Add a seccomp filter to prevent creating setuid/setgid binaries

This prevents builders from setting the S_ISUID or S_ISGID bits,
preventing users from using a nixbld* user to create a setuid/setgid
binary to interfere with subsequent builds under the same nixbld* uid.

This is based on aszlig's seccomp code
(47f587700d646f5b03a42f2fa57c28875a31efbe).

Reported by Linus Heckemann.

1 files changed, 39 insertions, 0 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 44cae3431f..06d2596066 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -46,6 +46,7 @@
 #include <sys/param.h>
 #include <sys/mount.h>
 #include <sys/syscall.h>
+#include <seccomp.h>
 #define pivot_root(new_root, put_old) (syscall(SYS_pivot_root, new_root, put_old))
 #endif
 
@@ -2298,6 +2299,42 @@ void DerivationGoal::doExportReferencesGraph()
 }
 
 
+void setupSeccomp()
+{
+#if __linux__
+    scmp_filter_ctx ctx;
+
+    if (!(ctx = seccomp_init(SCMP_ACT_ALLOW)))
+        throw SysError("unable to initialize seccomp mode 2");
+
+    Finally cleanup([&]() {
+        seccomp_release(ctx);
+    });
+
+    if (seccomp_arch_add(ctx, SCMP_ARCH_X86) != 0)
+        throw SysError("unable to add 32-bit seccomp architecture");
+
+    for (int perm : { S_ISUID, S_ISGID }) {
+        // TODO: test chmod and fchmod.
+        if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(chmod), 1,
+                SCMP_A1(SCMP_CMP_MASKED_EQ, perm, perm)) != 0)
+            throw SysError("unable to add seccomp rule");
+
+        if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(fchmod), 1,
+                SCMP_A1(SCMP_CMP_MASKED_EQ, perm, perm)) != 0)
+            throw SysError("unable to add seccomp rule");
+
+        if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(fchmodat), 1,
+                SCMP_A2(SCMP_CMP_MASKED_EQ, perm, perm)) != 0)
+            throw SysError("unable to add seccomp rule");
+    }
+
+    if (seccomp_load(ctx) != 0)
+        throw SysError("unable to load seccomp BPF program");
+#endif
+}
+
+
 void DerivationGoal::runChild()
 {
     /* Warning: in the child we should absolutely not make any SQLite
@@ -2307,6 +2344,8 @@ void DerivationGoal::runChild()
 
         commonChildInit(builderOut);
 
+        setupSeccomp();
+
         bool setUser = true;
 
         /* Make the contents of netrc available to builtin:fetchurl