Add missing signedness check in Keymaster bufferandroid-12.1.0_r9android-12.1.0_r8android-12.1.0_r7android-12.1.0_r22android-12.1.0_r21android-12.1.0_r20android-12.1.0_r19android-12.1.0_r11android-12.1.0_r10android12L-devandroid12-qpr3-s7-releaseandroid12-qpr3-s6-releaseandroid12-qpr3-s5-releaseandroid12-qpr3-s4-releaseandroid12-qpr3-s3-releaseandroid12-qpr3-s2-releaseandroid12-qpr3-s1-releaseandroid12-qpr3-release

Add a check in the Serializable Buffer implementation of Keymaster for
the signedness of the input parameter to advance_read and advance_write.
Both methods take a distance of type int, and add it to the buffer
position regardless of whether it's positive or negative.

This leads to violation of buffer state invariants (specifically
read_position_) and (ultimately) to reading from an invalid
memory region.

In this change:
* advance_read is removed as it's not used.
* advance_write is moved out of the header file.
* Guards against negative distance values and wrapping are added.
* A method for validating buffer state is added and used in reserve()

Ignore-AOSP-First: Security fix
Bug: 173567719
Test: Run libkeymaster_fuzz_buffer on clusterfuzz-testcase-minimized-libkeymaster_fuzz_buffer-5372592199434240
Merged-In: I15330a2f23c3461e23daad450af33e3f92e6730c
Change-Id: I15330a2f23c3461e23daad450af33e3f92e6730c
(cherry picked from commit 48edbcdb981c980b27f4826563b4ca46754df885)

3 files changed, 25 insertions, 18 deletions

diff --git a/android_keymaster/serializable.cpp b/android_keymaster/serializable.cpp
index b1f1e31..fe0d742 100644
--- a/android_keymaster/serializable.cpp
+++ b/android_keymaster/serializable.cpp
@@ -74,6 +74,10 @@ bool copy_size_and_data_from_buf(const uint8_t** buf_ptr, const uint8_t* end, si
 
 bool Buffer::reserve(size_t size) {
     if (available_write() < size) {
+        if (!valid_buffer_state()) {
+            return false;
+        }
+
         size_t new_size = buffer_size_ + size - available_write();
         uint8_t* new_buffer = new (std::nothrow) uint8_t[new_size];
         if (!new_buffer) return false;
@@ -121,6 +125,10 @@ size_t Buffer::available_read() const {
     return write_position_ - read_position_;
 }
 
+bool Buffer::valid_buffer_state() const {
+    return (buffer_size_ >= write_position_) && (write_position_ >= read_position_);
+}
+
 bool Buffer::write(const uint8_t* src, size_t write_length) {
     if (available_write() < write_length) return false;
     memcpy(buffer_.get() + write_position_, src, write_length);
@@ -135,6 +143,21 @@ bool Buffer::read(uint8_t* dest, size_t read_length) {
     return true;
 }
 
+bool Buffer::advance_write(int distance) {
+    if (distance < 0) {
+        return false;
+    }
+
+    const size_t validated_distance = static_cast<size_t>(distance);
+    const size_t new_write_position = write_position_ + validated_distance;
+
+    if (new_write_position <= buffer_size_ && new_write_position >= write_position_) {
+        write_position_ = new_write_position;
+        return true;
+    }
+    return false;
+}
+
 size_t Buffer::SerializedSize() const {
     return sizeof(uint32_t) + available_read();
 }
diff --git a/include/keymaster/serializable.h b/include/keymaster/serializable.h
index fdc97f1..4532485 100644
--- a/include/keymaster/serializable.h
+++ b/include/keymaster/serializable.h
@@ -238,26 +238,13 @@ class Buffer : public Serializable {
     size_t available_write() const;
     size_t available_read() const;
     size_t buffer_size() const { return buffer_size_; }
+    bool valid_buffer_state() const;
 
     bool write(const uint8_t* src, size_t write_length);
     bool read(uint8_t* dest, size_t read_length);
     const uint8_t* peek_read() const { return buffer_.get() + read_position_; }
-    bool advance_read(int distance) {
-        if (static_cast<size_t>(read_position_ + distance) <= write_position_) {
-            read_position_ += distance;
-            return true;
-        }
-        return false;
-    }
     uint8_t* peek_write() { return buffer_.get() + write_position_; }
-    bool advance_write(int distance) {
-        if (static_cast<size_t>(write_position_ + distance) <= buffer_size_) {
-            write_position_ += distance;
-            return true;
-        }
-        return false;
-    }
-
+    bool advance_write(int distance);
     size_t SerializedSize() const;
     uint8_t* Serialize(uint8_t* buf, const uint8_t* end) const;
     bool Deserialize(const uint8_t** buf_ptr, const uint8_t* end);
diff --git a/tests/fuzzers/buffer_fuzz.cpp b/tests/fuzzers/buffer_fuzz.cpp
index 0b02b3f..e0928cd 100644
--- a/tests/fuzzers/buffer_fuzz.cpp
+++ b/tests/fuzzers/buffer_fuzz.cpp
@@ -37,9 +37,6 @@ std::vector<std::function<void(keymaster::Buffer*, FuzzedDataProvider*)>> operat
         buf->reserve(fdp->ConsumeIntegralInRange<int>(kMinBufferSize, kMaxBufferSize));
     },
     [](keymaster::Buffer* buf, FuzzedDataProvider* fdp) -> void {
-        buf->advance_read(fdp->ConsumeIntegral<int>());
-    },
-    [](keymaster::Buffer* buf, FuzzedDataProvider* fdp) -> void {
         buf->advance_write(fdp->ConsumeIntegral<int>());
     },
     [](keymaster::Buffer* buf, FuzzedDataProvider* fdp) -> void {