Re: [Qemu-devel] [for-2.10 PATCH v4] 9pfs: local: fix fchmodat_nofollow() limitations

[Greg Kurz]

On Wed, 09 Aug 2017 18:40:57 +0200
Greg Kurz <address@hidden> 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 <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> ---
> 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;
>  }
> 
>

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