Subject: [PATCH v2 3/9] daemon: Remount inputs as read-only.

From: Ludovic Courtès <[email-masked]>
Date: Thu, 13 Feb 2025 13:13:06 +0100

* nix/libstore/build.cc (DerivationGoal::runChild): Remount ‘target’ as
read-only.

Reported-by: Reepca Russelstein <[email-masked]>
Change-Id: Ib7201bcf4363be566f205d23d17fe2f55d3ad666
---
 nix/libstore/build.cc | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index f4cd2131c84..7151bb6c6f1 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -2094,8 +2094,15 @@ void DerivationGoal::runChild()
                     createDirs(dirOf(target));
                     writeFile(target, "");
                 }
+
+		/* Extra flags passed with MS_BIND are ignored, hence the
+		   extra MS_REMOUNT.  */
                 if (mount(source.c_str(), target.c_str(), "", MS_BIND, 0) == -1)
                     throw SysError(format("bind mount from `%1%' to `%2%' failed") % source % target);
+		if (source != tmpDir) {
+		     if (mount(source.c_str(), target.c_str(), "", MS_BIND | MS_REMOUNT | MS_RDONLY, 0) == -1)
+			  throw SysError(format("read-only remount of `%1%' failed") % target);
+		}
             }
 
             /* Bind a new instance of procfs on /proc to reflect our
-- 
2.48.1