On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote: > > > > On Nov 10, 2018, at 6:38 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > >> On Sat, Nov 10, 2018 at 02:18:23PM -0800, Andy Lutomirski wrote: > >> > >>>> On Nov 10, 2018, at 2:09 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > >>>> > >>>>> On Sat, Nov 10, 2018 at 11:11:27AM -0800, Daniel Colascione wrote: > >>>>>> On Sat, Nov 10, 2018 at 10:45 AM, Daniel Colascione <dancol@xxxxxxxxxx> wrote: > >>>>>> On Sat, Nov 10, 2018 at 10:24 AM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > >>>>>> Thanks Andy for your thoughts, my comments below: > >>>> [snip] > >>>>>> I don't see it as warty, different seals will work differently. It works > >>>>>> quite well for our usecase, and since Linux is all about solving real > >>>>>> problems in the real work, it would be useful to have it. > >>>>>> > >>>>>>> - causes a probably-observable effect in the file mode in F_GETFL. > >>>>>> > >>>>>> Wouldn't that be the right thing to observe anyway? > >>>>>> > >>>>>>> - causes reopen to fail. > >>>>>> > >>>>>> So this concern isn't true anymore if we make reopen fail only for WRITE > >>>>>> opens as Daniel suggested. I will make this change so that the security fix > >>>>>> is a clean one. > >>>>>> > >>>>>>> - does *not* affect other struct files that may already exist on the same inode. > >>>>>> > >>>>>> TBH if you really want to block all writes to the file, then you want > >>>>>> F_SEAL_WRITE, not this seal. The usecase we have is the fd is sent over IPC > >>>>>> to another process and we want to prevent any new writes in the receiver > >>>>>> side. There is no way this other receiving process can have an existing fd > >>>>>> unless it was already sent one without the seal applied. The proposed seal > >>>>>> could be renamed to F_SEAL_FD_WRITE if that is preferred. > >>>>>> > >>>>>>> - mysteriously malfunctions if you try to set it again on another struct > >>>>>>> file that already exists > >>>>>>> > >>>>>> > >>>>>> I didn't follow this, could you explain more? > >>>>>> > >>>>>>> - probably is insecure when used on hugetlbfs. > >>>>>> > >>>>>> The usecase is not expected to prevent all writes, indeed the usecase > >>>>>> requires existing mmaps to continue to be able to write into the memory map. > >>>>>> So would you call that a security issue too? The use of the seal wants to > >>>>>> allow existing mmap regions to be continue to be written into (I mentioned > >>>>>> more details in the cover letter). > >>>>>> > >>>>>>> I see two reasonable solutions: > >>>>>>> > >>>>>>> 1. Don’t fiddle with the struct file at all. Instead make the inode flag > >>>>>>> work by itself. > >>>>>> > >>>>>> Currently, the various VFS paths check only the struct file's f_mode to deny > >>>>>> writes of already opened files. This would mean more checking in all those > >>>>>> paths (and modification of all those paths). > >>>>>> > >>>>>> Anyway going with that idea, we could > >>>>>> 1. call deny_write_access(file) from the memfd's seal path which decrements > >>>>>> the inode::i_writecount. > >>>>>> 2. call get_write_access(inode) in the various VFS paths in addition to > >>>>>> checking for FMODE_*WRITE and deny the write (incase i_writecount is negative) > >>>>>> > >>>>>> That will prevent both reopens, and writes from succeeding. However I worry a > >>>>>> bit about 2 not being too familiar with VFS internals, about what the > >>>>>> consequences of doing that may be. > >>>>> > >>>>> IMHO, modifying both the inode and the struct file separately is fine, > >>>>> since they mean different things. In regular filesystems, it's fine to > >>>>> have a read-write open file description for a file whose inode grants > >>>>> write permission to nobody. Speaking of which: is fchmod enough to > >>>>> prevent this attack? > >>>> > >>>> Well, yes and no. fchmod does prevent reopening the file RW, but > >>>> anyone with permissions (owner, CAP_FOWNER) can just fchmod it back. A > >>>> seal is supposed to be irrevocable, so fchmod-as-inode-seal probably > >>>> isn't sufficient by itself. While it might be good enough for Android > >>>> (in the sense that it'll prevent RW-reopens from other security > >>>> contexts to which we send an open memfd file), it's still conceptually > >>>> ugly, IMHO. Let's go with the original approach of just tweaking the > >>>> inode so that open-for-write is permanently blocked. > >>> > >>> Agreed with the idea of modifying both file and inode flags. I was thinking > >>> modifying i_mode may do the trick but as you pointed it probably could be > >>> reverted by chmod or some other attribute setting calls. > >>> > >>> OTOH, I don't think deny_write_access(file) can be reverted from any > >>> user-facing path so we could do that from the seal to prevent the future > >>> opens in write mode. I'll double check and test that out tomorrow. > >>> > >>> > >> > >> This seems considerably more complicated and more fragile than needed. Just > >> add a new F_SEAL_WRITE_FUTURE. Grep for F_SEAL_WRITE and make the _FUTURE > >> variant work exactly like it with two exceptions: > >> > >> - shmem_mmap and maybe its hugetlbfs equivalent should check for it and act > >> accordingly. > > > > There's more to it than that, we also need to block future writes through > > write syscall, so we have to hook into the write path too once the seal is > > set, not just the mmap. That means we have to add code in mm/shmem.c to do > > that in all those handlers, to check for the seal (and hope we didn't miss a > > file_operations handler). Is that what you are proposing? > > The existing code already does this. That’s why I suggested grepping :) Ahh sorry I see your point now. Ok let me try this approach. Thank you! Probably we can make this work this way and it is sufficient. > > > > Also, it means we have to keep CONFIG_TMPFS enabled so that the > > shmem_file_operations write handlers like write_iter are hooked up. Currently > > memfd works even with !CONFIG_TMPFS. > > If so, that sounds like it may already be a bug. Actually, its not a bug. If CONFIG_TMPFS is disabled, then IIRC write syscall will be prevented anyway and then the mmap is the only way. I'll double check that once I work on this idea. > > > >> - add_seals won’t need the wait_for_pins and mapping_deny_write logic. > >> > >> That really should be all that’s needed. > > > > It seems a fair idea what you're saying. But I don't see how its less > > complex.. IMO its far more simple to have VFS do the denial of the operations > > based on the flags of its datastructures.. and if it works (which I will test > > to be sure it will), then we should be good. > > I agree it’s complicated, but the code is already written. You should just > need to adjust some masks. > Right. > > > > Btw by any chance, are you also coming by LPC conference next week? > > > > No. I’d like to, but I can’t make the trip this year. Ok, no worries. thanks, - Joel
