vmexec: Set perms on stdio (#174)

We don't chown the container's stdio today.

vminitd/Sources/vmexec/ExecCommand.swift

@@ -100,7 +100,10 @@ struct ExecCommand: ParsableCommand {
             try App.applyCloseExecOnFDs()
             try App.setRLimits(rlimits: process.rlimits)
 
-            // set uid, gid, and supplementary groups
+            // Change stdio to be owned by the requested user.
+            try App.fixStdioPerms(user: process.user)
+
+            // Set uid, gid, and supplementary groups
             try App.setPermissions(user: process.user)
 
             if process.terminal {

vminitd/Sources/vmexec/RunCommand.swift

@@ -109,7 +109,10 @@ struct RunCommand: ParsableCommand {
 
             try App.setRLimits(rlimits: process.rlimits)
 
-            // set uid, gid, and supplementary groups
+            // Change stdio to be owned by the requested user.
+            try App.fixStdioPerms(user: process.user)
+
+            // Set uid, gid, and supplementary groups.
             try App.setPermissions(user: process.user)
 
             if process.terminal {

vminitd/Sources/vmexec/vmexec.swift

@@ -104,6 +104,24 @@ extension App {
         }
     }
 
+    static func fixStdioPerms(user: ContainerizationOCI.User) throws {
+        for i in 0...2 {
+            var fdStat = stat()
+            try withUnsafeMutablePointer(to: &fdStat) { pointer in
+                guard fstat(Int32(i), pointer) == 0 else {
+                    throw App.Errno(stage: "fstat(fd)")
+                }
+            }
+
+            let desired = uid_t(user.uid)
+            if fdStat.st_uid != desired {
+                guard fchown(Int32(i), desired, fdStat.st_gid) != -1 else {
+                    throw App.Errno(stage: "fchown(\(i))")
+                }
+            }
+        }
+    }
+
     static func setRLimits(rlimits: [ContainerizationOCI.POSIXRlimit]) throws {
         for rl in rlimits {
             var limit = rlimit(rlim_cur: rl.soft, rlim_max: rl.hard)