From 28ba2e8f3eeee6bd81e5af431d79a612f85e11bf Mon Sep 17 00:00:00 2001 From: dec05eba Date: Sun, 6 Oct 2024 22:26:47 +0200 Subject: gsr-kms-server 'security': only allow gpu-screen-recorder to get framebuffer --- kms/client/kms_client.c | 83 ++++++++++++++++++++++++-------------------- kms/server/kms_server.c | 92 ++++++++++++++++++++++++++++++++++++++++++++----- 2 files changed, 129 insertions(+), 46 deletions(-) (limited to 'kms') diff --git a/kms/client/kms_client.c b/kms/client/kms_client.c index 468e3a6..b72c64b 100644 --- a/kms/client/kms_client.c +++ b/kms/client/kms_client.c @@ -146,47 +146,41 @@ static bool create_socket_path(char *output_path, size_t output_path_size) { return true; } -static void string_copy(char *dst, const char *src, int len) { - int src_len = strlen(src); - int min_len = src_len; - if(len - 1 < min_len) - min_len = len - 1; - memcpy(dst, src, min_len); - dst[min_len] = '\0'; -} - -static bool find_program_in_path(const char *program_name, char *filepath, int filepath_len) { - const char *path = getenv("PATH"); - if(!path) +static bool readlink_realpath(const char *filepath, char *buffer) { + char symlinked_path[PATH_MAX]; + ssize_t bytes_written = readlink(filepath, symlinked_path, sizeof(symlinked_path) - 1); + if(bytes_written == -1 && errno == EINVAL) { + /* Not a symlink */ + snprintf(symlinked_path, sizeof(symlinked_path), "%s", filepath); + } else if(bytes_written == -1) { return false; + } else { + symlinked_path[bytes_written] = '\0'; + } - int program_name_len = strlen(program_name); - const char *end = path + strlen(path); - while(path != end) { - const char *part_end = strchr(path, ':'); - const char *next = part_end; - if(part_end) { - next = part_end + 1; - } else { - part_end = end; - next = end; - } + if(!realpath(symlinked_path, buffer)) + return false; - int len = part_end - path; - if(len + 1 + program_name_len < filepath_len) { - memcpy(filepath, path, len); - filepath[len] = '/'; - memcpy(filepath + len + 1, program_name, program_name_len); - filepath[len + 1 + program_name_len] = '\0'; + return true; +} - if(access(filepath, F_OK) == 0) - return true; - } +static bool strcat_safe(char *str, int size, const char *str_to_add) { + const int str_len = strlen(str); + const int str_to_add_len = strlen(str_to_add); + if(str_len + str_to_add_len + 1 >= size) + return false; - path = next; - } + memcpy(str + str_len, str_to_add, str_to_add_len); + str[str_len + str_to_add_len] = '\0'; + return true; +} - return false; +static void file_get_directory(char *filepath) { + char *end = strrchr(filepath, '/'); + if(end == NULL) + filepath[0] = '\0'; + else + *end = '\0'; } int gsr_kms_client_init(gsr_kms_client *self, const char *card_path) { @@ -206,11 +200,24 @@ int gsr_kms_client_init(gsr_kms_client *self, const char *card_path) { } char server_filepath[PATH_MAX]; - if(!find_program_in_path("gsr-kms-server", server_filepath, sizeof(server_filepath))) { - fprintf(stderr, "gsr error: gsr_kms_client_init: gsr-kms-server is not installed\n"); + if(!readlink_realpath("/proc/self/exe", server_filepath)) { + fprintf(stderr, "gsr error: gsr_kms_client_init: failed to resolve /proc/self/exe\n"); + return -1; + } + file_get_directory(server_filepath); + + if(!strcat_safe(server_filepath, sizeof(server_filepath), "/gsr-kms-server")) { + fprintf(stderr, "gsr error: gsr_kms_client_init: gsr-kms-server path too long\n"); return -1; } + if(access(server_filepath, F_OK) != 0) { + fprintf(stderr, "gsr error: gsr_kms_client_init: gsr-kms-server is not installed (%s not found)\n", server_filepath); + return -1; + } + + fprintf(stderr, "gsr info: gsr_kms_client_init: setting up connection to %s\n", server_filepath); + const bool inside_flatpak = getenv("FLATPAK_ID") != NULL; const char *home = getenv("HOME"); if(!home) @@ -251,7 +258,7 @@ int gsr_kms_client_init(gsr_kms_client *self, const char *card_path) { } local_addr.sun_family = AF_UNIX; - string_copy(local_addr.sun_path, self->initial_socket_path, sizeof(local_addr.sun_path)); + snprintf(local_addr.sun_path, sizeof(local_addr.sun_path), "%s", (const char*)self->initial_socket_path); const mode_t prev_mask = umask(0000); const int bind_res = bind(self->initial_socket_fd, (struct sockaddr*)&local_addr, sizeof(local_addr.sun_family) + strlen(local_addr.sun_path)); diff --git a/kms/server/kms_server.c b/kms/server/kms_server.c index c6460ad..53770b4 100644 --- a/kms/server/kms_server.c +++ b/kms/server/kms_server.c @@ -1,3 +1,7 @@ +#ifndef _GNU_SOURCE +#define _GNU_SOURCE +#endif + #include "../kms_shared.h" #include @@ -6,6 +10,7 @@ #include #include +#include #include #include #include @@ -413,13 +418,79 @@ static double clock_get_monotonic_seconds(void) { return (double)ts.tv_sec + (double)ts.tv_nsec * 0.000000001; } -static void string_copy(char *dst, const char *src, int len) { - int src_len = strlen(src); - int min_len = src_len; - if(len - 1 < min_len) - min_len = len - 1; - memcpy(dst, src, min_len); - dst[min_len] = '\0'; +static bool readlink_realpath(const char *filepath, char *buffer) { + char symlinked_path[PATH_MAX]; + ssize_t bytes_written = readlink(filepath, symlinked_path, sizeof(symlinked_path) - 1); + if(bytes_written == -1 && errno == EINVAL) { + /* Not a symlink */ + snprintf(symlinked_path, sizeof(symlinked_path), "%s", filepath); + } else if(bytes_written == -1) { + return false; + } else { + symlinked_path[bytes_written] = '\0'; + } + + if(!realpath(symlinked_path, buffer)) + return false; + + return true; +} + +static void file_get_directory(char *filepath) { + char *end = strrchr(filepath, '/'); + if(end == NULL) + filepath[0] = '\0'; + else + *end = '\0'; +} + +static bool string_ends_with(const char *str, const char *ends_with) { + const int len = strlen(str); + const int ends_with_len = strlen(ends_with); + return len >= ends_with_len && memcmp(str + len - ends_with_len, ends_with, ends_with_len) == 0; +} + +// This is not foolproof, but the assumption is that gsr-kms-server and gpu-screen-recorder are installed in the same directory +// in a location that only the root user can write to (usually /usr/bin or /usr/local/bin) and if the client runs from that location +// and is called gpu-screen-recorder then gsr-kms-server can only be used by a malicious program if the malicious program +// had root access, to modify that program install directory. +static bool is_remote_peer_program_gpu_screen_recorder(int socket_fd) { + // TODO: Use SO_PEERPIDFD on kernel >= 6.5 to avoid a race condition in the /proc/ check + struct ucred cred; + socklen_t ucred_len = sizeof(cred); + if(getsockopt(socket_fd, SOL_SOCKET, SO_PEERCRED, &cred, &ucred_len) == -1) { + fprintf(stderr, "kms server error: failed to get peer credentials, error: %s\n", strerror(errno)); + return false; + } + + char self_directory[PATH_MAX]; + if(!readlink_realpath("/proc/self/exe", self_directory)) { + fprintf(stderr, "kms server error: failed to resolve /proc/self/exe\n"); + return false; + } + file_get_directory(self_directory); + + char peer_directory[PATH_MAX]; + char peer_exe_path[PATH_MAX]; + snprintf(peer_exe_path, sizeof(peer_exe_path), "/proc/%d/exe", (int)cred.pid); + if(!readlink_realpath(peer_exe_path, peer_directory)) { + fprintf(stderr, "kms server error: failed to resolve /proc/self/exe\n"); + return false; + } + + if(!string_ends_with(peer_directory, "/gpu-screen-recorder")) { + fprintf(stderr, "kms server error: only gpu-screen-recorder can use gsr-kms-server. client program location is %s\n", peer_directory); + return false; + } + + file_get_directory(peer_directory); + + if(strcmp(self_directory, peer_directory) != 0) { + fprintf(stderr, "kms server error: the client program is in directory %s but only programs in %s can run gsr-kms-server\n", peer_directory, self_directory); + return false; + } + + return true; } int main(int argc, char **argv) { @@ -478,7 +549,7 @@ int main(int argc, char **argv) { while(clock_get_monotonic_seconds() - start_time < connect_timeout_sec) { struct sockaddr_un remote_addr = {0}; remote_addr.sun_family = AF_UNIX; - string_copy(remote_addr.sun_path, domain_socket_path, sizeof(remote_addr.sun_path)); + snprintf(remote_addr.sun_path, sizeof(remote_addr.sun_path), "%s", domain_socket_path); // TODO: Check if parent disconnected if(connect(socket_fd, (struct sockaddr*)&remote_addr, sizeof(remote_addr.sun_family) + strlen(remote_addr.sun_path)) == -1) { if(errno == ECONNREFUSED || errno == ENOENT) { @@ -505,6 +576,11 @@ int main(int argc, char **argv) { goto done; } + if(!is_remote_peer_program_gpu_screen_recorder(socket_fd)) { + res = 3; + goto done; + } + for(;;) { gsr_kms_request request; request.version = 0; -- cgit v1.2.3