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Linux sandbox: Use /build instead of /tmp as $TMPDIR

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There is a security issue when a build accidentally stores its $TMPDIR
in some critical place, such as an RPATH. If
TMPDIR=/tmp/nix-build-..., then any user on the system can recreate
that directory and inject libraries into the RPATH of programs
executed by other users. Since /build probably doesn't exist (or isn't
world-writable), this mitigates the issue.
This commit is contained in:
Eelco Dolstra 2017-05-04 16:57:03 +02:00
parent 2da6a42448
commit eba840c8a1
No known key found for this signature in database
GPG key ID: 8170B4726D7198DE
1 changed files with 15 additions and 5 deletions
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20
src/libstore/build.cc
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@ -1661,6 +1661,9 @@ int childEntry(void * arg)
} }
const std::string buildDir = "/build";
void DerivationGoal::startBuilder() void DerivationGoal::startBuilder()
{ {
auto f = format( auto f = format(
@ -1721,7 +1724,14 @@ void DerivationGoal::startBuilder()
/* In a sandbox, for determinism, always use the same temporary /* In a sandbox, for determinism, always use the same temporary
directory. */ directory. */
#if __linux__
tmpDirInSandbox = useChroot ? buildDir : tmpDir;
#elif __APPLE__
// On Darwin, we canonize /tmp because its probably a symlink to /private/tmp.
tmpDirInSandbox = useChroot ? canonPath("/tmp", true) + "/nix-build-" + drvName + "-0" : tmpDir; tmpDirInSandbox = useChroot ? canonPath("/tmp", true) + "/nix-build-" + drvName + "-0" : tmpDir;
#else
tmpDirInSandbox = tmpDir;
#endif
chownToBuilder(tmpDir); chownToBuilder(tmpDir);
/* Substitute output placeholders with the actual output paths. */ /* Substitute output placeholders with the actual output paths. */
@ -1829,11 +1839,11 @@ void DerivationGoal::startBuilder()
Samba-in-QEMU. */ Samba-in-QEMU. */
createDirs(chrootRootDir + "/etc"); createDirs(chrootRootDir + "/etc");
writeFile(chrootRootDir + "/etc/passwd", writeFile(chrootRootDir + "/etc/passwd", fmt(
(format( "root:x:0:0:Nix build user:%3%:/noshell\n"
"root:x:0:0:Nix build user:/:/noshell\n" "nixbld:x:%1%:%2%:Nix build user:%3%:/noshell\n"
"nixbld:x:%1%:%2%:Nix build user:/:/noshell\n" "nobody:x:65534:65534:Nobody:/:/noshell\n",
"nobody:x:65534:65534:Nobody:/:/noshell\n") % sandboxUid % sandboxGid).str()); sandboxUid, sandboxGid, buildDir));
/* Declare the build user's group so that programs get a consistent /* Declare the build user's group so that programs get a consistent
view of the system (e.g., "id -gn"). */ view of the system (e.g., "id -gn"). */