Add FileExec flag to OpenOptions

This allow callers to say whether the file is being
opened to be executed, so that the proper checks can
be done from FilesystemImpl.OpenAt()

Updates #1623

PiperOrigin-RevId: 295042595

3 files changed, 36 insertions, 7 deletions

diff --git a/pkg/sentry/vfs/options.go b/pkg/sentry/vfs/options.go
index b7774bf28..fdf8be157 100644
--- a/pkg/sentry/vfs/options.go
+++ b/pkg/sentry/vfs/options.go
@@ -72,6 +72,11 @@ type OpenOptions struct {
 	// If FilesystemImpl.OpenAt() creates a file, Mode is the file mode for the
 	// created file.
 	Mode linux.FileMode
+
+	// FileExec is set when the file is being opened to be executed.
+	// VirtualFilesystem.OpenAt() checks that the caller has execute permissions
+	// on the file, and that the file is a regular file.
+	FileExec bool
 }
 
 // ReadOptions contains options to FileDescription.PRead(),
diff --git a/pkg/sentry/vfs/permissions.go b/pkg/sentry/vfs/permissions.go
index f664581f4..8e250998a 100644
--- a/pkg/sentry/vfs/permissions.go
+++ b/pkg/sentry/vfs/permissions.go
@@ -103,17 +103,22 @@ func GenericCheckPermissions(creds *auth.Credentials, ats AccessTypes, isDir boo
 // AccessTypesForOpenFlags returns MayRead|MayWrite in this case.
 //
 // Use May{Read,Write}FileWithOpenFlags() for these checks instead.
-func AccessTypesForOpenFlags(flags uint32) AccessTypes {
-	switch flags & linux.O_ACCMODE {
+func AccessTypesForOpenFlags(opts *OpenOptions) AccessTypes {
+	ats := AccessTypes(0)
+	if opts.FileExec {
+		ats |= MayExec
+	}
+
+	switch opts.Flags & linux.O_ACCMODE {
 	case linux.O_RDONLY:
-		if flags&linux.O_TRUNC != 0 {
-			return MayRead | MayWrite
+		if opts.Flags&linux.O_TRUNC != 0 {
+			return ats | MayRead | MayWrite
 		}
-		return MayRead
+		return ats | MayRead
 	case linux.O_WRONLY:
-		return MayWrite
+		return ats | MayWrite
 	default:
-		return MayRead | MayWrite
+		return ats | MayRead | MayWrite
 	}
 }
 
diff --git a/pkg/sentry/vfs/vfs.go b/pkg/sentry/vfs/vfs.go
index 908c69f91..9629afee9 100644
--- a/pkg/sentry/vfs/vfs.go
+++ b/pkg/sentry/vfs/vfs.go
@@ -379,6 +379,25 @@ func (vfs *VirtualFilesystem) OpenAt(ctx context.Context, creds *auth.Credential
 		fd, err := rp.mount.fs.impl.OpenAt(ctx, rp, *opts)
 		if err == nil {
 			vfs.putResolvingPath(rp)
+
+			// TODO(gvisor.dev/issue/1193): Move inside fsimpl to avoid another call
+			// to FileDescription.Stat().
+			if opts.FileExec {
+				// Only a regular file can be executed.
+				stat, err := fd.Stat(ctx, StatOptions{Mask: linux.STATX_TYPE})
+				if err != nil {
+					return nil, err
+				}
+				if stat.Mask&linux.STATX_TYPE != 0 {
+					// This shouldn't happen, but if type can't be retrieved, file can't
+					// be executed.
+					return nil, syserror.EACCES
+				}
+				if linux.FileMode(stat.Mode).FileType() != linux.ModeRegular {
+					return nil, syserror.EACCES
+				}
+			}
+
 			return fd, nil
 		}
 		if !rp.handleError(err) {