1 files changed, 84 insertions, 2 deletions

diff --git a/pkg/sentry/vfs/permissions.go b/pkg/sentry/vfs/permissions.go
index f8e74355c..d279d05ca 100644
--- a/pkg/sentry/vfs/permissions.go
+++ b/pkg/sentry/vfs/permissions.go
@@ -30,6 +30,26 @@ const (
 	MayExec              = 1
 )
 
+// OnlyRead returns true if access _only_ allows read.
+func (a AccessTypes) OnlyRead() bool {
+	return a == MayRead
+}
+
+// MayRead returns true if access allows read.
+func (a AccessTypes) MayRead() bool {
+	return a&MayRead != 0
+}
+
+// MayWrite returns true if access allows write.
+func (a AccessTypes) MayWrite() bool {
+	return a&MayWrite != 0
+}
+
+// MayExec returns true if access allows exec.
+func (a AccessTypes) MayExec() bool {
+	return a&MayExec != 0
+}
+
 // GenericCheckPermissions checks that creds has the given access rights on a
 // file with the given permissions, UID, and GID, subject to the rules of
 // fs/namei.c:generic_permission(). isDir is true if the file is a directory.
@@ -53,7 +73,7 @@ func GenericCheckPermissions(creds *auth.Credentials, ats AccessTypes, isDir boo
 	}
 	// CAP_DAC_READ_SEARCH allows the caller to read and search arbitrary
 	// directories, and read arbitrary non-directory files.
-	if (isDir && (ats&MayWrite == 0)) || ats == MayRead {
+	if (isDir && !ats.MayWrite()) || ats.OnlyRead() {
 		if creds.HasCapability(linux.CAP_DAC_READ_SEARCH) {
 			return nil
 		}
@@ -61,7 +81,7 @@ func GenericCheckPermissions(creds *auth.Credentials, ats AccessTypes, isDir boo
 	// CAP_DAC_OVERRIDE allows arbitrary access to directories, read/write
 	// access to non-directory files, and execute access to non-directory files
 	// for which at least one execute bit is set.
-	if isDir || (ats&MayExec == 0) || (mode&0111 != 0) {
+	if isDir || !ats.MayExec() || (mode&0111 != 0) {
 		if creds.HasCapability(linux.CAP_DAC_OVERRIDE) {
 			return nil
 		}
@@ -119,3 +139,65 @@ func MayWriteFileWithOpenFlags(flags uint32) bool {
 		return false
 	}
 }
+
+// CheckSetStat checks that creds has permission to change the metadata of a
+// file with the given permissions, UID, and GID as specified by stat, subject
+// to the rules of Linux's fs/attr.c:setattr_prepare().
+func CheckSetStat(creds *auth.Credentials, stat *linux.Statx, mode uint16, kuid auth.KUID, kgid auth.KGID) error {
+	if stat.Mask&linux.STATX_MODE != 0 {
+		if !CanActAsOwner(creds, kuid) {
+			return syserror.EPERM
+		}
+		// TODO(b/30815691): "If the calling process is not privileged (Linux:
+		// does not have the CAP_FSETID capability), and the group of the file
+		// does not match the effective group ID of the process or one of its
+		// supplementary group IDs, the S_ISGID bit will be turned off, but
+		// this will not cause an error to be returned." - chmod(2)
+	}
+	if stat.Mask&linux.STATX_UID != 0 {
+		if !((creds.EffectiveKUID == kuid && auth.KUID(stat.UID) == kuid) ||
+			HasCapabilityOnFile(creds, linux.CAP_CHOWN, kuid, kgid)) {
+			return syserror.EPERM
+		}
+	}
+	if stat.Mask&linux.STATX_GID != 0 {
+		if !((creds.EffectiveKUID == kuid && creds.InGroup(auth.KGID(stat.GID))) ||
+			HasCapabilityOnFile(creds, linux.CAP_CHOWN, kuid, kgid)) {
+			return syserror.EPERM
+		}
+	}
+	if stat.Mask&(linux.STATX_ATIME|linux.STATX_MTIME|linux.STATX_CTIME) != 0 {
+		if !CanActAsOwner(creds, kuid) {
+			if (stat.Mask&linux.STATX_ATIME != 0 && stat.Atime.Nsec != linux.UTIME_NOW) ||
+				(stat.Mask&linux.STATX_MTIME != 0 && stat.Mtime.Nsec != linux.UTIME_NOW) ||
+				(stat.Mask&linux.STATX_CTIME != 0 && stat.Ctime.Nsec != linux.UTIME_NOW) {
+				return syserror.EPERM
+			}
+			// isDir is irrelevant in the following call to
+			// GenericCheckPermissions since ats == MayWrite means that
+			// CAP_DAC_READ_SEARCH does not apply, and CAP_DAC_OVERRIDE
+			// applies, regardless of isDir.
+			if err := GenericCheckPermissions(creds, MayWrite, false /* isDir */, mode, kuid, kgid); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// CanActAsOwner returns true if creds can act as the owner of a file with the
+// given owning UID, consistent with Linux's
+// fs/inode.c:inode_owner_or_capable().
+func CanActAsOwner(creds *auth.Credentials, kuid auth.KUID) bool {
+	if creds.EffectiveKUID == kuid {
+		return true
+	}
+	return creds.HasCapability(linux.CAP_FOWNER) && creds.UserNamespace.MapFromKUID(kuid).Ok()
+}
+
+// HasCapabilityOnFile returns true if creds has the given capability with
+// respect to a file with the given owning UID and GID, consistent with Linux's
+// kernel/capability.c:capable_wrt_inode_uidgid().
+func HasCapabilityOnFile(creds *auth.Credentials, cp linux.Capability, kuid auth.KUID, kgid auth.KGID) bool {
+	return creds.HasCapability(cp) && creds.UserNamespace.MapFromKUID(kuid).Ok() && creds.UserNamespace.MapFromKGID(kgid).Ok()
+}