On Thu, Apr 16, 2020 at 10:31:06AM +0100, Mark Rutland wrote:
> On Wed, Apr 15, 2020 at 08:26:05PM +0100, Will Deacon wrote:
> > I think abusing a relaxed concurrency primitive for this is
> > not the right thing to do, particularly when the __no_sanitize_address
> > annotation is available. I fact, it's almost an argument in favour
> > of removing READ_ONCE_NOCHECK() so that people use the annotation instead!
>
> Arguably we *are* using it as a relaxed concurrency primitive, to get a
> snapshot of a varaible undergoing concurrent modification.
That's fair, so it's only the checksum code that was abusing this, which
I've fixed.
> FWIW, for the arm64 unwind code we could add a helper to snapshot the
> frame record, and mark that as __no_sanitize_address, e.g.
>
> /*
> * Get a snapshot of a frame record that might be undergoing concurrent
> * modification (and hence we must also avoid a KASAN splat).
> */
> static __no_sanitize_address snapshot_frame(struct stackframe *frame,
> unsigned long fp)
> {
> frame->fp = READ_ONCE(*(unsigned long *)(fp));
> frame->pc = READ_ONCE(*(unsigned long *)(fp + 8));
> }
>
> ... we'd need to do likewied in a few bits of unwind code:
>
> arch/s390/kernel/unwind_bc.c: READ_ONCE_NOCHECK(regs->psw.mask) & PSW_MASK_PSTATE;
> arch/s390/kernel/unwind_bc.c: ip = READ_ONCE_NOCHECK(sf->gprs[8]);
> arch/s390/kernel/unwind_bc.c: sp = READ_ONCE_NOCHECK(sf->back_chain);
> arch/s390/kernel/unwind_bc.c: ip = READ_ONCE_NOCHECK(sf->gprs[8]);
> arch/s390/kernel/unwind_bc.c: ip = READ_ONCE_NOCHECK(regs->psw.addr);
> arch/s390/kernel/unwind_bc.c: sp = READ_ONCE_NOCHECK(regs->gprs[15]);
> arch/s390/kernel/unwind_bc.c: ip = READ_ONCE_NOCHECK(sf->gprs[8]);
> arch/x86/include/asm/atomic.h: * Note for KASAN: we deliberately don't use READ_ONCE_NOCHECK() here,
> arch/x86/include/asm/unwind.h: val = READ_ONCE_NOCHECK(x); \
> arch/x86/kernel/dumpstack.c: unsigned long addr = READ_ONCE_NOCHECK(*stack);
> arch/x86/kernel/process.c: fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
> arch/x86/kernel/process.c: ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
> arch/x86/kernel/process.c: fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
> arch/x86/kernel/unwind_frame.c: word = READ_ONCE_NOCHECK(*sp);
> arch/x86/kernel/unwind_guess.c: addr = READ_ONCE_NOCHECK(*state->sp);
> arch/x86/kernel/unwind_guess.c: unsigned long addr = READ_ONCE_NOCHECK(*state->sp);
> arch/x86/kernel/unwind_orc.c: *val = READ_ONCE_NOCHECK(*(unsigned long *)addr);
> arch/x86/kernel/unwind_orc.c: state->bp = READ_ONCE_NOCHECK(frame->bp);
> arch/x86/kernel/unwind_orc.c: state->ip = READ_ONCE_NOCHECK(frame->ret_addr);
> include/linux/compiler.h: * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
> include/linux/compiler.h:#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
> kernel/trace/trace_stack.c: * The READ_ONCE_NOCHECK is used to let KASAN know that
> kernel/trace/trace_stack.c: if ((READ_ONCE_NOCHECK(*p)) == stack_dump_trace[i]) {
Indeed. For now, I'm going to keep this simple with the change below, but
I'll revisit this later on because I have another series removing
smp_read_barrier_depends() which makes this a lot simpler.
Will
--->8
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 00a68063d9d5..c363d8debc43 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -212,18 +212,12 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
(typeof(x))__x; \
})
-/*
- * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
- * to hide memory access from KASAN.
- */
-#define READ_ONCE_NOCHECK(x) \
+#define READ_ONCE(x) \
({ \
compiletime_assert_rwonce_type(x); \
__READ_ONCE_SCALAR(x); \
})
-#define READ_ONCE(x) READ_ONCE_NOCHECK(x)
-
#define __WRITE_ONCE(x, val) \
do { \
*(volatile typeof(x) *)&(x) = (val); \
@@ -247,6 +241,24 @@ do { \
# define __no_kasan_or_inline __always_inline
#endif
+static __no_kasan_or_inline
+unsigned long __read_once_word_nocheck(const void *addr)
+{
+ return __READ_ONCE(*(unsigned long *)addr);
+}
+
+/*
+ * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
+ * word from memory atomically but without telling KASAN. This is usually
+ * used by unwinding code when walking the stack of a running process.
+ */
+#define READ_ONCE_NOCHECK(x) \
+({ \
+ unsigned long __x = __read_once_word_nocheck(&(x)); \
+ smp_read_barrier_depends(); \
+ __x; \
+})
+
static __no_kasan_or_inline
unsigned long read_word_at_a_time(const void *addr)
{
