Use a tmpfs file for shared anonymous and /dev/zero mmap on VFS2.

This is more consistent with Linux (see comment on MM.NewSharedAnonMappable()).
We don't do the same thing on VFS1 for reasons documented by the updated
comment.

PiperOrigin-RevId: 332514849

pkg/sentry/devices/memdev/BUILD

@@ -18,9 +18,10 @@ go_library(
         "//pkg/rand",
         "//pkg/safemem",
         "//pkg/sentry/fsimpl/devtmpfs",
+        "//pkg/sentry/fsimpl/tmpfs",
+        "//pkg/sentry/kernel",
+        "//pkg/sentry/kernel/auth",
         "//pkg/sentry/memmap",
-        "//pkg/sentry/mm",
-        "//pkg/sentry/pgalloc",
         "//pkg/sentry/vfs",
         "//pkg/syserror",
         "//pkg/usermem",

pkg/sentry/devices/memdev/zero.go

@@ -16,9 +16,10 @@ package memdev
 
 import (
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/memmap"
-	"gvisor.dev/gvisor/pkg/sentry/mm"
-	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
@@ -79,11 +80,22 @@ func (fd *zeroFD) Seek(ctx context.Context, offset int64, whence int32) (int64,
 
 // ConfigureMMap implements vfs.FileDescriptionImpl.ConfigureMMap.
 func (fd *zeroFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) error {
-	m, err := mm.NewSharedAnonMappable(opts.Length, pgalloc.MemoryFileProviderFromContext(ctx))
+	if opts.Private || !opts.MaxPerms.Write {
+		// This mapping will never permit writing to the "underlying file" (in
+		// Linux terms, it isn't VM_SHARED), so implement it as an anonymous
+		// mapping, but back it with fd; this is what Linux does, and is
+		// actually application-visible because the resulting VMA will show up
+		// in /proc/[pid]/maps with fd.vfsfd.VirtualDentry()'s path rather than
+		// "/dev/zero (deleted)".
+		opts.Offset = 0
+		opts.MappingIdentity = &fd.vfsfd
+		opts.MappingIdentity.IncRef()
+		return nil
+	}
+	tmpfsFD, err := tmpfs.NewZeroFile(ctx, auth.CredentialsFromContext(ctx), kernel.KernelFromContext(ctx).ShmMount(), opts.Length)
 	if err != nil {
 		return err
 	}
-	opts.MappingIdentity = m
-	opts.Mappable = m
-	return nil
+	defer tmpfsFD.DecRef(ctx)
+	return tmpfsFD.ConfigureMMap(ctx, opts)
 }

pkg/sentry/fsimpl/tmpfs/filesystem.go

@@ -865,8 +865,16 @@ func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDe
 		}
 		if d.parent == nil {
 			if d.name != "" {
-				// This must be an anonymous memfd file.
+				// This file must have been created by
+				// newUnlinkedRegularFileDescription(). In Linux,
+				// mm/shmem.c:__shmem_file_setup() =>
+				// fs/file_table.c:alloc_file_pseudo() sets the created
+				// dentry's dentry_operations to anon_ops, for which d_dname ==
+				// simple_dname. fs/d_path.c:simple_dname() defines the
+				// dentry's pathname to be its name, prefixed with "/" and
+				// suffixed with " (deleted)".
 				b.PrependComponent("/" + d.name)
+				b.AppendString(" (deleted)")
 				return vfs.PrependPathSyntheticError{}
 			}
 			return vfs.PrependPathAtNonMountRootError{}

pkg/sentry/fsimpl/tmpfs/regular_file.go

@@ -42,6 +42,10 @@ type regularFile struct {
 	// memFile is a platform.File used to allocate pages to this regularFile.
 	memFile *pgalloc.MemoryFile
 
+	// memoryUsageKind is the memory accounting category under which pages backing
+	// this regularFile's contents are accounted.
+	memoryUsageKind usage.MemoryKind
+
 	// mapsMu protects mappings.
 	mapsMu sync.Mutex `state:"nosave"`
 
@@ -86,14 +90,75 @@ type regularFile struct {
 
 func (fs *filesystem) newRegularFile(kuid auth.KUID, kgid auth.KGID, mode linux.FileMode) *inode {
 	file := &regularFile{
-		memFile: fs.memFile,
-		seals:   linux.F_SEAL_SEAL,
+		memFile:         fs.memFile,
+		memoryUsageKind: usage.Tmpfs,
+		seals:           linux.F_SEAL_SEAL,
 	}
 	file.inode.init(file, fs, kuid, kgid, linux.S_IFREG|mode)
 	file.inode.nlink = 1 // from parent directory
 	return &file.inode
 }
 
+// newUnlinkedRegularFileDescription creates a regular file on the tmpfs
+// filesystem represented by mount and returns an FD representing that file.
+// The new file is not reachable by path traversal from any other file.
+//
+// newUnlinkedRegularFileDescription is analogous to Linux's
+// mm/shmem.c:__shmem_file_setup().
+//
+// Preconditions: mount must be a tmpfs mount.
+func newUnlinkedRegularFileDescription(ctx context.Context, creds *auth.Credentials, mount *vfs.Mount, name string) (*regularFileFD, error) {
+	fs, ok := mount.Filesystem().Impl().(*filesystem)
+	if !ok {
+		panic("tmpfs.newUnlinkedRegularFileDescription() called with non-tmpfs mount")
+	}
+
+	inode := fs.newRegularFile(creds.EffectiveKUID, creds.EffectiveKGID, 0777)
+	d := fs.newDentry(inode)
+	defer d.DecRef(ctx)
+	d.name = name
+
+	fd := &regularFileFD{}
+	fd.Init(&inode.locks)
+	flags := uint32(linux.O_RDWR)
+	if err := fd.vfsfd.Init(fd, flags, mount, &d.vfsd, &vfs.FileDescriptionOptions{}); err != nil {
+		return nil, err
+	}
+	return fd, nil
+}
+
+// NewZeroFile creates a new regular file and file description as for
+// mmap(MAP_SHARED | MAP_ANONYMOUS). The file has the given size and is
+// initially (implicitly) filled with zeroes.
+//
+// Preconditions: mount must be a tmpfs mount.
+func NewZeroFile(ctx context.Context, creds *auth.Credentials, mount *vfs.Mount, size uint64) (*vfs.FileDescription, error) {
+	// Compare mm/shmem.c:shmem_zero_setup().
+	fd, err := newUnlinkedRegularFileDescription(ctx, creds, mount, "dev/zero")
+	if err != nil {
+		return nil, err
+	}
+	rf := fd.inode().impl.(*regularFile)
+	rf.memoryUsageKind = usage.Anonymous
+	rf.size = size
+	return &fd.vfsfd, err
+}
+
+// NewMemfd creates a new regular file and file description as for
+// memfd_create.
+//
+// Preconditions: mount must be a tmpfs mount.
+func NewMemfd(ctx context.Context, creds *auth.Credentials, mount *vfs.Mount, allowSeals bool, name string) (*vfs.FileDescription, error) {
+	fd, err := newUnlinkedRegularFileDescription(ctx, creds, mount, name)
+	if err != nil {
+		return nil, err
+	}
+	if allowSeals {
+		fd.inode().impl.(*regularFile).seals = 0
+	}
+	return &fd.vfsfd, nil
+}
+
 // truncate grows or shrinks the file to the given size. It returns true if the
 // file size was updated.
 func (rf *regularFile) truncate(newSize uint64) (bool, error) {
@@ -226,7 +291,7 @@ func (rf *regularFile) Translate(ctx context.Context, required, optional memmap.
 		optional.End = pgend
 	}
 
-	cerr := rf.data.Fill(ctx, required, optional, rf.memFile, usage.Tmpfs, func(_ context.Context, dsts safemem.BlockSeq, _ uint64) (uint64, error) {
+	cerr := rf.data.Fill(ctx, required, optional, rf.memFile, rf.memoryUsageKind, func(_ context.Context, dsts safemem.BlockSeq, _ uint64) (uint64, error) {
 		// Newly-allocated pages are zeroed, so we don't need to do anything.
 		return dsts.NumBytes(), nil
 	})
@@ -575,7 +640,7 @@ func (rw *regularFileReadWriter) WriteFromBlocks(srcs safemem.BlockSeq) (uint64,
 		case gap.Ok():
 			// Allocate memory for the write.
 			gapMR := gap.Range().Intersect(pgMR)
-			fr, err := rw.file.memFile.Allocate(gapMR.Length(), usage.Tmpfs)
+			fr, err := rw.file.memFile.Allocate(gapMR.Length(), rw.file.memoryUsageKind)
 			if err != nil {
 				retErr = err
 				goto exitLoop

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -746,37 +746,6 @@ func (fd *fileDescription) RemoveXattr(ctx context.Context, name string) error {
 	return nil
 }
 
-// NewMemfd creates a new tmpfs regular file and file description that can back
-// an anonymous fd created by memfd_create.
-func NewMemfd(ctx context.Context, creds *auth.Credentials, mount *vfs.Mount, allowSeals bool, name string) (*vfs.FileDescription, error) {
-	fs, ok := mount.Filesystem().Impl().(*filesystem)
-	if !ok {
-		panic("NewMemfd() called with non-tmpfs mount")
-	}
-
-	// Per Linux, mm/shmem.c:__shmem_file_setup(), memfd inodes are set up with
-	// S_IRWXUGO.
-	inode := fs.newRegularFile(creds.EffectiveKUID, creds.EffectiveKGID, 0777)
-	rf := inode.impl.(*regularFile)
-	if allowSeals {
-		rf.seals = 0
-	}
-
-	d := fs.newDentry(inode)
-	defer d.DecRef(ctx)
-	d.name = name
-
-	// Per Linux, mm/shmem.c:__shmem_file_setup(), memfd files are set up with
-	// FMODE_READ | FMODE_WRITE.
-	var fd regularFileFD
-	fd.Init(&inode.locks)
-	flags := uint32(linux.O_RDWR)
-	if err := fd.vfsfd.Init(&fd, flags, mount, &d.vfsd, &vfs.FileDescriptionOptions{}); err != nil {
-		return nil, err
-	}
-	return &fd.vfsfd, nil
-}
-
 // LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
 func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, start, length uint64, whence int16, block fslock.Blocker) error {
 	return fd.Locks().LockPOSIX(ctx, &fd.vfsfd, uid, t, start, length, whence, block)

pkg/sentry/mm/mm.go

@@ -242,7 +242,7 @@ type MemoryManager struct {
 // +stateify savable
 type vma struct {
 	// mappable is the virtual memory object mapped by this vma. If mappable is
-	// nil, the vma represents a private anonymous mapping.
+	// nil, the vma represents an anonymous mapping.
 	mappable memmap.Mappable
 
 	// off is the offset into mappable at which this vma begins. If mappable is

pkg/sentry/mm/mm_test.go

@@ -51,7 +51,8 @@ func TestUsageASUpdates(t *testing.T) {
 	defer mm.DecUsers(ctx)
 
 	addr, err := mm.MMap(ctx, memmap.MMapOpts{
-		Length: 2 * usermem.PageSize,
+		Length:  2 * usermem.PageSize,
+		Private: true,
 	})
 	if err != nil {
 		t.Fatalf("MMap got err %v want nil", err)

pkg/sentry/mm/special_mappable.go

@@ -136,9 +136,12 @@ func (m *SpecialMappable) Length() uint64 {
 // NewSharedAnonMappable returns a SpecialMappable that implements the
 // semantics of mmap(MAP_SHARED|MAP_ANONYMOUS) and mappings of /dev/zero.
 //
-// TODO(jamieliu): The use of SpecialMappable is a lazy code reuse hack. Linux
-// uses an ephemeral file created by mm/shmem.c:shmem_zero_setup(); we should
-// do the same to get non-zero device and inode IDs.
+// TODO(gvisor.dev/issue/1624): Linux uses an ephemeral file created by
+// mm/shmem.c:shmem_zero_setup(), and VFS2 does something analogous. VFS1 uses
+// a SpecialMappable instead, incorrectly getting device and inode IDs of zero
+// and causing memory for shared anonymous mappings to be allocated up-front
+// instead of on first touch; this is to avoid exacerbating the fs.MountSource
+// leak (b/143656263). Delete this function along with VFS1.
 func NewSharedAnonMappable(length uint64, mfp pgalloc.MemoryFileProvider) (*SpecialMappable, error) {
 	if length == 0 {
 		return nil, syserror.EINVAL

pkg/sentry/mm/syscalls.go

@@ -24,7 +24,6 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/kernel/futex"
 	"gvisor.dev/gvisor/pkg/sentry/limits"
 	"gvisor.dev/gvisor/pkg/sentry/memmap"
-	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
 	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
@@ -93,18 +92,6 @@ func (mm *MemoryManager) MMap(ctx context.Context, opts memmap.MMapOpts) (userme
 		}
 	} else {
 		opts.Offset = 0
-		if !opts.Private {
-			if opts.MappingIdentity != nil {
-				return 0, syserror.EINVAL
-			}
-			m, err := NewSharedAnonMappable(opts.Length, pgalloc.MemoryFileProviderFromContext(ctx))
-			if err != nil {
-				return 0, err
-			}
-			defer m.DecRef(ctx)
-			opts.MappingIdentity = m
-			opts.Mappable = m
-		}
 	}
 
 	if opts.Addr.RoundDown() != opts.Addr {

pkg/sentry/syscalls/linux/sys_mmap.go

@@ -100,6 +100,15 @@ func Mmap(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallC
 		if err := file.ConfigureMMap(t, &opts); err != nil {
 			return 0, nil, err
 		}
+	} else if shared {
+		// Back shared anonymous mappings with a special mappable.
+		opts.Offset = 0
+		m, err := mm.NewSharedAnonMappable(opts.Length, t.Kernel())
+		if err != nil {
+			return 0, nil, err
+		}
+		opts.MappingIdentity = m // transfers ownership of m to opts
+		opts.Mappable = m
 	}
 
 	rv, err := t.MemoryManager().MMap(t, opts)

pkg/sentry/syscalls/linux/vfs2/mmap.go

@@ -17,6 +17,7 @@ package vfs2
 import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -82,6 +83,17 @@ func Mmap(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallC
 			opts.MaxPerms.Write = false
 		}
 
+		if err := file.ConfigureMMap(t, &opts); err != nil {
+			return 0, nil, err
+		}
+	} else if shared {
+		// Back shared anonymous mappings with an anonymous tmpfs file.
+		opts.Offset = 0
+		file, err := tmpfs.NewZeroFile(t, t.Credentials(), t.Kernel().ShmMount(), opts.Length)
+		if err != nil {
+			return 0, nil, err
+		}
+		defer file.DecRef(t)
 		if err := file.ConfigureMMap(t, &opts); err != nil {
 			return 0, nil, err
 		}

test/syscalls/linux/BUILD

@@ -1673,6 +1673,7 @@ cc_binary(
         gtest,
         "//test/util:memory_util",
         "//test/util:posix_error",
+        "//test/util:proc_util",
         "//test/util:temp_path",
         "//test/util:test_util",
         "//test/util:thread_util",

test/syscalls/linux/proc.cc

@@ -63,6 +63,7 @@
 #include "test/util/fs_util.h"
 #include "test/util/memory_util.h"
 #include "test/util/posix_error.h"
+#include "test/util/proc_util.h"
 #include "test/util/temp_path.h"
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
@@ -672,6 +673,23 @@ TEST(ProcSelfMaps, Mprotect) {
                                                    3 * kPageSize, PROT_READ)));
 }
 
+TEST(ProcSelfMaps, SharedAnon) {
+  const Mapping m = ASSERT_NO_ERRNO_AND_VALUE(
+      MmapAnon(kPageSize, PROT_READ, MAP_SHARED | MAP_ANONYMOUS));
+
+  const auto proc_self_maps =
+      ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/self/maps"));
+  for (const auto& line : absl::StrSplit(proc_self_maps, '\n')) {
+    const auto entry = ASSERT_NO_ERRNO_AND_VALUE(ParseProcMapsLine(line));
+    if (entry.start <= m.addr() && m.addr() < entry.end) {
+      // cf. proc(5), "/proc/[pid]/map_files/"
+      EXPECT_EQ(entry.filename, "/dev/zero (deleted)");
+      return;
+    }
+  }
+  FAIL() << "no maps entry containing mapping at " << m.ptr();
+}
+
 TEST(ProcSelfFd, OpenFd) {
   int pipe_fds[2];
   ASSERT_THAT(pipe2(pipe_fds, O_CLOEXEC), SyscallSucceeds());