On Tue, Oct 17, 2023 at 11:20 AM Theo de Raadt <deraadt@xxxxxxxxxxx> wrote: > > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, 17 Oct 2023 at 02:08, Jeff Xu <jeffxu@xxxxxxxxxx> wrote: > > > > > > It is probably worth noting that I choose to check one and only > > > one sealing type per syscall. i.e. munmap(2) checks > > > MM_SEAL_MUNMAP only. > > > > Yeah, this is wrong. > > > > It's wrong exactly because other system calls will unmap things too. > > > > Using mmap() to over-map something will unmap the old one. > > > > Same goes for mremap() to move over an existing mapping. > > > > So the whole "do things by the name of the system call" is not workable. > > > > All that matters is what the system calls *do*, not what their name is. > > I agree completely... > > mseal() is a clone of mimmutable(2), but with an extremely > over-complicated API based upon dubious arguments. > > I designed mimmutable(2) [1] in OpenBSD, it took about a year to get all > the components working correctly. There were many intermediate API > during development, but in the end the API is simply: > > int mimmutable(void *addr, size_t len); > > The kernel code for mimmutable() traverses the specified VA range. In > that range, it will find unmapped sub-regions (which are are ignored) > and mapped sub-regions. For these mapped regions, it does not care what > the permissions are, it just marks each sub-region as immutable. > > Later on, when any VM operation request upon a VA range attempts to > (1) change the permissions > (2) to re-map on top > (3) or dispose of the mapping, > that operation is refused with errno EPERM. We don't care where the > request comes from (ie. what system call). It is a behaviour of the > VM system, when asked to act upon a VA sub-range mapping. > > Very simple semantics. > > The only case where the immutable marker is ignored is during address space > teardown as a result of process termination. > May I ask, for BSD's implementation of immutable(), do you cover things such as mlock(), madvice() ? or just the protection bit (WRX) + remap() + unmap(). In other words: Is BSD's definition of immutable equivalent to MM_SEAL_MPROTECT|MM_SEAL_MUNMAP|MM_SEAL_MREMAP|MM_SEAL_MMAP, of this patch set ? I hesitate to introduce the concept of immutable into linux because I don't know all the scenarios present in linux where VMAs's metadata can be modified. As Jann's email pointed out, There could be quite a few things we still need to deal with, to completely block the possibility, e.g. malicious code attempting to write to a RO memory or change RW memory to RWX. If, as part of immutable, I also block madvice(), mlock(), which also updates VMA's metadata, so by definition, I could. What if the user wants the features in madvice() and at the same time, also wants their .text protected ? Also, if linux introduces a new syscall that depends on a new metadata of VMA, say msecret(), (for discussion purpose), should immutable automatically support that ? Without those questions answered, I couldn't choose the route of immutable() yet. -Jeff > > In his submission of this API, Jeff Xu makes three claims I find dubious; > > > Also, Chrome wants to adopt this feature for their CFI work [2] and this > > patchset has been designed to be compatible with the Chrome use case. > > I specifically designed mimmutable(2) with chrome in mind, and the > chrome binary running on OpenBSD is full of immutable mappings. All the > library regions automatically become immutable because ld.so can infer > it and do the mimmutable calls for the right subregions. > > So this chrome work has already been done by OpenBSD, and it is dead > simple. During early development I thought mimmutable(2) would be > called by applications or libraries, but I was dead wrong: 99.9% of > calls are from ld.so, and no applications need to call it, these are the > two exceptions: > > In OpenBSD, mimmutable() is used in libc malloc() to lock-down some data > structures at initialization time, so they canoot be attacked to create > an invariant for use in ROP return-to-libc style methods. > > In Chrome, there is a v8_flags variable rounded out to a full page, and > placed in .data. Chrome initialized this variable, and wants to mprotect > PROT_READ, but .data has been made immutable by ld.so. So we force this > page into a new ELF section called "openbsd.mutable" which also behaves RW > like .data. Where chrome does the mprotect PROT_READ, it now also performs > mimmutable() on that page. > > > Having a seal type per syscall type helps to add the feature incrementally. > > Yet, somehow OpenBSD didn't do it per syscall, and we managed to make our > entire base operating system and 10,000+ applications automatically receive > the benefits. In one year's effort. The only application which cared about > it was chrome, described in the previous paragraph. > > I think Jeff's idea here is super dangerous. What will actually happen > is people will add a few mseal() sub-operations and think the job is done. > It isn't done. They need all the mseal() requests, or the mapping are > not safe. > > It is very counterproductive to provide developers a complex API that has > insecure suboperations. > > > Applications also know exactly what is sealed. > > Actually applicatins won't know because there is no tooling to inspect this -- > but I will argue further that applications don't need to know. Immutable > marking is a system decision, not a program decision. > > > I'll close by asking for a new look at the mimmutable(2) API we settled > on for OpenBSD. I think there is nothing wrong with it. I'm willing to > help guide glibc / ld.so / musl teams through the problems they may find > along the way, I know where the skeletons are buried. Two in > particular: -znow RELRO already today, and xonly-text in the future. > > > [1] https://man.openbsd.org/mimmutable.2 >
