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Forbid removing ro/noexec/nodev/nosuid flags on locked mounts. #13907

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Forbid removing ro/noexec/nodev/nosuid flags on locked mounts. #13907

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56 changes: 51 additions & 5 deletions pkg/sentry/vfs/mount.go
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,24 @@ const (
cgroup2FsName = "cgroup2"
)

// mountLockFlags records which of a Mount's flags a remount may not clear. It
// is analogous to the MNT_LOCK_* flags in Linux.
//
// +stateify savable
type mountLockFlags struct {
// readOnly is analogous to MNT_LOCK_READONLY.
readOnly bool

// noExec is analogous to MNT_LOCK_NOEXEC.
noExec bool

// noDev is analogous to MNT_LOCK_NODEV.
noDev bool

// noSUID is analogous to MNT_LOCK_NOSUID.
noSUID bool
}

// A Mount is a replacement of a Dentry (Mount.key.point) from one Filesystem
// (Mount.key.parent.fs) with a Dentry (Mount.root) from another Filesystem
// (Mount.fs), which applies to path resolution in the context of a particular
Expand Down Expand Up @@ -123,6 +141,10 @@ type Mount struct {
// namespace. It is analogous to MNT_LOCKED in Linux.
locked bool

// lockedFlags contains the flags that RemountAt may not clear. lockedFlags
// is protected by VirtualFilesystem.mountMu.
lockedFlags mountLockFlags

// The lower 63 bits of writers is the number of calls to
// Mount.CheckBeginWrite() that have not yet been paired with a call to
// Mount.EndWrite(). The MSB of writers is set if MS_RDONLY is in effect.
Expand Down Expand Up @@ -160,14 +182,31 @@ func (mnt *Mount) Options() MountOptions {
}
}

// canChangeLockedFlags returns false if applying opts to mnt would clear any
// of mnt's locked flags.
//
// Preconditions:
// - vfs.mountMu must be locked.
func (mnt *Mount) canChangeLockedFlags(opts *MountOptions) bool {
locked := mnt.lockedFlags
switch {
case locked.readOnly && !opts.ReadOnly:
return false
case locked.noExec && !opts.Flags.NoExec:
return false
case locked.noDev && !opts.Flags.NoDev:
return false
case locked.noSUID && !opts.Flags.NoSUID:
return false
}
return true
}

// setMountOptions sets mnt's options to the given opts.
//
// Preconditions:
// - vfs.mountMu must be locked.
func (mnt *Mount) setMountOptions(opts *MountOptions) error {
if opts == nil {
return linuxerr.EINVAL
}
if err := mnt.setReadOnlyLocked(opts.ReadOnly); err != nil {
return err
}
Expand Down Expand Up @@ -370,8 +409,11 @@ func (vfs *VirtualFilesystem) attachTreeLocked(ctx context.Context, mnt *Mount,
// +checklocks:vfs.mountMu
func (vfs *VirtualFilesystem) lockMountTree(mnt *Mount) {
for _, m := range mnt.submountsLocked() {
// TODO(b/315839347): Add equivalents for MNT_LOCK_ATIME,
// MNT_LOCK_READONLY, etc.
// TODO(b/315839347): Add equivalents for MNT_LOCK_ATIME.
m.lockedFlags.readOnly = m.lockedFlags.readOnly || m.ReadOnlyLocked()
m.lockedFlags.noExec = m.lockedFlags.noExec || m.flags.NoExec
m.lockedFlags.noDev = m.lockedFlags.noDev || m.flags.NoDev
m.lockedFlags.noSUID = m.lockedFlags.noSUID || m.flags.NoSUID
m.locked = true
}
}
Expand Down Expand Up @@ -617,6 +659,7 @@ func (vfs *VirtualFilesystem) cloneMount(mnt *Mount, root *Dentry, mopts *MountO
}
clone.isShared = mnt.isShared
clone.locked = mnt.locked
clone.lockedFlags = mnt.lockedFlags
if cloneType&makeFollowerClone != 0 || (cloneType&sharedToFollowerClone != 0 && mnt.isShared) {
mnt.followerList.PushFront(clone)
clone.leader = mnt
Expand Down Expand Up @@ -788,6 +831,9 @@ func (vfs *VirtualFilesystem) RemountAt(ctx context.Context, creds *auth.Credent
if !vfs.validInMountNS(ctx, mnt) {
return linuxerr.EINVAL
}
if !mnt.canChangeLockedFlags(opts) {
return linuxerr.EPERM
}
if err := mnt.setMountOptions(opts); err != nil {
return err
}
Expand Down
79 changes: 79 additions & 0 deletions test/syscalls/linux/mount.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -2608,6 +2608,85 @@ TEST(MountTest, ChangeMountFlags) {
ASSERT_EQ(st.f_flags & flag, 0);
}

constexpr int kLockableFlags = MS_RDONLY | MS_NOEXEC | MS_NODEV | MS_NOSUID;

// Sets kLockableFlags on the mount at `path`.
void SetLockableFlags(const std::string& path) {
ASSERT_THAT(mount("", path.c_str(), nullptr,
MS_REMOUNT | MS_BIND | kLockableFlags, ""),
SyscallSucceeds());
}

TEST(MountTest, ChangeMountFlagsOfLockedMount) {
SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));

const TempPath dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
const Cleanup mount_cleanup =
ASSERT_NO_ERRNO_AND_VALUE(Mount("", dir.path(), kTmpfs, 0, "", 0));
ASSERT_NO_FATAL_FAILURE(SetLockableFlags(dir.path()));
const std::string path = dir.path();
const std::string file_path = JoinPath(path, "foo");

const std::function<void()> child = [&] {
// The mount was copied into a new user namespace, so none of its flags may
// be cleared.
for (const int flag : {MS_RDONLY, MS_NOEXEC, MS_NODEV, MS_NOSUID}) {
TEST_CHECK_ERRNO(mount("", path.c_str(), nullptr,
MS_REMOUNT | MS_BIND | (kLockableFlags & ~flag),
""),
EPERM);
}
TEST_CHECK_ERRNO(mount("", path.c_str(), nullptr, MS_REMOUNT | MS_BIND, ""),
EPERM);
// The mount is still read-only.
TEST_CHECK_ERRNO(open(file_path.c_str(), O_CREAT | O_RDWR, 0666), EROFS);
// A remount that keeps all of the locked flags set is allowed.
TEST_CHECK_SUCCESS(mount("", path.c_str(), nullptr,
MS_REMOUNT | MS_BIND | kLockableFlags, ""));
};
EXPECT_THAT(InForkedUserMountNamespace([] {}, child),
IsPosixErrorOkAndHolds(0));

// The flags are not locked in the namespace that owns the mount.
EXPECT_THAT(mount("", path.c_str(), nullptr, MS_REMOUNT | MS_BIND, ""),
SyscallSucceeds());
struct statfs st;
ASSERT_THAT(statfs(path.c_str(), &st), SyscallSucceeds());
EXPECT_EQ(st.f_flags & kLockableFlags, 0);
}

TEST(MountTest, LockedMountFlagsAreInheritedByBind) {
SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));

const TempPath dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
const Cleanup mount_cleanup =
ASSERT_NO_ERRNO_AND_VALUE(Mount("", dir.path(), kTmpfs, 0, "", 0));
ASSERT_NO_FATAL_FAILURE(SetLockableFlags(dir.path()));
const TempPath target = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
const std::string path = dir.path();
const std::string target_path = target.path();

const std::function<void()> child = [&] {
// Binding the locked mount elsewhere must not result in a mount for which
// flags can be cleared.
TEST_CHECK_SUCCESS(
mount(path.c_str(), target_path.c_str(), "", MS_BIND, ""));
for (const int flag : {MS_RDONLY, MS_NOEXEC, MS_NODEV, MS_NOSUID}) {
TEST_CHECK_ERRNO(mount("", target_path.c_str(), nullptr,
MS_REMOUNT | MS_BIND | (kLockableFlags & ~flag),
""),
EPERM);
}
TEST_CHECK_SUCCESS(mount("", target_path.c_str(), nullptr,
MS_REMOUNT | MS_BIND | kLockableFlags, ""));
// The bind mount itself was created in this namespace, so unlike the mount
// it was cloned from, it can be unmounted.
TEST_CHECK_SUCCESS(umount2(target_path.c_str(), MNT_DETACH));
};
EXPECT_THAT(InForkedUserMountNamespace([] {}, child),
IsPosixErrorOkAndHolds(0));
}

TEST(MountTest, RemountUnmounted) {
SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));

Expand Down
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