From deae98ef45e060b8412a5edd195fb0c7c14f0321 Mon Sep 17 00:00:00 2001 From: Emmanuel Gil Peyrot Date: Mon, 22 Jul 2019 15:06:20 +0200 Subject: [PATCH] shared: Use memfd_create() when available MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This (so-far) Linux-only API lets users create file descriptors purely in memory, without any backing file on the filesystem and the race condition which could ensue when unlink()ing it. It also allows seals to be placed on the file, ensuring to every other process that we won’t be allowed to shrink the contents, potentially causing a SIGBUS when they try reading it. This patch is best viewed with the -w option of git log -p. It is an almost exact copy of Wayland commit 6908c8c85a2e33e5654f64a55cd4f847bf385cae, see https://gitlab.freedesktop.org/wayland/wayland/merge_requests/4 Signed-off-by: Emmanuel Gil Peyrot --- meson.build | 2 +- shared/os-compatibility.c | 53 ++++++++++++++++------ tests/bad-buffer-test.c | 95 ++++++++++++++++++++++++++++++++++++++- 3 files changed, 134 insertions(+), 16 deletions(-) diff --git a/meson.build b/meson.build index 3d5aa5fb..9fa787b9 100644 --- a/meson.build +++ b/meson.build @@ -78,7 +78,7 @@ elif cc.has_header_symbol('sys/mkdev.h', 'major') endif optional_libc_funcs = [ - 'mkostemp', 'strchrnul', 'initgroups', 'posix_fallocate' + 'mkostemp', 'strchrnul', 'initgroups', 'posix_fallocate', 'memfd_create' ] foreach func : optional_libc_funcs if cc.has_function(func) diff --git a/shared/os-compatibility.c b/shared/os-compatibility.c index e19fb61b..aeea780f 100644 --- a/shared/os-compatibility.c +++ b/shared/os-compatibility.c @@ -34,6 +34,10 @@ #include #include +#ifdef HAVE_MEMFD_CREATE +#include +#endif + #include "os-compatibility.h" int @@ -147,6 +151,13 @@ create_tmpfile_cloexec(char *tmpname) * given size. If disk space is insufficient, errno is set to ENOSPC. * If posix_fallocate() is not supported, program may receive * SIGBUS on accessing mmap()'ed file contents instead. + * + * If the C library implements memfd_create(), it is used to create the + * file purely in memory, without any backing file name on the file + * system, and then sealing off the possibility of shrinking it. This + * can then be checked before accessing mmap()'ed file contents, to + * make sure SIGBUS can't happen. It also avoids requiring + * XDG_RUNTIME_DIR. */ int os_create_anonymous_file(off_t size) @@ -157,25 +168,39 @@ os_create_anonymous_file(off_t size) int fd; int ret; - path = getenv("XDG_RUNTIME_DIR"); - if (!path) { - errno = ENOENT; - return -1; - } +#ifdef HAVE_MEMFD_CREATE + fd = memfd_create("weston-shared", MFD_CLOEXEC | MFD_ALLOW_SEALING); + if (fd >= 0) { + /* We can add this seal before calling posix_fallocate(), as + * the file is currently zero-sized anyway. + * + * There is also no need to check for the return value, we + * couldn't do anything with it anyway. + */ + fcntl(fd, F_ADD_SEALS, F_SEAL_SHRINK); + } else +#endif + { + path = getenv("XDG_RUNTIME_DIR"); + if (!path) { + errno = ENOENT; + return -1; + } - name = malloc(strlen(path) + sizeof(template)); - if (!name) - return -1; + name = malloc(strlen(path) + sizeof(template)); + if (!name) + return -1; - strcpy(name, path); - strcat(name, template); + strcpy(name, path); + strcat(name, template); - fd = create_tmpfile_cloexec(name); + fd = create_tmpfile_cloexec(name); - free(name); + free(name); - if (fd < 0) - return -1; + if (fd < 0) + return -1; + } #ifdef HAVE_POSIX_FALLOCATE do { diff --git a/tests/bad-buffer-test.c b/tests/bad-buffer-test.c index ee7b2231..b7abda23 100644 --- a/tests/bad-buffer-test.c +++ b/tests/bad-buffer-test.c @@ -26,11 +26,104 @@ #include "config.h" +#include +#include +#include +#include #include #include "shared/os-compatibility.h" #include "weston-test-client-helper.h" +/* These three functions are copied from shared/os-compatibility.c in order to + * behave like older clients, and allow ftruncate() to shrink the file’s size, + * so SIGBUS can still happen. + * + * There is no reason not to use os_create_anonymous_file() otherwise. */ + +#ifndef HAVE_MKOSTEMP +static int +set_cloexec_or_close(int fd) +{ + if (os_fd_set_cloexec(fd) != 0) { + close(fd); + return -1; + } + return fd; +} +#endif + +static int +create_tmpfile_cloexec(char *tmpname) +{ + int fd; + +#ifdef HAVE_MKOSTEMP + fd = mkostemp(tmpname, O_CLOEXEC); + if (fd >= 0) + unlink(tmpname); +#else + fd = mkstemp(tmpname); + if (fd >= 0) { + fd = set_cloexec_or_close(fd); + unlink(tmpname); + } +#endif + + return fd; +} + +static int +create_anonymous_file_without_seals(off_t size) +{ + static const char template[] = "/weston-test-XXXXXX"; + const char *path; + char *name; + int fd; + int ret; + + path = getenv("XDG_RUNTIME_DIR"); + if (!path) { + errno = ENOENT; + return -1; + } + + name = malloc(strlen(path) + sizeof(template)); + if (!name) + return -1; + + strcpy(name, path); + strcat(name, template); + + fd = create_tmpfile_cloexec(name); + + free(name); + + if (fd < 0) + return -1; + +#ifdef HAVE_POSIX_FALLOCATE + do { + ret = posix_fallocate(fd, 0, size); + } while (ret == EINTR); + if (ret != 0) { + close(fd); + errno = ret; + return -1; + } +#else + do { + ret = ftruncate(fd, size); + } while (ret < 0 && errno == EINTR); + if (ret < 0) { + close(fd); + return -1; + } +#endif + + return fd; +} + /* tests, that attempt to crash the compositor on purpose */ static struct wl_buffer * @@ -43,7 +136,7 @@ create_bad_shm_buffer(struct client *client, int width, int height) struct wl_buffer *buffer; int fd; - fd = os_create_anonymous_file(size); + fd = create_anonymous_file_without_seals(size); assert(fd >= 0); pool = wl_shm_create_pool(shm, fd, size);