAN metadata residing in virt_to_page(kaddr)->shadow always accompanies an access to `kaddr` itself, so if there is a race on a PFN then the access to `kaddr` will probably also trigger a fault. Third, KMSAN metadata accesses are inherently non-atomic, and even if we ensure pfn_valid() is returning a consistent value for a single memory access, calling it twice may already return different results. Considering the above, how bad would it be to drop synchronization for KMSAN's version of pfn_valid() called from kmsan_virt_addr_valid()?
