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.
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.
Signed-off-by: Greg Kurz <address@hidden>
---
hw/9pfs/9p-local.c | 44 ++++++++++++++++++++++++++++----------------
hw/9pfs/9p-util.h | 10 +++++++---
2 files changed, 35 insertions(+), 19 deletions(-)
diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index 6e478f4765ef..b178d627c764 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -333,30 +333,42 @@ update_map_file:
static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode)
{
+ struct stat stbuf;
int fd, ret;
+ char *proc_path;
/* 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);
- if (fd == -1) {
- /* In case the file is writable-only and isn't a directory. */
- if (errno == EACCES) {
- fd = openat_file(dirfd, name, O_WRONLY, 0);
- }
- if (fd == -1 && errno == EISDIR) {
- errno = EACCES;
- }
+ if (fstatat(dirfd, name, &stbuf, AT_SYMLINK_NOFOLLOW)) {
+ return -1;
}
+
+ if (S_ISLNK(stbuf.st_mode)) {
+ errno = ELOOP;
+ return -1;
+ }
+
+ fd = openat_file(dirfd, name, O_RDONLY | O_PATH, 0);
if (fd == -1) { > return -1;
}
- ret = fchmod(fd, mode);
+
+ ret = fstat(fd, &stbuf);
+ if (ret) {
+ goto out;
+ }
+
+ if (S_ISLNK(stbuf.st_mode)) {
+ errno = ELOOP;
+ ret = -1;
+ goto out;
+ }
+
+ proc_path = g_strdup_printf("/proc/self/fd/%d", fd);
+ ret = chmod(proc_path, mode);
+ g_free(proc_path);
+out:
close_preserve_errno(fd);
return ret;
}
diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index 91299a24b8af..7c1c8fb1e35c 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -43,9 +43,13 @@ 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 it ignores O_NONBLOCK.
+ */