From 408f3d2cd64cae6b2f76a940c76236e9841c095f Mon Sep 17 00:00:00 2001
From: Dean Deng <deandeng@google.com>
Date: Thu, 18 Jun 2020 22:00:56 -0700
Subject: Fix vfs2 tmpfs link permission checks.

Updates #2923.

PiperOrigin-RevId: 317246916
---
 pkg/sentry/fsimpl/tmpfs/filesystem.go | 16 ++++++++++------
 pkg/sentry/vfs/permissions.go         | 31 +++++++++++++++++++++++++++++++
 test/syscalls/BUILD                   |  1 +
 test/syscalls/linux/link.cc           | 15 +++++++++++++--
 test/util/test_util.h                 |  1 +
 5 files changed, 56 insertions(+), 8 deletions(-)

diff --git a/pkg/sentry/fsimpl/tmpfs/filesystem.go b/pkg/sentry/fsimpl/tmpfs/filesystem.go
index 72399b321..ac359cf7b 100644
--- a/pkg/sentry/fsimpl/tmpfs/filesystem.go
+++ b/pkg/sentry/fsimpl/tmpfs/filesystem.go
@@ -237,18 +237,22 @@ func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 			return syserror.EXDEV
 		}
 		d := vd.Dentry().Impl().(*dentry)
-		if d.inode.isDir() {
+		i := d.inode
+		if i.isDir() {
 			return syserror.EPERM
 		}
-		if d.inode.nlink == 0 {
+		if err := vfs.MayLink(auth.CredentialsFromContext(ctx), linux.FileMode(atomic.LoadUint32(&i.mode)), auth.KUID(atomic.LoadUint32(&i.uid)), auth.KGID(atomic.LoadUint32(&i.gid))); err != nil {
+			return err
+		}
+		if i.nlink == 0 {
 			return syserror.ENOENT
 		}
-		if d.inode.nlink == maxLinks {
+		if i.nlink == maxLinks {
 			return syserror.EMLINK
 		}
-		d.inode.incLinksLocked()
-		d.inode.watches.Notify("", linux.IN_ATTRIB, 0, vfs.InodeEvent)
-		parentDir.insertChildLocked(fs.newDentry(d.inode), name)
+		i.incLinksLocked()
+		i.watches.Notify("", linux.IN_ATTRIB, 0, vfs.InodeEvent)
+		parentDir.insertChildLocked(fs.newDentry(i), name)
 		return nil
 	})
 }
diff --git a/pkg/sentry/vfs/permissions.go b/pkg/sentry/vfs/permissions.go
index f9647f90e..afe2be8d7 100644
--- a/pkg/sentry/vfs/permissions.go
+++ b/pkg/sentry/vfs/permissions.go
@@ -94,6 +94,37 @@ func GenericCheckPermissions(creds *auth.Credentials, ats AccessTypes, mode linu
 	return syserror.EACCES
 }
 
+// MayLink determines whether creating a hard link to a file with the given
+// mode, kuid, and kgid is permitted.
+//
+// This corresponds to Linux's fs/namei.c:may_linkat.
+func MayLink(creds *auth.Credentials, mode linux.FileMode, kuid auth.KUID, kgid auth.KGID) error {
+	// Source inode owner can hardlink all they like; otherwise, it must be a
+	// safe source.
+	if CanActAsOwner(creds, kuid) {
+		return nil
+	}
+
+	// Only regular files can be hard linked.
+	if mode.FileType() != linux.S_IFREG {
+		return syserror.EPERM
+	}
+
+	// Setuid files should not get pinned to the filesystem.
+	if mode&linux.S_ISUID != 0 {
+		return syserror.EPERM
+	}
+
+	// Executable setgid files should not get pinned to the filesystem, but we
+	// don't support S_IXGRP anyway.
+
+	// Hardlinking to unreadable or unwritable sources is dangerous.
+	if err := GenericCheckPermissions(creds, MayRead|MayWrite, mode, kuid, kgid); err != nil {
+		return syserror.EPERM
+	}
+	return nil
+}
+
 // AccessTypesForOpenFlags returns the access types required to open a file
 // with the given OpenOptions.Flags. Note that this is NOT the same thing as
 // the set of accesses permitted for the opened file:
diff --git a/test/syscalls/BUILD b/test/syscalls/BUILD
index 1638a11c7..65a6a7f37 100644
--- a/test/syscalls/BUILD
+++ b/test/syscalls/BUILD
@@ -305,6 +305,7 @@ syscall_test(
     add_overlay = True,
     test = "//test/syscalls/linux:link_test",
     use_tmpfs = True,  # gofer needs CAP_DAC_READ_SEARCH to use AT_EMPTY_PATH with linkat(2)
+    vfs2 = "True",
 )
 
 syscall_test(
diff --git a/test/syscalls/linux/link.cc b/test/syscalls/linux/link.cc
index e74fa2ed5..544681168 100644
--- a/test/syscalls/linux/link.cc
+++ b/test/syscalls/linux/link.cc
@@ -79,8 +79,13 @@ TEST(LinkTest, PermissionDenied) {
 
   // Make the file "unsafe" to link by making it only readable, but not
   // writable.
-  const auto oldfile =
+  const auto unwriteable_file =
       ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFileMode(0400));
+  const std::string special_path = NewTempAbsPath();
+  ASSERT_THAT(mkfifo(special_path.c_str(), 0666), SyscallSucceeds());
+  const auto setuid_file =
+      ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateFileMode(0666 | S_ISUID));
+
   const std::string newname = NewTempAbsPath();
 
   // Do setuid in a separate thread so that after finishing this test, the
@@ -97,8 +102,14 @@ TEST(LinkTest, PermissionDenied) {
     EXPECT_THAT(syscall(SYS_setuid, absl::GetFlag(FLAGS_scratch_uid)),
                 SyscallSucceeds());
 
-    EXPECT_THAT(link(oldfile.path().c_str(), newname.c_str()),
+    EXPECT_THAT(link(unwriteable_file.path().c_str(), newname.c_str()),
+                SyscallFailsWithErrno(EPERM));
+    EXPECT_THAT(link(special_path.c_str(), newname.c_str()),
                 SyscallFailsWithErrno(EPERM));
+    if (!IsRunningWithVFS1()) {
+      EXPECT_THAT(link(setuid_file.path().c_str(), newname.c_str()),
+                  SyscallFailsWithErrno(EPERM));
+    }
   });
 }
 
diff --git a/test/util/test_util.h b/test/util/test_util.h
index 8e3245b27..e635827e6 100644
--- a/test/util/test_util.h
+++ b/test/util/test_util.h
@@ -220,6 +220,7 @@ constexpr char kKVM[] = "kvm";
 bool IsRunningOnGvisor();
 const std::string GvisorPlatform();
 bool IsRunningWithHostinet();
+// TODO(gvisor.dev/issue/1624): Delete once VFS1 is gone.
 bool IsRunningWithVFS1();
 
 #ifdef __linux__
-- 
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