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Use high FD numbers in the forkserver to avoid collision with FDs mapped by the user

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PiperOrigin-RevId: 242106285
Change-Id: I0f4bd130f8e66e6b47ad1d7311e0fff519aa9e90
This commit is contained in:
Kevin Hamacher 2019-04-05 04:51:20 -07:00 committed by Copybara-Service
parent 6a65e63eae
commit 77ad64ac30
2 changed files with 29 additions and 8 deletions
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33
sandboxed_api/sandbox2/forkserver.cc
View File

@ -57,6 +57,25 @@
#include "sandboxed_api/sandbox2/util/strerror.h"
#include "sandboxed_api/util/raw_logging.h"
namespace {
// "Moves" the old FD to the new FD number.
// The old FD will be closed, the new one is marked as CLOEXEC.
void MoveToFdNumber(int* old_fd, int new_fd) {
if (dup2(*old_fd, new_fd) == -1) {
SAPI_RAW_PLOG(FATAL, "Moving temporary to proper FD failed.");
}
close(*old_fd);
// Try to mark that FD as CLOEXEC.
int flags = fcntl(new_fd, F_GETFD);
if (flags == -1 || fcntl(new_fd, F_SETFD, flags | O_CLOEXEC) != 0) {
SAPI_RAW_PLOG(ERROR, "Marking FD as CLOEXEC failed");
}
*old_fd = new_fd;
}
} // namespace
namespace sandbox2 {
pid_t ForkClient::SendRequest(const ForkRequest& request, int exec_fd,
@ -314,6 +333,10 @@ void ForkServer::LaunchChild(const ForkRequest& request, int execve_fd,
}
pid_t ForkServer::ServeRequest() const {
// Keep the low FD numbers clean so that client FD mappings don't interfer
// with us.
constexpr int kTargetExecFd = 1022;
ForkRequest fork_request;
if (!comms_->RecvProtoBuf(&fork_request)) {
if (comms_->IsTerminated()) {
@ -334,6 +357,9 @@ pid_t ForkServer::ServeRequest() const {
if (!comms_->RecvFD(&exec_fd)) {
SAPI_RAW_LOG(FATAL, "Failed to receive Exec FD");
}
// We're duping to a high number here to avoid colliding with the IPC FDs.
MoveToFdNumber(&exec_fd, kTargetExecFd);
}
// Make the kernel notify us with SIGCHLD when the process terminates.
@ -381,12 +407,6 @@ pid_t ForkServer::ServeRequest() const {
if (sandboxee_pid == 0) {
LaunchChild(fork_request, exec_fd, comms_fd, uid, gid, user_ns_fd,
fd_closer1.get());
// comms_fd has been remapped to 1023 (kSandbox2ClientCommsFD), so the
// original FD is not required anymore.
if (comms_fd != Comms::kSandbox2ClientCommsFD) {
close(comms_fd);
}
return sandboxee_pid;
}
@ -483,6 +503,7 @@ void ForkServer::SanitizeEnvironment(int client_fd) {
// Duplicate client's CommsFD onto fd=Comms::kSandbox2ClientCommsFD (1023).
SAPI_RAW_CHECK(dup2(client_fd, Comms::kSandbox2ClientCommsFD) != -1,
"while remapping client comms fd");
close(client_fd);
// Mark all file descriptors, except the standard ones (needed
// for proper sandboxed process operations), as close-on-exec.
if (!sanitizer::SanitizeCurrentProcess(

4
sandboxed_api/sandbox2/global_forkclient.cc
View File

@ -87,8 +87,8 @@ static void StartGlobalForkServer() {
// Make sure the logs go stderr.
google::LogToStderr();
// Child.
close(sv[1]);
// Close all non-essential FDs to keep newly opened FD numbers consistent.
sandbox2::sanitizer::CloseAllFDsExcept({0, 1, 2, sv[0]});
// Make the process' name easily recognizable with ps/pstree.
if (prctl(PR_SET_NAME, "S2-FORK-SERV", 0, 0, 0) != 0) {