cfi: Initial support for cfi-icall in QEMU

LLVM/Clang, supports runtime checks for forward-edge Control-Flow
Integrity (CFI).

CFI on indirect function calls (cfi-icall) ensures that, in indirect
function calls, the function called is of the right signature for the
pointer type defined at compile time.

For this check to work, the code must always respect the function
signature when using function pointer, the function must be defined
at compile time, and be compiled with link-time optimization.

This rules out, for example, shared libraries that are dynamically loaded
(given that functions are not known at compile time), and code that is
dynamically generated at run-time.

This patch:

1) Introduces the CONFIG_CFI flag to support cfi in QEMU

2) Introduces a decorator to allow the definition of "sensitive"
functions, where a non-instrumented function may be called at runtime
through a pointer. The decorator will take care of disabling cfi-icall
checks on such functions, when cfi is enabled.

3) Marks functions currently in QEMU that exhibit such behavior,
in particular:
- The function in TCG that calls pre-compiled TBs
- The function in TCI that interprets instructions
- Functions in the plugin infrastructures that jump to callbacks
- Functions in util that directly call a signal handler

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org
Message-Id: <20201204230615.2392-3-dbuono@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/tcg/cpu-exec.c

@@ -26,6 +26,7 @@
 #include "exec/exec-all.h"
 #include "tcg/tcg.h"
 #include "qemu/atomic.h"
+#include "qemu/compiler.h"
 #include "sysemu/qtest.h"
 #include "qemu/timer.h"
 #include "qemu/rcu.h"
@@ -144,6 +145,16 @@ static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
 #endif /* CONFIG USER ONLY */
 
 /* Execute a TB, and fix up the CPU state afterwards if necessary */
+/*
+ * Disable CFI checks.
+ * TCG creates binary blobs at runtime, with the transformed code.
+ * A TB is a blob of binary code, created at runtime and called with an
+ * indirect function call. Since such function did not exist at compile time,
+ * the CFI runtime has no way to verify its signature and would fail.
+ * TCG is not considered a security-sensitive part of QEMU so this does not
+ * affect the impact of CFI in environment with high security requirements
+ */
+QEMU_DISABLE_CFI
 static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 {
     CPUArchState *env = cpu->env_ptr;