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. thanks, - Joel
