[PATCH 1/2] linux-user: add a plugin API to filter syscalls

[Ziyang Zhang <functioner@sjtu.edu.cn>]

This commit adds a syscall filter API to the TCG plugin API set.
Plugins can register a filter callback to QEMU to decide whether
to intercept a syscall, process it and bypass the QEMU syscall
handler.

Signed-off-by: Ziyang Zhang <functioner@sjtu.edu.cn>
Co-authored-by: Mingyuan Xia <xiamy@ultrarisc.com>
---
 include/qemu/plugin-event.h  |  1 +
 include/qemu/plugin.h        | 29 +++++++++++++++++++++--------
 include/qemu/qemu-plugin.h   | 24 ++++++++++++++++++++++++
 include/user/syscall-trace.h | 17 +++++++++++++++++
 linux-user/syscall.c         |  7 +++++--
 plugins/api.c                |  7 +++++++
 plugins/core.c               | 36 ++++++++++++++++++++++++++++++++++++
 7 files changed, 111 insertions(+), 10 deletions(-)

diff --git a/include/qemu/plugin-event.h b/include/qemu/plugin-event.h
index 7056d8427b..bbb1c2b91f 100644
--- a/include/qemu/plugin-event.h
+++ b/include/qemu/plugin-event.h
@@ -20,6 +20,7 @@ enum qemu_plugin_event {
     QEMU_PLUGIN_EV_VCPU_SYSCALL_RET,
     QEMU_PLUGIN_EV_FLUSH,
     QEMU_PLUGIN_EV_ATEXIT,
+    QEMU_PLUGIN_EV_VCPU_SYSCALL_FILTER,
     QEMU_PLUGIN_EV_MAX, /* total number of plugin events we support */
 };
 
diff --git a/include/qemu/plugin.h b/include/qemu/plugin.h
index f355c7cb8a..9f90a233e7 100644
--- a/include/qemu/plugin.h
+++ b/include/qemu/plugin.h
@@ -55,14 +55,15 @@ void qemu_plugin_opt_parse(const char *optstr, QemuPluginList *head);
 int qemu_plugin_load_list(QemuPluginList *head, Error **errp);
 
 union qemu_plugin_cb_sig {
-    qemu_plugin_simple_cb_t          simple;
-    qemu_plugin_udata_cb_t           udata;
-    qemu_plugin_vcpu_simple_cb_t     vcpu_simple;
-    qemu_plugin_vcpu_udata_cb_t      vcpu_udata;
-    qemu_plugin_vcpu_tb_trans_cb_t   vcpu_tb_trans;
-    qemu_plugin_vcpu_mem_cb_t        vcpu_mem;
-    qemu_plugin_vcpu_syscall_cb_t    vcpu_syscall;
-    qemu_plugin_vcpu_syscall_ret_cb_t vcpu_syscall_ret;
+    qemu_plugin_simple_cb_t              simple;
+    qemu_plugin_udata_cb_t               udata;
+    qemu_plugin_vcpu_simple_cb_t         vcpu_simple;
+    qemu_plugin_vcpu_udata_cb_t          vcpu_udata;
+    qemu_plugin_vcpu_tb_trans_cb_t       vcpu_tb_trans;
+    qemu_plugin_vcpu_mem_cb_t            vcpu_mem;
+    qemu_plugin_vcpu_syscall_cb_t        vcpu_syscall;
+    qemu_plugin_vcpu_syscall_ret_cb_t    vcpu_syscall_ret;
+    qemu_plugin_vcpu_syscall_filter_cb_t vcpu_syscall_filter;
     void *generic;
 };
 
@@ -165,6 +166,11 @@ qemu_plugin_vcpu_syscall(CPUState *cpu, int64_t num, uint64_t a1,
                          uint64_t a2, uint64_t a3, uint64_t a4, uint64_t a5,
                          uint64_t a6, uint64_t a7, uint64_t a8);
 void qemu_plugin_vcpu_syscall_ret(CPUState *cpu, int64_t num, int64_t ret);
+bool
+qemu_plugin_vcpu_syscall_filter(CPUState *cpu, int64_t num, uint64_t a1,
+                                uint64_t a2, uint64_t a3, uint64_t a4,
+                                uint64_t a5, uint64_t a6, uint64_t a7,
+                                uint64_t a8, uint64_t *ret);
 
 void qemu_plugin_vcpu_mem_cb(CPUState *cpu, uint64_t vaddr,
                              uint64_t value_low,
@@ -267,6 +273,13 @@ static inline
 void qemu_plugin_vcpu_syscall_ret(CPUState *cpu, int64_t num, int64_t ret)
 { }
 
+static inline bool
+qemu_plugin_vcpu_syscall_filter(CPUState *cpu, int64_t num, uint64_t a1,
+                                uint64_t a2, uint64_t a3, uint64_t a4,
+                                uint64_t a5, uint64_t a6, uint64_t a7,
+                                uint64_t a8, uint64_t *ret)
+{ }
+
 static inline void qemu_plugin_vcpu_mem_cb(CPUState *cpu, uint64_t vaddr,
                                            uint64_t value_low,
                                            uint64_t value_high,
diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
index c450106af1..cdd0b2c4da 100644
--- a/include/qemu/qemu-plugin.h
+++ b/include/qemu/qemu-plugin.h
@@ -738,6 +738,25 @@ typedef void
                                  uint64_t a3, uint64_t a4, uint64_t a5,
                                  uint64_t a6, uint64_t a7, uint64_t a8);
 
+/*
+ * typedef qemu_plugin_vcpu_syscall_filter_cb_t - vCPU syscall filter callback
+ * function type
+ * @vcpu_index: the executing vCPU
+ * @num: the syscall number
+ * @a1-a8: the syscall arguments
+ * @ret: the address of the syscall return value, set this if filtered
+ *
+ * Returns: true if you want to filter this syscall (i.e. stop it being
+ * handled further), otherwise return false.
+ */
+typedef bool
+(*qemu_plugin_vcpu_syscall_filter_cb_t)(qemu_plugin_id_t id,
+                                        unsigned int vcpu_index,
+                                        int64_t num, uint64_t a1, uint64_t a2,
+                                        uint64_t a3, uint64_t a4, uint64_t a5,
+                                        uint64_t a6, uint64_t a7, uint64_t a8,
+                                        uint64_t *ret);
+
 QEMU_PLUGIN_API
 void qemu_plugin_register_vcpu_syscall_cb(qemu_plugin_id_t id,
                                           qemu_plugin_vcpu_syscall_cb_t cb);
@@ -751,6 +770,11 @@ void
 qemu_plugin_register_vcpu_syscall_ret_cb(qemu_plugin_id_t id,
                                          qemu_plugin_vcpu_syscall_ret_cb_t cb);
 
+QEMU_PLUGIN_API
+void
+qemu_plugin_register_vcpu_syscall_filter_cb(qemu_plugin_id_t id,
+                                            qemu_plugin_vcpu_syscall_filter_cb_t cb);
+
 
 /**
  * qemu_plugin_insn_disas() - return disassembly string for instruction
diff --git a/include/user/syscall-trace.h b/include/user/syscall-trace.h
index 9bd7ca19c8..61cdbd7583 100644
--- a/include/user/syscall-trace.h
+++ b/include/user/syscall-trace.h
@@ -39,5 +39,22 @@ static inline void record_syscall_return(CPUState *cpu, int num, abi_long ret)
     gdb_syscall_return(cpu, num);
 }
 
+static bool send_through_syscall_filters(CPUState *cpu, int num,
+                                         abi_long arg1, abi_long arg2,
+                                         abi_long arg3, abi_long arg4,
+                                         abi_long arg5, abi_long arg6,
+                                         abi_long arg7, abi_long arg8,
+                                         abi_long *ret)
+{
+    uint64_t sysret64 = 0;
+    bool filtered = qemu_plugin_vcpu_syscall_filter(cpu, num, arg1, arg2,
+                                                    arg3, arg4, arg5, arg6,
+                                                    arg7, arg8, &sysret64);
+    if (filtered) {
+        *ret = sysret64;
+    }
+    return filtered;
+}
+
 
 #endif /* SYSCALL_TRACE_H */
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index d78b2029fa..1fb99dcf0c 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -14084,8 +14084,11 @@ abi_long do_syscall(CPUArchState *cpu_env, int num, abi_long arg1,
         print_syscall(cpu_env, num, arg1, arg2, arg3, arg4, arg5, arg6);
     }
 
-    ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
-                      arg5, arg6, arg7, arg8);
+    if (!send_through_syscall_filters(cpu, num, arg1, arg2, arg3, arg4, arg5,
+                                      arg6, arg7, arg8, &ret)) {
+        ret = do_syscall1(cpu_env, num, arg1, arg2, arg3, arg4,
+                          arg5, arg6, arg7, arg8);
+    }
 
     if (unlikely(qemu_loglevel_mask(LOG_STRACE))) {
         print_syscall_ret(cpu_env, num, ret, arg1, arg2,
diff --git a/plugins/api.c b/plugins/api.c
index eac04cc1f6..478d0c8889 100644
--- a/plugins/api.c
+++ b/plugins/api.c
@@ -208,6 +208,13 @@ qemu_plugin_register_vcpu_syscall_ret_cb(qemu_plugin_id_t id,
     plugin_register_cb(id, QEMU_PLUGIN_EV_VCPU_SYSCALL_RET, cb);
 }
 
+void
+qemu_plugin_register_vcpu_syscall_filter_cb(qemu_plugin_id_t id,
+                                            qemu_plugin_vcpu_syscall_filter_cb_t cb)
+{
+    plugin_register_cb(id, QEMU_PLUGIN_EV_VCPU_SYSCALL_FILTER, cb);
+}
+
 /*
  * Plugin Queries
  *
diff --git a/plugins/core.c b/plugins/core.c
index ead09fd2f1..8cd773cbca 100644
--- a/plugins/core.c
+++ b/plugins/core.c
@@ -538,6 +538,42 @@ void qemu_plugin_vcpu_syscall_ret(CPUState *cpu, int64_t num, int64_t ret)
     }
 }
 
+/*
+ * Disable CFI checks.
+ * The callback function has been loaded from an external library so we do not
+ * have type information
+ */
+QEMU_DISABLE_CFI
+bool
+qemu_plugin_vcpu_syscall_filter(CPUState *cpu, int64_t num, uint64_t a1,
+                                uint64_t a2, uint64_t a3, uint64_t a4,
+                                uint64_t a5, uint64_t a6, uint64_t a7,
+                                uint64_t a8, uint64_t *ret)
+{
+    struct qemu_plugin_cb *cb, *next;
+    enum qemu_plugin_event ev = QEMU_PLUGIN_EV_VCPU_SYSCALL_FILTER;
+
+    if (!test_bit(ev, cpu->plugin_state->event_mask)) {
+        return false;
+    }
+
+    bool filtered = false;
+    QLIST_FOREACH_SAFE_RCU(cb, &plugin.cb_lists[ev], entry, next) {
+        qemu_plugin_vcpu_syscall_filter_cb_t func = cb->f.vcpu_syscall_filter;
+
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
+        if (func(cb->ctx->id, cpu->cpu_index, num, a1, a2, a3, a4,
+                 a5, a6, a7, a8, ret)) {
+            filtered = true;
+        }
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
+
+        if (filtered)
+            break;
+    }
+    return filtered;
+}
+
 void qemu_plugin_vcpu_idle_cb(CPUState *cpu)
 {
     /* idle and resume cb may be called before init, ignore in this case */
-- 
2.34.1