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Security: Don't allow builders to change permissions on files they do…

…n't own

It turns out that in multi-user Nix, a builder may be able to do

  ln /etc/shadow $out/foo

Afterwards, canonicalisePathMetaData() will be applied to $out/foo,
causing /etc/shadow's mode to be set to 444 (readable by everybody but
writable by nobody).  That's obviously Very Bad.

Fortunately, this fails in NixOS's default configuration because
/nix/store is a bind mount, so "ln" will fail with "Invalid
cross-device link".  It also fails if hard-link restrictions are
enabled, so a workaround is:

  echo 1 > /proc/sys/fs/protected_hardlinks

The solution is to check that all files in $out are owned by the build
user.  This means that innocuous operations like "ln
${pkgs.foo}/some-file $out/" are now rejected, but that already failed
in chroot builds anyway.
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edolstra committed Feb 26, 2013
1 parent dadf7a5 commit 5526a282b5b44e9296e61e07d7d2626a79141ac4
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@@ -2287,7 +2287,7 @@ void DerivationGoal::computeClosure()
}
/* Get rid of all weird permissions. */
canonicalisePathMetaData(path);
canonicalisePathMetaData(path, buildUser.enabled() ? buildUser.getUID() : -1);
/* For this output path, find the references to other paths
contained in it. Compute the SHA-256 NAR hash at the same
@@ -2839,7 +2839,7 @@ void SubstitutionGoal::finished()
return;
}
canonicalisePathMetaData(destPath);
canonicalisePathMetaData(destPath, -1);
worker.store.optimisePath(destPath); // FIXME: combine with hashPath()
@@ -465,7 +465,7 @@ void LocalStore::makeStoreWritable()
const time_t mtimeStore = 1; /* 1 second into the epoch */
void canonicalisePathMetaData(const Path & path, bool recurse)
void canonicalisePathMetaData(const Path & path, bool recurse, uid_t fromUid)
{
checkInterrupt();
@@ -477,6 +477,9 @@ void canonicalisePathMetaData(const Path & path, bool recurse)
has already been checked in dumpPath(). */
assert(S_ISREG(st.st_mode) || S_ISDIR(st.st_mode) || S_ISLNK(st.st_mode));
if (fromUid != (uid_t) -1 && st.st_uid != fromUid)
throw BuildError(format("invalid ownership on file `%1%'") % path);
/* Change ownership to the current uid. If it's a symlink, use
lchown if available, otherwise don't bother. Wrong ownership
of a symlink doesn't matter, since the owning user can't change
@@ -529,14 +532,14 @@ void canonicalisePathMetaData(const Path & path, bool recurse)
if (recurse && S_ISDIR(st.st_mode)) {
Strings names = readDirectory(path);
foreach (Strings::iterator, i, names)
canonicalisePathMetaData(path + "/" + *i, true);
canonicalisePathMetaData(path + "/" + *i, true, fromUid);
}
}
void canonicalisePathMetaData(const Path & path)
void canonicalisePathMetaData(const Path & path, uid_t fromUid)
{
canonicalisePathMetaData(path, true);
canonicalisePathMetaData(path, true, fromUid);
/* On platforms that don't have lchown(), the top-level path can't
be a symlink, since we can't change its ownership. */
@@ -1198,7 +1201,7 @@ Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
} else
writeFile(dstPath, dump);
canonicalisePathMetaData(dstPath);
canonicalisePathMetaData(dstPath, -1);
/* Register the SHA-256 hash of the NAR serialisation of
the path in the database. We may just have computed it
@@ -1263,7 +1266,7 @@ Path LocalStore::addTextToStore(const string & name, const string & s,
writeFile(dstPath, s);
canonicalisePathMetaData(dstPath);
canonicalisePathMetaData(dstPath, -1);
HashResult hash = hashPath(htSHA256, dstPath);
@@ -1498,7 +1501,7 @@ Path LocalStore::importPath(bool requireSignature, Source & source)
throw SysError(format("cannot move `%1%' to `%2%'")
% unpacked % dstPath);
canonicalisePathMetaData(dstPath);
canonicalisePathMetaData(dstPath, -1);
/* !!! if we were clever, we could prevent the hashPath()
here. */
@@ -307,9 +307,9 @@ private:
without execute permission; setuid bits etc. are cleared)
- the owner and group are set to the Nix user and group, if we're
in a setuid Nix installation. */
void canonicalisePathMetaData(const Path & path);
void canonicalisePathMetaData(const Path & path, uid_t fromUid);
void canonicalisePathMetaData(const Path & path, bool recurse);
void canonicalisePathMetaData(const Path & path, bool recurse, uid_t fromUid);
MakeError(PathInUse, Error);
@@ -32,7 +32,7 @@ struct MakeReadOnly
{
try {
/* This will make the path read-only. */
if (path != "") canonicalisePathMetaData(path, false);
if (path != "") canonicalisePathMetaData(path, false, -1);
} catch (...) {
ignoreException();
}
View
@@ -238,20 +238,14 @@ static void printTree(const Path & path,
PathSet references;
store->queryReferences(path, references);
#if 0
for (PathSet::iterator i = drv.inputSrcs.begin();
i != drv.inputSrcs.end(); ++i)
cout << format("%1%%2%\n") % (tailPad + treeConn) % *i;
#endif
/* Topologically sort under the relation A < B iff A \in
closure(B). That is, if derivation A is an (possibly indirect)
input of B, then A is printed first. This has the effect of
flattening the tree, preventing deeply nested structures. */
Paths sorted = topoSortPaths(*store, references);
reverse(sorted.begin(), sorted.end());
for (Paths::iterator i = sorted.begin(); i != sorted.end(); ++i) {
foreach (Paths::iterator, i, sorted) {
Paths::iterator j = i; ++j;
printTree(*i, tailPad + treeConn,
j == sorted.end() ? tailPad + treeNull : tailPad + treeLine,
@@ -521,7 +515,7 @@ static void registerValidity(bool reregister, bool hashGiven, bool canonicalise)
if (!store->isValidPath(info.path) || reregister) {
/* !!! races */
if (canonicalise)
canonicalisePathMetaData(info.path);
canonicalisePathMetaData(info.path, -1);
if (!hashGiven) {
HashResult hash = hashPath(htSHA256, info.path);
info.hash = hash.first;

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