On 08/10/2017 07:46 AM, Greg Kurz wrote:
This function has to ensure it doesn't follow a symlink that could be used
to escape the virtfs directory. This could be easily achieved if fchmodat()
on linux honored the AT_SYMLINK_NOFOLLOW flag as described in POSIX, but
it doesn't. There was a tentative to implement a new fchmodat2() syscall
with the correct semantics:
https://patchwork.kernel.org/patch/9596301/
but it didn't gain much momentum. Also it was suggested to look at an O_PATH
based solution in the first place.
The current implementation covers most use-cases, but it notably fails if:
- the target path has access rights equal to 0000 (openat() returns EPERM),
=> once you've done chmod(0000) on a file, you can never chmod() again
- the target path is UNIX domain socket (openat() returns ENXIO)
=> bind() of UNIX domain sockets fails if the file is on 9pfs
The solution is to use O_PATH: openat() now succeeds in both cases, and we
can ensure the path isn't a symlink with fstat(). The associated entry in
"/proc/self/fd" can hence be safely passed to the regular chmod() syscall.
The previous behavior is kept for older systems that don't have O_PATH.
Signed-off-by: Greg Kurz <gr...@kaod.org>
Reviewed-by: Eric Blake <ebl...@redhat.com>
Tested-by: Zhi Yong Wu <zhiyong...@ucloud.cn>
---
v5: - added missing fd == -1 check
v4: - fixed #if condition
- moved out: label above #endif
- fixed typo in changelog
- added Eric's r-b
v3: - O_PATH in a separate block of code
- added a reference to the fchmodat2() tentative in the changelog
v2: - renamed OPENAT_DIR_O_PATH to O_PATH_9P_UTIL and use it as a replacement
for O_PATH to avoid build breaks on O_PATH-less systems
- keep current behavior for O_PATH-less systems
- added comments
- TODO in 2.11: add _nofollow suffix to openat_dir() and openat_file()
---
hw/9pfs/9p-local.c | 47 ++++++++++++++++++++++++++++++++++++++++-------
hw/9pfs/9p-util.h | 24 +++++++++++++++---------
2 files changed, 55 insertions(+), 16 deletions(-)
diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index 6e478f4765ef..cb0b5f1fc41c 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -333,17 +333,27 @@ update_map_file:
static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode)
{
+ struct stat stbuf;
int fd, ret;
/* FIXME: this should be handled with fchmodat(AT_SYMLINK_NOFOLLOW).
- * Unfortunately, the linux kernel doesn't implement it yet. As an
- * alternative, let's open the file and use fchmod() instead. This
- * may fail depending on the permissions of the file, but it is the
- * best we can do to avoid TOCTTOU. We first try to open read-only
- * in case name points to a directory. If that fails, we try write-only
- * in case name doesn't point to a directory.
+ * Unfortunately, the linux kernel doesn't implement it yet.
*/
- fd = openat_file(dirfd, name, O_RDONLY, 0);
+
+ /* First, we clear non-racing symlinks out of the way. */
+ if (fstatat(dirfd, name, &stbuf, AT_SYMLINK_NOFOLLOW)) {
+ return -1;
+ }
+ if (S_ISLNK(stbuf.st_mode)) {
+ errno = ELOOP;
+ return -1;
+ }
+
+ /* Access modes are ignored when O_PATH is supported. We try O_RDONLY and
+ * O_WRONLY for old-systems that don't support O_PATH.
+ */
+ fd = openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL, 0);
+#if O_PATH_9P_UTIL == 0
if (fd == -1) {
/* In case the file is writable-only and isn't a directory. */
if (errno == EACCES) {
@@ -357,6 +367,29 @@ static int fchmodat_nofollow(int dirfd, const char *name,
mode_t mode)
return -1;
}
ret = fchmod(fd, mode);
+#else
+ if (fd == -1) {
+ return -1;
+ }
+
+ /* Now we handle racing symlinks. */
+ ret = fstat(fd, &stbuf);
the following ...
+ if (ret) {
+ goto out;
+ }
+ if (S_ISLNK(stbuf.st_mode)) {
+ errno = ELOOP;
+ ret = -1;
+ goto out;
+ }
+
+ {
+ char *proc_path = g_strdup_printf("/proc/self/fd/%d", fd);
+ ret = chmod(proc_path, mode);
+ g_free(proc_path);
+ }
+out:
can be refactored as:
if (!ret) {
if (S_ISLNK(stbuf.st_mode)) {
errno = ELOOP;
ret = -1;
} else {
char *proc_path = g_strdup_printf("/proc/self/fd/%d", fd);
ret = chmod(proc_path, mode);
g_free(proc_path);
}
}
which look (at least to me) easier to follow, and remove the 'out' label
this way:
Acked-by: Philippe Mathieu-Daudé <f4...@amsat.org>
+#endif
close_preserve_errno(fd);
return ret;
}
diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index 91299a24b8af..dc0d2e29aa3b 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -13,6 +13,12 @@
#ifndef QEMU_9P_UTIL_H
#define QEMU_9P_UTIL_H
+#ifdef O_PATH
+#define O_PATH_9P_UTIL O_PATH
+#else
+#define O_PATH_9P_UTIL 0
+#endif
+
static inline void close_preserve_errno(int fd)
{
int serrno = errno;
@@ -22,13 +28,8 @@ static inline void close_preserve_errno(int fd)
static inline int openat_dir(int dirfd, const char *name)
{
-#ifdef O_PATH
-#define OPENAT_DIR_O_PATH O_PATH
-#else
-#define OPENAT_DIR_O_PATH 0
-#endif
return openat(dirfd, name,
- O_DIRECTORY | O_RDONLY | O_NOFOLLOW | OPENAT_DIR_O_PATH);
+ O_DIRECTORY | O_RDONLY | O_NOFOLLOW | O_PATH_9P_UTIL);
}
static inline int openat_file(int dirfd, const char *name, int flags,
@@ -43,9 +44,14 @@ static inline int openat_file(int dirfd, const char *name,
int flags,
}
serrno = errno;
- /* O_NONBLOCK was only needed to open the file. Let's drop it. */
- ret = fcntl(fd, F_SETFL, flags);
- assert(!ret);
+ /* O_NONBLOCK was only needed to open the file. Let's drop it. We don't
+ * do that with O_PATH since fcntl(F_SETFL) isn't supported, and openat()
+ * ignored it anyway.
+ */
+ if (!(flags & O_PATH_9P_UTIL)) {
+ ret = fcntl(fd, F_SETFL, flags);
+ assert(!ret);
+ }
errno = serrno;
return fd;
}
