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9pfs: local: fix fchmodat_nofollow() limitations

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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 <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Zhi Yong Wu <zhiyong.wu@ucloud.cn>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
This commit is contained in:
Greg Kurz 2017-08-10 14:21:04 +02:00
parent b38df311c1
commit 4751fd5328
2 changed files with 50 additions and 16 deletions
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24
hw/9pfs/9p-util.h
View file

@ -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;
}