Merge "capabilities: Check ambient caps, last valid runtime cap."

init/capabilities.cpp

@@ -25,8 +25,7 @@
 
 #define CAP_MAP_ENTRY(cap) { #cap, CAP_##cap }
 
-namespace {
-const std::map<std::string, int> cap_map = {
+static const std::map<std::string, int> cap_map = {
     CAP_MAP_ENTRY(CHOWN),
     CAP_MAP_ENTRY(DAC_OVERRIDE),
     CAP_MAP_ENTRY(DAC_READ_SEARCH),
@@ -69,9 +68,30 @@ const std::map<std::string, int> cap_map = {
 
 static_assert(CAP_LAST_CAP == CAP_AUDIT_READ, "CAP_LAST_CAP is not CAP_AUDIT_READ");
 
-bool DropBoundingSet(const CapSet& to_keep) {
-    for (size_t cap = 0; cap < to_keep.size(); ++cap) {
-        if (to_keep.test(cap)) {
+static bool ComputeCapAmbientSupported() {
+    return prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_CHOWN, 0, 0) >= 0;
+}
+
+static unsigned int ComputeLastValidCap() {
+    // Android does not support kernels < 3.8. 'CAP_WAKE_ALARM' has been present since 3.0, see
+    // http://lxr.free-electrons.com/source/include/linux/capability.h?v=3.0#L360.
+    unsigned int last_valid_cap = CAP_WAKE_ALARM;
+    for (; prctl(PR_CAPBSET_READ, last_valid_cap, 0, 0, 0) >= 0; ++last_valid_cap);
+
+    // |last_valid_cap| will be the first failing value.
+    return last_valid_cap - 1;
+}
+
+static bool DropBoundingSet(const CapSet& to_keep) {
+    unsigned int last_valid_cap = GetLastValidCap();
+    // When dropping the bounding set, attempt to drop capabilities reported at
+    // run-time, not at compile-time.
+    // If the run-time kernel is older than the compile-time headers, this
+    // avoids dropping an invalid capability. If the run-time kernel is newer
+    // than the headers, this guarantees all capabilities (even those unknown at
+    // compile time) will be dropped.
+    for (size_t cap = 0; cap <= last_valid_cap; ++cap) {
+        if (cap < to_keep.size() && to_keep.test(cap)) {
             // No need to drop this capability.
             continue;
         }
@@ -83,14 +103,14 @@ bool DropBoundingSet(const CapSet& to_keep) {
     return true;
 }
 
-bool SetProcCaps(const CapSet& to_keep, bool add_setpcap) {
+static bool SetProcCaps(const CapSet& to_keep, bool add_setpcap) {
     cap_t caps = cap_init();
     auto deleter = [](cap_t* p) { cap_free(*p); };
     std::unique_ptr<cap_t, decltype(deleter)> ptr_caps(&caps, deleter);
 
     cap_clear(caps);
     cap_value_t value[1];
-    for (size_t cap = 0; cap <= to_keep.size(); ++cap) {
+    for (size_t cap = 0; cap < to_keep.size(); ++cap) {
         if (to_keep.test(cap)) {
             value[0] = cap;
             if (cap_set_flag(caps, CAP_INHERITABLE, arraysize(value), value, CAP_SET) != 0 ||
@@ -117,7 +137,7 @@ bool SetProcCaps(const CapSet& to_keep, bool add_setpcap) {
     return true;
 }
 
-bool SetAmbientCaps(const CapSet& to_raise) {
+static bool SetAmbientCaps(const CapSet& to_raise) {
     for (size_t cap = 0; cap < to_raise.size(); ++cap) {
         if (to_raise.test(cap)) {
             if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap, 0, 0) != 0) {
@@ -129,8 +149,6 @@ bool SetAmbientCaps(const CapSet& to_raise) {
     return true;
 }
 
-}  // namespace anonymous
-
 int LookupCap(const std::string& cap_name) {
     auto e = cap_map.find(cap_name);
     if (e != cap_map.end()) {
@@ -140,6 +158,16 @@ int LookupCap(const std::string& cap_name) {
     }
 }
 
+bool CapAmbientSupported() {
+    static bool cap_ambient_supported = ComputeCapAmbientSupported();
+    return cap_ambient_supported;
+}
+
+unsigned int GetLastValidCap() {
+    static unsigned int last_valid_cap = ComputeLastValidCap();
+    return last_valid_cap;
+}
+
 bool SetCapsForExec(const CapSet& to_keep) {
     // Need to keep SETPCAP to drop bounding set below.
     bool add_setpcap = true;

init/capabilities.h

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#ifndef _INIT_CAPABILITIES_H
+#define _INIT_CAPABILITIES_H
+
 #include <linux/capability.h>
 
 #include <bitset>
@@ -20,4 +23,8 @@
 using CapSet = std::bitset<CAP_LAST_CAP + 1>;
 
 int LookupCap(const std::string& cap_name);
+bool CapAmbientSupported();
+unsigned int GetLastValidCap();
 bool SetCapsForExec(const CapSet& to_keep);
+
+#endif  // _INIT_CAPABILITIES_H

init/service.cpp

@@ -312,13 +312,28 @@ void Service::DumpState() const {
 bool Service::ParseCapabilities(const std::vector<std::string>& args, std::string* err) {
     capabilities_ = 0;
 
+    if (!CapAmbientSupported()) {
+        *err = "capabilities requested but the kernel does not support ambient capabilities";
+        return false;
+    }
+
+    unsigned int last_valid_cap = GetLastValidCap();
+    if (last_valid_cap >= capabilities_.size()) {
+        LOG(WARNING) << "last valid run-time capability is larger than CAP_LAST_CAP";
+    }
+
     for (size_t i = 1; i < args.size(); i++) {
         const std::string& arg = args[i];
-        int cap = LookupCap(arg);
-        if (cap == -1) {
+        int res = LookupCap(arg);
+        if (res < 0) {
             *err = StringPrintf("invalid capability '%s'", arg.c_str());
             return false;
         }
+        unsigned int cap = static_cast<unsigned int>(res);  // |res| is >= 0.
+        if (cap > last_valid_cap) {
+            *err = StringPrintf("capability '%s' not supported by the kernel", arg.c_str());
+            return false;
+        }
         capabilities_[cap] = true;
     }
     return true;