On Wed, 9 Aug 2017 11:19:42 -0500 Eric Blake <ebl...@redhat.com> wrote:
> On 08/09/2017 11:00 AM, Greg Kurz 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 a O_PATH > > > > s/a O_PATH/an O_PATH/ > Fixed. > > 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. > > My late-breaking question from v2 remains: fstat() on O_PATH only works Yeah I saw your mail just after sending the v3 :) > in kernel 3.6 and newer; are we worried about kernels in the window of > 2.6.39 (when O_PATH was introduced) and 3.5? Or at this point, are we > reasonably sure that platforms are either too old for O_PATH at all > (Hello RHEL 6, with 2.6.32), or else new enough that we aren't going to > have spurious failures due to fstat() not doing what we want? > > I don't actually know the failure mode of fstat() on kernel 3.5, so if > someone cares about that working (presumably because they are on a > platform with such a kernel), please speak up. (Or even run my test > program included on the v1 thread, to show us what happens) > That seems reasonable to me. > > + fd = openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL, 0); > > +#ifndef O_PATH > > Please make this '#if O_PATH' or even '#if O_PATH_9P_UTIL'; as it might > be feasible for someone to > > #ifndef O_PATH > #define O_PATH 0 > #endif > > where the macro is defined but the feature is not present, messing up > our code if we only check for a definition. > Ok, I'll do that. > > +#else > > + /* Now we handle racing symlinks. */ > > + ret = fstat(fd, &stbuf); > > + if (ret) { > > + goto out; > > This may leave errno at an unusual value for fchmodat(), if we are on > kernel 3.5. But until someone speaks up that it matters, I'm okay > saving any cleanup work in that area for a followup patch. > Agreed. > > + } > > + 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); > > + } > > +#endif > > +out: > > Swap these two lines - your only use of 'goto out' are under the O_PATH > branch, and therefore you get a compilation failure about unused label > on older glibc. > Oops. > With the #if condition fixed and the scope of the #endif fixed, > > Reviewed-by: Eric Blake <ebl...@redhat.com> > Thanks !
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