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Merge pull request #9497 from edolstra/move-access-control

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Move restricted/pure-eval access control out of the evaluator and into the accessor
This commit is contained in:
Robert Hensing 2023-12-08 22:21:50 +01:00 • committed by GitHub
commit d4f6b1d38b
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23 changed files with 412 additions and 305 deletions
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106
src/libexpr/eval.cc
View file

@ -14,6 +14,7 @@
#include "profiles.hh"
#include "print.hh"
#include "fs-input-accessor.hh"
#include "filtering-input-accessor.hh"
#include "memory-input-accessor.hh"
#include "signals.hh"
#include "gc-small-vector.hh"
@ -509,7 +510,16 @@ EvalState::EvalState(
, sOutputSpecified(symbols.create("outputSpecified"))
, repair(NoRepair)
, emptyBindings(0)
, rootFS(makeFSInputAccessor(CanonPath::root))
, rootFS(
evalSettings.restrictEval || evalSettings.pureEval
? ref<InputAccessor>(AllowListInputAccessor::create(makeFSInputAccessor(CanonPath::root), {},
[](const CanonPath & path) -> RestrictedPathError {
auto modeInformation = evalSettings.pureEval
? "in pure evaluation mode (use '--impure' to override)"
: "in restricted mode";
throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", path, modeInformation);
}))
: makeFSInputAccessor(CanonPath::root))
, corepkgsFS(makeMemoryInputAccessor())
, internalFS(makeMemoryInputAccessor())
, derivationInternal{corepkgsFS->addFile(
@ -551,28 +561,10 @@ EvalState::EvalState(
searchPath.elements.emplace_back(SearchPath::Elem::parse(i));
}
if (evalSettings.restrictEval || evalSettings.pureEval) {
allowedPaths = PathSet();
for (auto & i : searchPath.elements) {
auto r = resolveSearchPathPath(i.path);
if (!r) continue;
auto path = std::move(*r);
if (store->isInStore(path)) {
try {
StorePathSet closure;
store->computeFSClosure(store->toStorePath(path).first, closure);
for (auto & path : closure)
allowPath(path);
} catch (InvalidPath &) {
allowPath(path);
}
} else
allowPath(path);
}
}
/* Allow access to all paths in the search path. */
if (rootFS.dynamic_pointer_cast<AllowListInputAccessor>())
for (auto & i : searchPath.elements)
resolveSearchPathPath(i.path, true);
corepkgsFS->addFile(
CanonPath("fetchurl.nix"),
@ -590,14 +582,14 @@ EvalState::~EvalState()
void EvalState::allowPath(const Path & path)
{
if (allowedPaths)
allowedPaths->insert(path);
if (auto rootFS2 = rootFS.dynamic_pointer_cast<AllowListInputAccessor>())
rootFS2->allowPath(CanonPath(path));
}
void EvalState::allowPath(const StorePath & storePath)
{
if (allowedPaths)
allowedPaths->insert(store->toRealPath(storePath));
if (auto rootFS2 = rootFS.dynamic_pointer_cast<AllowListInputAccessor>())
rootFS2->allowPath(CanonPath(store->toRealPath(storePath)));
}
void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value & v)
@ -607,54 +599,6 @@ void EvalState::allowAndSetStorePathString(const StorePath & storePath, Value &
mkStorePathString(storePath, v);
}
SourcePath EvalState::checkSourcePath(const SourcePath & path_)
{
// Don't check non-rootFS accessors, they're in a different namespace.
if (path_.accessor != ref<InputAccessor>(rootFS)) return path_;
if (!allowedPaths) return path_;
auto i = resolvedPaths.find(path_.path.abs());
if (i != resolvedPaths.end())
return i->second;
bool found = false;
/* First canonicalize the path without symlinks, so we make sure an
* attacker can't append ../../... to a path that would be in allowedPaths
* and thus leak symlink targets.
*/
Path abspath = canonPath(path_.path.abs());
for (auto & i : *allowedPaths) {
if (isDirOrInDir(abspath, i)) {
found = true;
break;
}
}
if (!found) {
auto modeInformation = evalSettings.pureEval
? "in pure eval mode (use '--impure' to override)"
: "in restricted mode";
throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", abspath, modeInformation);
}
/* Resolve symlinks. */
debug("checking access to '%s'", abspath);
SourcePath path = rootPath(CanonPath(canonPath(abspath, true)));
for (auto & i : *allowedPaths) {
if (isDirOrInDir(path.path.abs(), i)) {
resolvedPaths.insert_or_assign(path_.path.abs(), path);
return path;
}
}
throw RestrictedPathError("access to canonical path '%1%' is forbidden in restricted mode", path);
}
void EvalState::checkURI(const std::string & uri)
{
if (!evalSettings.restrictEval) return;
@ -674,12 +618,14 @@ void EvalState::checkURI(const std::string & uri)
/* If the URI is a path, then check it against allowedPaths as
well. */
if (hasPrefix(uri, "/")) {
checkSourcePath(rootPath(CanonPath(uri)));
if (auto rootFS2 = rootFS.dynamic_pointer_cast<AllowListInputAccessor>())
rootFS2->checkAccess(CanonPath(uri));
return;
}
if (hasPrefix(uri, "file://")) {
checkSourcePath(rootPath(CanonPath(std::string(uri, 7))));
if (auto rootFS2 = rootFS.dynamic_pointer_cast<AllowListInputAccessor>())
rootFS2->checkAccess(CanonPath(uri.substr(7)));
return;
}
@ -1181,10 +1127,8 @@ Value * ExprPath::maybeThunk(EvalState & state, Env & env)
}
void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial)
void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
{
auto path = checkSourcePath(path_);
FileEvalCache::iterator i;
if ((i = fileEvalCache.find(path)) != fileEvalCache.end()) {
v = i->second;
@ -1205,7 +1149,7 @@ void EvalState::evalFile(const SourcePath & path_, Value & v, bool mustBeTrivial
e = j->second;
if (!e)
e = parseExprFromFile(checkSourcePath(resolvedPath));
e = parseExprFromFile(resolvedPath);
fileParseCache[resolvedPath] = e;

28
src/libexpr/eval.hh
View file

@ -30,7 +30,6 @@ class EvalState;
class StorePath;
struct SingleDerivedPath;
enum RepairFlag : bool;
struct FSInputAccessor;
struct MemoryInputAccessor;
@ -217,18 +216,12 @@ public:
*/
RepairFlag repair;
/**
* The allowed filesystem paths in restricted or pure evaluation
* mode.
*/
std::optional<PathSet> allowedPaths;
Bindings emptyBindings;
/**
* The accessor for the root filesystem.
*/
const ref<FSInputAccessor> rootFS;
const ref<InputAccessor> rootFS;
/**
* The in-memory filesystem for <nix/...> paths.
@ -396,12 +389,6 @@ public:
*/
void allowAndSetStorePathString(const StorePath & storePath, Value & v);
/**
* Check whether access to a path is allowed and throw an error if
* not. Otherwise return the canonicalised path.
*/
SourcePath checkSourcePath(const SourcePath & path);
void checkURI(const std::string & uri);
/**
@ -445,13 +432,15 @@ public:
SourcePath findFile(const SearchPath & searchPath, const std::string_view path, const PosIdx pos = noPos);
/**
* Try to resolve a search path value (not the optional key part)
* Try to resolve a search path value (not the optional key part).
*
* If the specified search path element is a URI, download it.
*
* If it is not found, return `std::nullopt`
*/
std::optional<std::string> resolveSearchPathPath(const SearchPath::Path & path);
std::optional<std::string> resolveSearchPathPath(
const SearchPath::Path & elem,
bool initAccessControl = false);
/**
* Evaluate an expression to normal form
@ -756,6 +745,13 @@ public:
*/
[[nodiscard]] StringMap realiseContext(const NixStringContext & context);
/* Call the binary path filter predicate used builtins.path etc. */
bool callPathFilter(
Value * filterFun,
const SourcePath & path,
std::string_view pathArg,
PosIdx pos);
private:
/**

30
src/libexpr/parser.y
View file

@ -692,16 +692,17 @@ SourcePath resolveExprPath(SourcePath path)
/* If `path' is a symlink, follow it. This is so that relative
path references work. */
while (true) {
while (!path.path.isRoot()) {
// Basic cycle/depth limit to avoid infinite loops.
if (++followCount >= maxFollow)
throw Error("too many symbolic links encountered while traversing the path '%s'", path);
if (path.lstat().type != InputAccessor::tSymlink) break;
path = {path.accessor, CanonPath(path.readLink(), path.path.parent().value_or(CanonPath::root))};
auto p = path.parent().resolveSymlinks() + path.baseName();
if (p.lstat().type != InputAccessor::tSymlink) break;
path = {path.accessor, CanonPath(p.readLink(), path.path.parent().value_or(CanonPath::root))};
}
/* If `path' refers to a directory, append `/default.nix'. */
if (path.lstat().type == InputAccessor::tDirectory)
if (path.resolveSymlinks().lstat().type == InputAccessor::tDirectory)
return path + "default.nix";
return path;
@ -716,7 +717,7 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path)
Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv)
{
auto buffer = path.readFile();
auto buffer = path.resolveSymlinks().readFile();
// readFile hopefully have left some extra space for terminators
buffer.append("\0\0", 2);
return parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
@ -783,7 +784,7 @@ SourcePath EvalState::findFile(const SearchPath & searchPath, const std::string_
}
std::optional<std::string> EvalState::resolveSearchPathPath(const SearchPath::Path & value0)
std::optional<std::string> EvalState::resolveSearchPathPath(const SearchPath::Path & value0, bool initAccessControl)
{
auto & value = value0.s;
auto i = searchPathResolved.find(value);
@ -800,7 +801,6 @@ std::optional<std::string> EvalState::resolveSearchPathPath(const SearchPath::Pa
logWarning({
.msg = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)
});
res = std::nullopt;
}
}
@ -814,6 +814,20 @@ std::optional<std::string> EvalState::resolveSearchPathPath(const SearchPath::Pa
else {
auto path = absPath(value);
/* Allow access to paths in the search path. */
if (initAccessControl) {
allowPath(path);
if (store->isInStore(path)) {
try {
StorePathSet closure;
store->computeFSClosure(store->toStorePath(path).first, closure);
for (auto & p : closure)
allowPath(p);
} catch (InvalidPath &) { }
}
}
if (pathExists(path))
res = { path };
else {
@ -829,7 +843,7 @@ std::optional<std::string> EvalState::resolveSearchPathPath(const SearchPath::Pa
else
debug("failed to resolve search path element '%s'", value);
searchPathResolved[value] = res;
searchPathResolved.emplace(value, res);
return res;
}

140
src/libexpr/primops.cc
View file

@ -15,6 +15,7 @@
#include "value-to-json.hh"
#include "value-to-xml.hh"
#include "primops.hh"
#include "fs-input-accessor.hh"
#include <boost/container/small_vector.hpp>
#include <nlohmann/json.hpp>
@ -90,9 +91,8 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
for (auto & [outputName, outputPath] : outputs) {
/* Add the output of this derivations to the allowed
paths. */
if (allowedPaths) {
allowPath(outputPath);
}
allowPath(store->toRealPath(outputPath));
/* Get all the output paths corresponding to the placeholders we had */
if (experimentalFeatureSettings.isEnabled(Xp::CaDerivations)) {
res.insert_or_assign(
@ -110,27 +110,19 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
return res;
}
struct RealisePathFlags {
// Whether to check that the path is allowed in pure eval mode
bool checkForPureEval = true;
};
static SourcePath realisePath(EvalState & state, const PosIdx pos, Value & v, const RealisePathFlags flags = {})
static SourcePath realisePath(EvalState & state, const PosIdx pos, Value & v, bool resolveSymlinks = true)
{
NixStringContext context;
auto path = state.coerceToPath(noPos, v, context, "while realising the context of a path");
try {
if (!context.empty()) {
if (!context.empty() && path.accessor == state.rootFS) {
auto rewrites = state.realiseContext(context);
auto realPath = state.toRealPath(rewriteStrings(path.path.abs(), rewrites), context);
return {path.accessor, CanonPath(realPath)};
path = {path.accessor, CanonPath(realPath)};
}
return flags.checkForPureEval
? state.checkSourcePath(path)
: path;
return resolveSymlinks ? path.resolveSymlinks() : path;
} catch (Error & e) {
e.addTrace(state.positions[pos], "while realising the context of path '%s'", path);
throw;
@ -170,7 +162,7 @@ static void mkOutputString(
argument. */
static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * vScope, Value & v)
{
auto path = realisePath(state, pos, vPath);
auto path = realisePath(state, pos, vPath, false);
auto path2 = path.path.abs();
// FIXME
@ -1493,7 +1485,7 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
}));
NixStringContext context;
auto path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to 'builtins.storePath'")).path;
auto path = state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to 'builtins.storePath'").path;
/* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
directly in the store. The latter condition is necessary so
e.g. nix-push does the right thing. */
@ -1533,29 +1525,19 @@ static RegisterPrimOp primop_storePath({
static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args, Value & v)
{
auto & arg = *args[0];
/* We don’t check the path right now, because we don’t want to
throw if the path isn’t allowed, but just return false (and we
can’t just catch the exception here because we still want to
throw if something in the evaluation of `arg` tries to
access an unauthorized path). */
auto path = realisePath(state, pos, arg, { .checkForPureEval = false });
/* SourcePath doesn't know about trailing slash. */
auto mustBeDir = arg.type() == nString
&& (arg.string_view().ends_with("/")
|| arg.string_view().ends_with("/."));
try {
auto checked = state.checkSourcePath(path);
auto st = checked.maybeLstat();
auto & arg = *args[0];
auto path = realisePath(state, pos, arg);
/* SourcePath doesn't know about trailing slash. */
auto mustBeDir = arg.type() == nString
&& (arg.string_view().ends_with("/")
|| arg.string_view().ends_with("/."));
auto st = path.maybeLstat();
auto exists = st && (!mustBeDir || st->type == SourceAccessor::tDirectory);
v.mkBool(exists);
} catch (SysError & e) {
/* Don't give away info from errors while canonicalising
‘path’ in restricted mode. */
v.mkBool(false);
} catch (RestrictedPathError & e) {
v.mkBool(false);
}
@ -1699,7 +1681,7 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
auto path = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.findFile");
v.mkPath(state.checkSourcePath(state.findFile(searchPath, path, pos)));
v.mkPath(state.findFile(searchPath, path, pos));
}
static RegisterPrimOp primop_findFile(PrimOp {
@ -1789,7 +1771,7 @@ static std::string_view fileTypeToString(InputAccessor::Type type)
static void prim_readFileType(EvalState & state, const PosIdx pos, Value * * args, Value & v)
{
auto path = realisePath(state, pos, *args[0]);
auto path = realisePath(state, pos, *args[0], false);
/* Retrieve the directory entry type and stringize it. */
v.mkString(fileTypeToString(path.lstat().type));
}
@ -2178,11 +2160,35 @@ static RegisterPrimOp primop_toFile({
.fun = prim_toFile,
});
bool EvalState::callPathFilter(
Value * filterFun,
const SourcePath & path,
std::string_view pathArg,
PosIdx pos)
{
auto st = path.lstat();
/* Call the filter function. The first argument is the path, the
second is a string indicating the type of the file. */
Value arg1;
arg1.mkString(pathArg);
Value arg2;
// assert that type is not "unknown"
arg2.mkString(fileTypeToString(st.type));
Value * args []{&arg1, &arg2};
Value res;
callFunction(*filterFun, 2, args, res, pos);
return forceBool(res, pos, "while evaluating the return value of the path filter function");
}
static void addPath(
EvalState & state,
const PosIdx pos,
std::string_view name,
Path path,
SourcePath path,
Value * filterFun,
FileIngestionMethod method,
const std::optional<Hash> expectedHash,
@ -2190,48 +2196,29 @@ static void addPath(
const NixStringContext & context)
{
try {
// FIXME: handle CA derivation outputs (where path needs to
// be rewritten to the actual output).
auto rewrites = state.realiseContext(context);
path = state.toRealPath(rewriteStrings(path, rewrites), context);
StorePathSet refs;
if (state.store->isInStore(path)) {
if (path.accessor == state.rootFS && state.store->isInStore(path.path.abs())) {
// FIXME: handle CA derivation outputs (where path needs to
// be rewritten to the actual output).
auto rewrites = state.realiseContext(context);
path = {state.rootFS, CanonPath(state.toRealPath(rewriteStrings(path.path.abs(), rewrites), context))};
try {
auto [storePath, subPath] = state.store->toStorePath(path);
auto [storePath, subPath] = state.store->toStorePath(path.path.abs());
// FIXME: we should scanForReferences on the path before adding it
refs = state.store->queryPathInfo(storePath)->references;
path = state.store->toRealPath(storePath) + subPath;
path = {state.rootFS, CanonPath(state.store->toRealPath(storePath) + subPath)};
} catch (Error &) { // FIXME: should be InvalidPathError
}
}
path = evalSettings.pureEval && expectedHash
? path
: state.checkSourcePath(state.rootPath(CanonPath(path))).path.abs();
PathFilter filter = filterFun ? ([&](const Path & path) {
auto st = lstat(path);
/* Call the filter function. The first argument is the path,
the second is a string indicating the type of the file. */
Value arg1;
arg1.mkString(path);
Value arg2;
arg2.mkString(
S_ISREG(st.st_mode) ? "regular" :
S_ISDIR(st.st_mode) ? "directory" :
S_ISLNK(st.st_mode) ? "symlink" :
"unknown" /* not supported, will fail! */);
Value * args []{&arg1, &arg2};
Value res;
state.callFunction(*filterFun, 2, args, res, pos);
return state.forceBool(res, pos, "while evaluating the return value of the path filter function");
}) : defaultPathFilter;
std::unique_ptr<PathFilter> filter;
if (filterFun)
filter = std::make_unique<PathFilter>([&](const Path & p) {
auto p2 = CanonPath(p);
return state.callPathFilter(filterFun, {path.accessor, p2}, p2.abs(), pos);
});
std::optional<StorePath> expectedStorePath;
if (expectedHash)
@ -2242,7 +2229,7 @@ static void addPath(
});
if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
auto dstPath = state.rootPath(CanonPath(path)).fetchToStore(state.store, name, method, &filter, state.repair);
auto dstPath = path.fetchToStore(state.store, name, method, filter.get(), state.repair);
if (expectedHash && expectedStorePath != dstPath)
state.debugThrowLastTrace(Error("store path mismatch in (possibly filtered) path added from '%s'", path));
state.allowAndSetStorePathString(dstPath, v);
@ -2261,7 +2248,8 @@ static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * arg
auto path = state.coerceToPath(pos, *args[1], context,
"while evaluating the second argument (the path to filter) passed to 'builtins.filterSource'");
state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filterSource");
addPath(state, pos, path.baseName(), path.path.abs(), args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
addPath(state, pos, path.baseName(), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
}
static RegisterPrimOp primop_filterSource({
@ -2356,7 +2344,7 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
if (name.empty())
name = path->baseName();
addPath(state, pos, name, path->path.abs(), filterFun, method, expectedHash, v, context);
addPath(state, pos, name, *path, filterFun, method, expectedHash, v, context);
}
static RegisterPrimOp primop_path({

7
src/libexpr/value.hh
View file

@ -423,10 +423,9 @@ public:
SourcePath path() const
{
assert(internalType == tPath);
return SourcePath {
.accessor = ref(_path.accessor->shared_from_this()),
.path = CanonPath(CanonPath::unchecked_t(), _path.path)
};
return SourcePath(
ref(_path.accessor->shared_from_this()),
CanonPath(CanonPath::unchecked_t(), _path.path));
}
std::string_view string_view() const