On Wed, 09 Aug 2017 18:40:57 +0200 Greg Kurz <gr...@kaod.org> 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> > --- > 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 | 43 ++++++++++++++++++++++++++++++++++++------- > hw/9pfs/9p-util.h | 24 +++++++++++++++--------- > 2 files changed, 51 insertions(+), 16 deletions(-) > > diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c > index 6e478f4765ef..d9ef57d343c9 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 +367,25 @@ static int fchmodat_nofollow(int dirfd, const char > *name, mode_t mode) > return -1; > } > ret = fchmod(fd, mode); > +#else
Oops, missing fd == -1 check...
> + /* Now we handle racing symlinks. */
> + ret = fstat(fd, &stbuf);
> + 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:
> +#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;
> }
>
>
pgp1JW3_Gk3Zr.pgp
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