fuse: check write() length correctness

This is consistent with Linux's fs/fuse/dev.c:fuse_do_dev_write().

PiperOrigin-RevId: 660096450

pkg/sentry/fsimpl/fuse/dev.go

@@ -225,12 +225,18 @@ func (fd *DeviceFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.
 		return 0, linuxerr.EPERM
 	}
 
+	var hdr linux.FUSEHeaderOut
+	if src.NumBytes() < int64(hdr.SizeBytes()) {
+		return 0, linuxerr.EINVAL
+	}
 	n, err := src.CopyIn(ctx, fd.writeBuf[:])
 	if err != nil {
 		return 0, err
 	}
-	var hdr linux.FUSEHeaderOut
 	hdr.UnmarshalBytes(fd.writeBuf[:])
+	if src.NumBytes() != int64(hdr.Len) {
+		return 0, linuxerr.EINVAL
+	}
 
 	fut, ok := fd.completions[hdr.Unique]
 	if !ok {

test/syscalls/linux/fuse.cc

@@ -14,6 +14,7 @@
 
 #include <fcntl.h>
 #include <linux/capability.h>
+#include <linux/fuse.h>
 #include <stdio.h>
 #include <sys/mount.h>
 #include <sys/stat.h>
@@ -32,10 +33,13 @@
 #include "test/util/file_descriptor.h"
 #include "test/util/fs_util.h"
 #include "test/util/linux_capability_util.h"
+#include "test/util/mount_util.h"
 #include "test/util/posix_error.h"
 #include "test/util/temp_path.h"
 #include "test/util/test_util.h"
 
+using ::testing::Ge;
+
 namespace gvisor {
 namespace testing {
 
@@ -49,21 +53,19 @@ TEST(FuseTest, RejectBadInit) {
   auto mount_point = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
   auto mount_opts =
       absl::StrFormat("fd=%d,user_id=0,group_id=0,rootmode=40000", fd.get());
+  auto mount = ASSERT_NO_ERRNO_AND_VALUE(
+      Mount("fuse", mount_point.path(), "fuse", MS_NODEV | MS_NOSUID,
+            mount_opts, 0 /* umountflags */));
 
-  EXPECT_THAT(mount("fuse", mount_point.path().c_str(), "fuse",
-                    MS_NODEV | MS_NOSUID, mount_opts.c_str()),
-              SyscallSucceeds());
-  mount_point.release();
+  // Read the init request so that we have the correct unique ID.
+  alignas(fuse_in_header) char req_buf[FUSE_MIN_READ_BUFFER];
+  ASSERT_THAT(read(fd.get(), req_buf, sizeof(req_buf)),
+              SyscallSucceedsWithValue(Ge(sizeof(fuse_in_header))));
 
-  struct response {
-    uint32_t len;
-    int32_t err;
-    uint64_t uid;
-  } resp;
-  // min value for length is 24 + sizeof(response)
-  resp.len = 24 + sizeof(resp) - 1;
-  resp.err = 0;
-  resp.uid = 2;
+  fuse_out_header resp;
+  resp.len = sizeof(resp) - 1;
+  resp.error = 0;
+  resp.unique = reinterpret_cast<fuse_in_header*>(req_buf)->unique;
 
   ASSERT_THAT(write(fd.get(), reinterpret_cast<char*>(&resp), sizeof(resp)),
               SyscallFailsWithErrno(EINVAL));