> 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. - add_seals won’t need the wait_for_pins and mapping_deny_write logic. That really should be all that’s needed.
