setgid directory support in overlayfs

PiperOrigin-RevId: 363276495

pkg/sentry/fsimpl/overlay/filesystem.go

@@ -689,13 +689,9 @@ func (fs *filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 			}
 			return err
 		}
-		creds := rp.Credentials()
+
 		if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
-			Stat: linux.Statx{
-				Mask: linux.STATX_UID | linux.STATX_GID,
-				UID:  uint32(creds.EffectiveKUID),
-				GID:  uint32(creds.EffectiveKGID),
-			},
+			Stat: parent.newChildOwnerStat(opts.Mode, rp.Credentials()),
 		}); err != nil {
 			if cleanupErr := vfsObj.RmdirAt(ctx, fs.creds, &pop); cleanupErr != nil {
 				panic(fmt.Sprintf("unrecoverable overlayfs inconsistency: failed to delete upper layer directory after MkdirAt metadata update failure: %v", cleanupErr))
@@ -750,11 +746,7 @@ func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 		}
 		creds := rp.Credentials()
 		if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
-			Stat: linux.Statx{
-				Mask: linux.STATX_UID | linux.STATX_GID,
-				UID:  uint32(creds.EffectiveKUID),
-				GID:  uint32(creds.EffectiveKGID),
-			},
+			Stat: parent.newChildOwnerStat(opts.Mode, creds),
 		}); err != nil {
 			if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &pop); cleanupErr != nil {
 				panic(fmt.Sprintf("unrecoverable overlayfs inconsistency: failed to delete upper layer file after MknodAt metadata update failure: %v", cleanupErr))
@@ -963,14 +955,11 @@ func (fs *filesystem) createAndOpenLocked(ctx context.Context, rp *vfs.Resolving
 		}
 		return nil, err
 	}
+
 	// Change the file's owner to the caller. We can't use upperFD.SetStat()
 	// because it will pick up creds from ctx.
 	if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
-		Stat: linux.Statx{
-			Mask: linux.STATX_UID | linux.STATX_GID,
-			UID:  uint32(creds.EffectiveKUID),
-			GID:  uint32(creds.EffectiveKGID),
-		},
+		Stat: parent.newChildOwnerStat(opts.Mode, creds),
 	}); err != nil {
 		if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &pop); cleanupErr != nil {
 			panic(fmt.Sprintf("unrecoverable overlayfs inconsistency: failed to delete upper layer file after OpenAt(O_CREAT) metadata update failure: %v", cleanupErr))

pkg/sentry/fsimpl/overlay/overlay.go

@@ -749,6 +749,27 @@ func (d *dentry) mayDelete(creds *auth.Credentials, child *dentry) error {
 	)
 }
 
+// newChildOwnerStat returns a Statx for configuring the UID, GID, and mode of
+// children.
+func (d *dentry) newChildOwnerStat(mode linux.FileMode, creds *auth.Credentials) linux.Statx {
+	stat := linux.Statx{
+		Mask: uint32(linux.STATX_UID | linux.STATX_GID),
+		UID:  uint32(creds.EffectiveKUID),
+		GID:  uint32(creds.EffectiveKGID),
+	}
+	// Set GID and possibly the SGID bit if the parent is an SGID directory.
+	d.copyMu.RLock()
+	defer d.copyMu.RUnlock()
+	if atomic.LoadUint32(&d.mode)&linux.ModeSetGID == linux.ModeSetGID {
+		stat.GID = atomic.LoadUint32(&d.gid)
+		if stat.Mode&linux.ModeDirectory == linux.ModeDirectory {
+			stat.Mode = uint16(mode) | linux.ModeSetGID
+			stat.Mask |= linux.STATX_MODE
+		}
+	}
+	return stat
+}
+
 // fileDescription is embedded by overlay implementations of
 // vfs.FileDescriptionImpl.
 //

pkg/sentry/fsimpl/overlay/regular_file.go

@@ -205,6 +205,20 @@ func (fd *regularFileFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) e
 	if err := wrappedFD.SetStat(ctx, opts); err != nil {
 		return err
 	}
+
+	// Changing owners may clear one or both of the setuid and setgid bits,
+	// so we may have to update opts before setting d.mode.
+	if opts.Stat.Mask&(linux.STATX_UID|linux.STATX_GID) != 0 {
+		stat, err := wrappedFD.Stat(ctx, vfs.StatOptions{
+			Mask: linux.STATX_MODE,
+		})
+		if err != nil {
+			return err
+		}
+		opts.Stat.Mode = stat.Mode
+		opts.Stat.Mask |= linux.STATX_MODE
+	}
+
 	d.updateAfterSetStatLocked(&opts)
 	if ev := vfs.InotifyEventFromStatMask(opts.Stat.Mask); ev != 0 {
 		d.InotifyWithParent(ctx, ev, 0, vfs.InodeEvent)
@@ -295,7 +309,11 @@ func (fd *regularFileFD) PWrite(ctx context.Context, src usermem.IOSequence, off
 		return 0, err
 	}
 	defer wrappedFD.DecRef(ctx)
-	return wrappedFD.PWrite(ctx, src, offset, opts)
+	n, err := wrappedFD.PWrite(ctx, src, offset, opts)
+	if err != nil {
+		return n, err
+	}
+	return fd.updateSetUserGroupIDs(ctx, wrappedFD, n)
 }
 
 // Write implements vfs.FileDescriptionImpl.Write.
@@ -307,7 +325,28 @@ func (fd *regularFileFD) Write(ctx context.Context, src usermem.IOSequence, opts
 	if err != nil {
 		return 0, err
 	}
-	return wrappedFD.Write(ctx, src, opts)
+	n, err := wrappedFD.Write(ctx, src, opts)
+	if err != nil {
+		return n, err
+	}
+	return fd.updateSetUserGroupIDs(ctx, wrappedFD, n)
+}
+
+func (fd *regularFileFD) updateSetUserGroupIDs(ctx context.Context, wrappedFD *vfs.FileDescription, written int64) (int64, error) {
+	// Writing can clear the setuid and/or setgid bits. We only have to
+	// check this if something was written and one of those bits was set.
+	dentry := fd.dentry()
+	if written == 0 || atomic.LoadUint32(&dentry.mode)&(linux.S_ISUID|linux.S_ISGID) == 0 {
+		return written, nil
+	}
+	stat, err := wrappedFD.Stat(ctx, vfs.StatOptions{Mask: linux.STATX_MODE})
+	if err != nil {
+		return written, err
+	}
+	dentry.copyMu.Lock()
+	defer dentry.copyMu.Unlock()
+	atomic.StoreUint32(&dentry.mode, uint32(stat.Mode))
+	return written, nil
 }
 
 // Seek implements vfs.FileDescriptionImpl.Seek.

test/syscalls/linux/BUILD

@@ -2162,6 +2162,7 @@ cc_binary(
         "//test/util:temp_path",
         "//test/util:test_main",
         "//test/util:test_util",
+        "@com_google_absl//absl/flags:flag",
         "@com_google_absl//absl/strings",
         gtest,
     ],

test/syscalls/linux/setgid.cc

@@ -17,6 +17,7 @@
 #include <unistd.h>
 
 #include "gtest/gtest.h"
+#include "absl/flags/flag.h"
 #include "test/util/capability_util.h"
 #include "test/util/cleanup.h"
 #include "test/util/fs_util.h"
@@ -24,6 +25,11 @@
 #include "test/util/temp_path.h"
 #include "test/util/test_util.h"
 
+ABSL_FLAG(std::vector<std::string>, groups, std::vector<std::string>({}),
+          "groups the test can use");
+
+constexpr gid_t kNobody = 65534;
+
 namespace gvisor {
 namespace testing {
 
@@ -46,6 +52,18 @@ PosixErrorOr<Cleanup> Setegid(gid_t egid) {
 
 // Returns a pair of groups that the user is a member of.
 PosixErrorOr<std::pair<gid_t, gid_t>> Groups() {
+  // Were we explicitly passed GIDs?
+  std::vector<std::string> flagged_groups = absl::GetFlag(FLAGS_groups);
+  if (flagged_groups.size() >= 2) {
+    int group1;
+    int group2;
+    if (!absl::SimpleAtoi(flagged_groups[0], &group1) ||
+        !absl::SimpleAtoi(flagged_groups[1], &group2)) {
+      return PosixError(EINVAL, "failed converting group flags to ints");
+    }
+    return std::pair<gid_t, gid_t>(group1, group2);
+  }
+
   // See whether the user is a member of at least 2 groups.
   std::vector<gid_t> groups(64);
   for (; groups.size() <= NGROUPS_MAX; groups.resize(groups.size() * 2)) {
@@ -58,26 +76,47 @@ PosixErrorOr<std::pair<gid_t, gid_t>> Groups() {
       return PosixError(errno, absl::StrFormat("getgroups(%d, %p)",
                                                groups.size(), groups.data()));
     }
-    if (ngroups >= 2) {
-      return std::pair<gid_t, gid_t>(groups[0], groups[1]);
+
+    if (ngroups < 2) {
+      // There aren't enough groups.
+      break;
     }
-    // There aren't enough groups.
-    break;
+
+    // TODO(b/181878080): Read /proc/sys/fs/overflowgid once it is supported in
+    // gVisor.
+    if (groups[0] == kNobody || groups[1] == kNobody) {
+      // These groups aren't mapped into our user namespace, so we can't use
+      // them.
+      break;
+    }
+    return std::pair<gid_t, gid_t>(groups[0], groups[1]);
   }
 
-  // If we're root in the root user namespace, we can set our GID to whatever we
-  // want. Try that before giving up.
-  constexpr gid_t kGID1 = 1111;
-  constexpr gid_t kGID2 = 2222;
-  auto cleanup1 = Setegid(kGID1);
+  // If we're running in gVisor and are root in the root user namespace, we can
+  // set our GID to whatever we want. Try that before giving up.
+  //
+  // This won't work in native tests, as despite having CAP_SETGID, the gofer
+  // process will be sandboxed and unable to change file GIDs.
+  if (!IsRunningOnGvisor()) {
+    return PosixError(EPERM, "no valid groups for native testing");
+  }
+  PosixErrorOr<bool> capable = HaveCapability(CAP_SETGID);
+  if (!capable.ok()) {
+    return capable.error();
+  }
+  if (!capable.ValueOrDie()) {
+    return PosixError(EPERM, "missing CAP_SETGID");
+  }
+  gid_t gid = getegid();
+  auto cleanup1 = Setegid(gid);
   if (!cleanup1.ok()) {
     return cleanup1.error();
   }
-  auto cleanup2 = Setegid(kGID2);
+  auto cleanup2 = Setegid(kNobody);
   if (!cleanup2.ok()) {
     return cleanup2.error();
   }
-  return std::pair<gid_t, gid_t>(kGID1, kGID2);
+  return std::pair<gid_t, gid_t>(gid, kNobody);
 }
 
 class SetgidDirTest : public ::testing::Test {
@@ -85,17 +124,20 @@ class SetgidDirTest : public ::testing::Test {
   void SetUp() override {
     original_gid_ = getegid();
 
-    // TODO(b/175325250): Enable when setgid directories are supported.
     SKIP_IF(IsRunningWithVFS1());
-    SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SETGID)));
 
     temp_dir_ = ASSERT_NO_ERRNO_AND_VALUE(
         TempPath::CreateDirWith(GetAbsoluteTestTmpdir(), 0777 /* mode */));
-    groups_ = ASSERT_NO_ERRNO_AND_VALUE(Groups());
+
+    // If we can't find two usable groups, we're in an unsupporting environment.
+    // Skip the test.
+    PosixErrorOr<std::pair<gid_t, gid_t>> groups = Groups();
+    SKIP_IF(!groups.ok());
+    groups_ = groups.ValueOrDie();
   }
 
   void TearDown() override {
-    ASSERT_THAT(setegid(original_gid_), SyscallSucceeds());
+    EXPECT_THAT(setegid(original_gid_), SyscallSucceeds());
   }
 
   void MkdirAsGid(gid_t gid, const std::string& path, mode_t mode) {
@@ -131,7 +173,7 @@ TEST_F(SetgidDirTest, Control) {
   ASSERT_NO_FATAL_FAILURE(MkdirAsGid(groups_.first, g1owned, 0777));
 
   // Set group to G2, create a file in g1owned, and confirm that G2 owns it.
-  ASSERT_THAT(setegid(groups_.second), SyscallSucceeds());
+  auto cleanup = ASSERT_NO_ERRNO_AND_VALUE(Setegid(groups_.second));
   FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(
       Open(JoinPath(g1owned, "g2owned").c_str(), O_CREAT | O_RDWR, 0777));
   struct stat stats = ASSERT_NO_ERRNO_AND_VALUE(Stat(fd));
@@ -146,7 +188,7 @@ TEST_F(SetgidDirTest, CreateFile) {
   ASSERT_THAT(chmod(g1owned.c_str(), kDirmodeSgid), SyscallSucceeds());
 
   // Set group to G2, create a file, and confirm that G1 owns it.
-  ASSERT_THAT(setegid(groups_.second), SyscallSucceeds());
+  auto cleanup = ASSERT_NO_ERRNO_AND_VALUE(Setegid(groups_.second));
   FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(
       Open(JoinPath(g1owned, "g2created").c_str(), O_CREAT | O_RDWR, 0666));
   struct stat stats = ASSERT_NO_ERRNO_AND_VALUE(Stat(fd));
@@ -194,7 +236,7 @@ TEST_F(SetgidDirTest, OldFile) {
   ASSERT_THAT(chmod(g1owned.c_str(), kDirmodeNoSgid), SyscallSucceeds());
 
   // Set group to G2, create a file, confirm that G2 owns it.
-  ASSERT_THAT(setegid(groups_.second), SyscallSucceeds());
+  auto cleanup = ASSERT_NO_ERRNO_AND_VALUE(Setegid(groups_.second));
   FileDescriptor fd = ASSERT_NO_ERRNO_AND_VALUE(
       Open(JoinPath(g1owned, "g2created").c_str(), O_CREAT | O_RDWR, 0666));
   struct stat stats = ASSERT_NO_ERRNO_AND_VALUE(Stat(fd));
@@ -217,7 +259,7 @@ TEST_F(SetgidDirTest, OldDir) {
   ASSERT_THAT(chmod(g1owned.c_str(), kDirmodeNoSgid), SyscallSucceeds());
 
   // Set group to G2, create a directory, confirm that G2 owns it.
-  ASSERT_THAT(setegid(groups_.second), SyscallSucceeds());
+  auto cleanup = ASSERT_NO_ERRNO_AND_VALUE(Setegid(groups_.second));
   auto g2created = JoinPath(g1owned, "g2created");
   ASSERT_NO_FATAL_FAILURE(MkdirAsGid(groups_.second, g2created, 0666));
   struct stat stats = ASSERT_NO_ERRNO_AND_VALUE(Stat(g2created));