On 03/03/2017 11:25 AM, Greg Kurz wrote: > We should pass O_NOFOLLOW otherwise openat() will follow symlinks and make > QEMU vulnerable. > > O_PATH was used as an optimization: the fd returned by openat_dir() is only > passed to openat() actually, so we don't really need to reach the underlying > filesystem. > > O_NOFOLLOW | O_PATH isn't an option: if name is a symlink, openat() will > return a fd, forcing us to do some other syscall to detect we have a > symlink. Also, O_PATH doesn't exist in glibc 2.13 and older.
But the very next use of openat(fd, ) should fail with EBADF if fd is not a directory, so you don't need any extra syscalls. I agree that we _need_ O_NOFOLLOW, but I'm not yet convinced that we must avoid O_PATH where it works. I'm in the middle of writing a test program to probe kernel behavior and demonstrate (at least to myself) whether there are scenarios where O_PATH makes it possible to open something where omitting it did not, while at the same time validating that O_NOFOLLOW doesn't cause problems if a symlink-fd is returned instead of a directory fd, based on our subsequent use of that fd in a *at call. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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