Unlock VFS.mountMu before FilesystemImpl calls for /proc/[pid]/{mounts,mountinfo}.

Also move VFS.MakeSyntheticMountpoint() (which is a utility wrapper around
VFS.MkdirAllAt(), itself a utility wrapper around VFS.MkdirAt()) to not be in
the middle of the implementation of these proc files.

Fixes #3878

PiperOrigin-RevId: 330843106

pkg/sentry/vfs/mount.go

@@ -18,14 +18,12 @@ import (
 	"bytes"
 	"fmt"
 	"math"
-	"path"
 	"sort"
 	"strings"
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
-	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/syserror"
 )
@@ -740,11 +738,23 @@ func (mntns *MountNamespace) Root() VirtualDentry {
 //
 // Preconditions: taskRootDir.Ok().
 func (vfs *VirtualFilesystem) GenerateProcMounts(ctx context.Context, taskRootDir VirtualDentry, buf *bytes.Buffer) {
-	vfs.mountMu.Lock()
-	defer vfs.mountMu.Unlock()
 	rootMnt := taskRootDir.mount
+
+	vfs.mountMu.Lock()
 	mounts := rootMnt.submountsLocked()
+	// Take a reference on mounts since we need to drop vfs.mountMu before
+	// calling vfs.PathnameReachable() (=> FilesystemImpl.PrependPath()).
+	for _, mnt := range mounts {
+		mnt.IncRef()
+	}
+	vfs.mountMu.Unlock()
+	defer func() {
+		for _, mnt := range mounts {
+			mnt.DecRef(ctx)
+		}
+	}()
 	sort.Slice(mounts, func(i, j int) bool { return mounts[i].ID < mounts[j].ID })
+
 	for _, mnt := range mounts {
 		// Get the path to this mount relative to task root.
 		mntRootVD := VirtualDentry{
@@ -755,7 +765,7 @@ func (vfs *VirtualFilesystem) GenerateProcMounts(ctx context.Context, taskRootDi
 		if err != nil {
 			// For some reason we didn't get a path. Log a warning
 			// and run with empty path.
-			ctx.Warningf("Error getting pathname for mount root %+v: %v", mnt.root, err)
+			ctx.Warningf("VFS.GenerateProcMounts: error getting pathname for mount root %+v: %v", mnt.root, err)
 			path = ""
 		}
 		if path == "" {
@@ -789,11 +799,25 @@ func (vfs *VirtualFilesystem) GenerateProcMounts(ctx context.Context, taskRootDi
 //
 // Preconditions: taskRootDir.Ok().
 func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRootDir VirtualDentry, buf *bytes.Buffer) {
-	vfs.mountMu.Lock()
-	defer vfs.mountMu.Unlock()
 	rootMnt := taskRootDir.mount
+
+	vfs.mountMu.Lock()
 	mounts := rootMnt.submountsLocked()
+	// Take a reference on mounts since we need to drop vfs.mountMu before
+	// calling vfs.PathnameReachable() (=> FilesystemImpl.PrependPath()) or
+	// vfs.StatAt() (=> FilesystemImpl.StatAt()).
+	for _, mnt := range mounts {
+		mnt.IncRef()
+	}
+	vfs.mountMu.Unlock()
+	defer func() {
+		for _, mnt := range mounts {
+			mnt.DecRef(ctx)
+		}
+	}()
 	sort.Slice(mounts, func(i, j int) bool { return mounts[i].ID < mounts[j].ID })
+
+	creds := auth.CredentialsFromContext(ctx)
 	for _, mnt := range mounts {
 		// Get the path to this mount relative to task root.
 		mntRootVD := VirtualDentry{
@@ -804,7 +828,7 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 		if err != nil {
 			// For some reason we didn't get a path. Log a warning
 			// and run with empty path.
-			ctx.Warningf("Error getting pathname for mount root %+v: %v", mnt.root, err)
+			ctx.Warningf("VFS.GenerateProcMountInfo: error getting pathname for mount root %+v: %v", mnt.root, err)
 			path = ""
 		}
 		if path == "" {
@@ -817,9 +841,10 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 			Root:  mntRootVD,
 			Start: mntRootVD,
 		}
-		statx, err := vfs.StatAt(ctx, auth.NewAnonymousCredentials(), pop, &StatOptions{})
+		statx, err := vfs.StatAt(ctx, creds, pop, &StatOptions{})
 		if err != nil {
 			// Well that's not good. Ignore this mount.
+			ctx.Warningf("VFS.GenerateProcMountInfo: failed to stat mount root %+v: %v", mnt.root, err)
 			break
 		}
 
@@ -831,6 +856,9 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 		fmt.Fprintf(buf, "%d ", mnt.ID)
 
 		// (2)  Parent ID (or this ID if there is no parent).
+		// Note that even if the call to mnt.parent() races with Mount
+		// destruction (which is possible since we're not holding vfs.mountMu),
+		// its Mount.ID will still be valid.
 		pID := mnt.ID
 		if p := mnt.parent(); p != nil {
 			pID = p.ID
@@ -879,30 +907,6 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 	}
 }
 
-// MakeSyntheticMountpoint creates parent directories of target if they do not
-// exist and attempts to create a directory for the mountpoint. If a
-// non-directory file already exists there then we allow it.
-func (vfs *VirtualFilesystem) MakeSyntheticMountpoint(ctx context.Context, target string, root VirtualDentry, creds *auth.Credentials) error {
-	mkdirOpts := &MkdirOptions{Mode: 0777, ForSyntheticMountpoint: true}
-
-	// Make sure the parent directory of target exists.
-	if err := vfs.MkdirAllAt(ctx, path.Dir(target), root, creds, mkdirOpts); err != nil {
-		return fmt.Errorf("failed to create parent directory of mountpoint %q: %w", target, err)
-	}
-
-	// Attempt to mkdir the final component. If a file (of any type) exists
-	// then we let allow mounting on top of that because we do not require the
-	// target to be an existing directory, unlike Linux mount(2).
-	if err := vfs.MkdirAt(ctx, creds, &PathOperation{
-		Root:  root,
-		Start: root,
-		Path:  fspath.Parse(target),
-	}, mkdirOpts); err != nil && err != syserror.EEXIST {
-		return fmt.Errorf("failed to create mountpoint %q: %w", target, err)
-	}
-	return nil
-}
-
 // manglePath replaces ' ', '\t', '\n', and '\\' with their octal equivalents.
 // See Linux fs/seq_file.c:mangle_path.
 func manglePath(p string) string {

pkg/sentry/vfs/vfs.go

@@ -819,6 +819,30 @@ func (vfs *VirtualFilesystem) MkdirAllAt(ctx context.Context, currentPath string
 	return nil
 }
 
+// MakeSyntheticMountpoint creates parent directories of target if they do not
+// exist and attempts to create a directory for the mountpoint. If a
+// non-directory file already exists there then we allow it.
+func (vfs *VirtualFilesystem) MakeSyntheticMountpoint(ctx context.Context, target string, root VirtualDentry, creds *auth.Credentials) error {
+	mkdirOpts := &MkdirOptions{Mode: 0777, ForSyntheticMountpoint: true}
+
+	// Make sure the parent directory of target exists.
+	if err := vfs.MkdirAllAt(ctx, path.Dir(target), root, creds, mkdirOpts); err != nil {
+		return fmt.Errorf("failed to create parent directory of mountpoint %q: %w", target, err)
+	}
+
+	// Attempt to mkdir the final component. If a file (of any type) exists
+	// then we let allow mounting on top of that because we do not require the
+	// target to be an existing directory, unlike Linux mount(2).
+	if err := vfs.MkdirAt(ctx, creds, &PathOperation{
+		Root:  root,
+		Start: root,
+		Path:  fspath.Parse(target),
+	}, mkdirOpts); err != nil && err != syserror.EEXIST {
+		return fmt.Errorf("failed to create mountpoint %q: %w", target, err)
+	}
+	return nil
+}
+
 // A VirtualDentry represents a node in a VFS tree, by combining a Dentry
 // (which represents a node in a Filesystem's tree) and a Mount (which
 // represents the Filesystem's position in a VFS mount tree).