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Fix symlink leak in restricted eval mode

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In EvalState::checkSourcePath, the path is checked against the list of
allowed paths first and later it's checked again *after* resolving
symlinks.

The resolving of the symlinks is done via canonPath, which also strips
out "../" and "./". However after the canonicalisation the error message
pointing out that the path is not allowed prints the symlink target in
the error message.

Even if we'd suppress the message, symlink targets could still be leaked
if the symlink target doesn't exist (in this case the error is thrown in
canonPath).

So instead, we now do canonPath() without symlink resolving first before
even checking against the list of allowed paths and then later do the
symlink resolving and checking the allowed paths again.

The first call to canonPath() should get rid of all the "../" and "./",
so in theory the only way to leak a symlink if the attacker is able to
put a symlink in one of the paths allowed by restricted evaluation mode.

For the latter I don't think this is part of the threat model, because
if the attacker can write to that path, the attack vector is even
larger.

Signed-off-by: aszlig <aszlig@nix.build>
This commit is contained in:
aszlig 2018-08-03 06:24:57 +02:00
parent aa64e95bc8
commit 43e28a1b75
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GPG key ID: 684089CE67EBB691
2 changed files with 21 additions and 4 deletions
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14
src/libexpr/eval.cc
View file

@ -349,19 +349,25 @@ Path EvalState::checkSourcePath(const Path & path_)
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_);
for (auto & i : *allowedPaths) {
if (isDirOrInDir(path_, i)) {
if (isDirOrInDir(abspath, i)) {
found = true;
break;
}
}
if (!found)
throw RestrictedPathError("access to path '%1%' is forbidden in restricted mode", path_);
throw RestrictedPathError("access to path '%1%' is forbidden in restricted mode", abspath);
/* Resolve symlinks. */
debug(format("checking access to '%s'") % path_);
Path path = canonPath(path_, true);
debug(format("checking access to '%s'") % abspath);
Path path = canonPath(abspath, true);
for (auto & i : *allowedPaths) {
if (isDirOrInDir(path, i)) {

11
tests/restricted.sh
View file

@ -38,3 +38,14 @@ ln -sfn $(pwd)/restricted.nix $TEST_ROOT/restricted.nix
nix-instantiate --eval --restrict-eval $TEST_ROOT/restricted.nix -I $TEST_ROOT -I .
[[ $(nix eval --raw --restrict-eval -I . '(builtins.readFile "${import ./simple.nix}/hello")') == 'Hello World!' ]]
# Check whether we can leak symlink information through directory traversal.
traverseDir="$(pwd)/restricted-traverse-me"
ln -sfn "$(pwd)/restricted-secret" "$(pwd)/restricted-innocent"
mkdir -p "$traverseDir"
goUp="..$(echo "$traverseDir" | sed -e 's,[^/]\+,..,g')"
output="$(nix eval --raw --restrict-eval -I "$traverseDir" \
"(builtins.readFile \"$traverseDir/$goUp$(pwd)/restricted-innocent\")" \
2>&1 || :)"
echo "$output" | grep "is forbidden"
! echo "$output" | grep -F restricted-secret