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

[Zhi Yong Wu]

Tested-by: Zhi Yong Wu <address@hidden>



Regards,


Zhi Yong Wu
At 2017-08-10 00:40:57, "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
>+    /* 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;
> }
>