On Sun, Oct 19, 2014 at 1:20 AM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > >> [Added Eric Biederman, since I think your tree might be a reasonable >> route forward for these patches.] >> >> On Thu, Jun 5, 2014 at 6:40 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote: >>> Resending, adding cc:linux-api. >>> >>> Also, it may help to add a little more background -- this patch is >>> needed as a (small) part of implementing Capsicum in the Linux kernel. >>> >>> Capsicum is a security framework that has been present in FreeBSD since >>> version 9.0 (Jan 2012), and is based on concepts from object-capability >>> security [1]. >>> >>> One of the features of Capsicum is capability mode, which locks down >>> access to global namespaces such as the filesystem hierarchy. In >>> capability mode, /proc is thus inaccessible and so fexecve(3) doesn't >>> work -- hence the need for a kernel-space >> >> I just found myself wanting this syscall for another reason: injecting >> programs into sandboxes or otherwise heavily locked-down namespaces. >> >> For example, I want to be able to reliably do something like nsenter >> --namespace-flags-here toybox sh. Toybox's shell is unusual in that >> it is more or less fully functional, so this should Just Work (tm), >> except that the toybox binary might not exist in the namespace being >> entered. If execveat were available, I could rig nsenter or a similar >> tool to open it with O_CLOEXEC, enter the namespace, and then call >> execveat. >> >> Is there any reason that these patches can't be merged more or less as >> is for 3.19? > > Yes. There is a silliness in how it implements fexecve. The fexecve > case should be use the empty string "" not a NULL pointer to indication > that. That change will then harmonize execveat with the other ...at > system calls and simplify the code and remove a special case. I believe > using the empty string "" requires implementing the AT_EMPTY_PATH flag. Good point -- I'll shift to "" + AT_EMPTY_PATH. > For sandboxes execveat seems to make a great deal of sense. I can > get the same functionality by passing in a directory file descriptor > calling fchdir and execve so this should not introduce any new security > holes. And using the final file descriptor removes a race. > > AT_SYMLINK_NOFOLLOW seems to have some limited utility as well, although > for exec I don't know what problems it can solve. > > Until I am done moving I won't have time to pick this up, and the code > clearly needs another revision but I will be happy to work to see that > we get a sane execveat implemented. If it helps, I can push out another revision in the next couple of days. > Eric > > p.s. I don't believe there are any namespaces issues where doing > something with execveat flags make sense. > > -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
