> On Nov 10, 2018, at 11:11 AM, Daniel Colascione <dancol@xxxxxxxxxx> 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. This should be straightforward. Just add a new seal type and wire it up. It should be considerably simpler than SEAL_WRITE.
