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DerivationBuilderImpl: Add more hardening for writeBuilderFile #15328

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Merged
Ericson2314 merged 1 commit into from
Feb 24, 2026
Merged

DerivationBuilderImpl: Add more hardening for writeBuilderFile #15328

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19 changes: 16 additions & 3 deletions src/libstore/unix/build/derivation-builder.cc
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
#include "nix/store/build/derivation-builder.hh"
#include "nix/util/file-system-at.hh"
#include "nix/util/file-system.hh"
#include "nix/store/local-store.hh"
#include "nix/util/processes.hh"
Expand Down Expand Up @@ -384,11 +385,16 @@ class DerivationBuilderImpl : public DerivationBuilder, public DerivationBuilder

/**
* Make a file owned by the builder addressed by its file descriptor.
*
* @param path Only used for error messages.
*/
void chownToBuilder(int fd, const std::filesystem::path & path);

/**
* Create a file in `tmpDir` owned by the builder.
*
* @param Name must not contain more than one path segment and none of them must be `..`, `.`
* Otherwise this function throws an Error.
*/
void writeBuilderFile(const std::string & name, std::string_view contents);

Expand Down Expand Up @@ -1264,9 +1270,16 @@ void DerivationBuilderImpl::chownToBuilder(int fd, const std::filesystem::path &

void DerivationBuilderImpl::writeBuilderFile(const std::string & name, std::string_view contents)
{
auto path = std::filesystem::path(tmpDir) / name;
AutoCloseFD fd{
openat(tmpDirFd.get(), name.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC | O_EXCL | O_NOFOLLOW, 0666)};
auto relPath = CanonPath(name);
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@xokdvium xokdvium Feb 24, 2026

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It might be worth adding a CanonFilename type to verify this in the constructor or something I suppose.

/* Path must be the same after normalisation. This is an additional sanity check in addition to
existing parsing checks for non-structured attrs exportReferencesGraph. In practice we only expect
a single path component without any `..`, `.` components. */
if (relPath.rel() != name || std::ranges::distance(relPath) != 1)
throw Error("invalid file name for a builder file: '%s'", name);

AutoCloseFD fd = openFileEnsureBeneathNoSymlinks(
tmpDirFd.get(), relPath, O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC | O_EXCL | O_NOFOLLOW, 0666);
auto path = tmpDir / relPath.rel(); /* This is used only for error messages. */
if (!fd)
throw SysError("creating file %s", PathFmt(path));
writeFile(fd.get(), contents);
Expand Down
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