On Sun, Nov 11, 2018 at 12:09 AM, Joel Fernandes <j...@joelfernandes.org> wrote: > On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote: > [...] >> >>>>>>> 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 :) >> >> > >> > 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.
Why shouldn't memfd work independently of CONFIG_TMPFS? In particular, write(2) on tmpfs FDs shouldn't work differently. If it does, that's a kernel implementation detail leaking out into userspace. >> >> - 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. >> > > Its actually not that bad and a great idea, I did something like the > following and it works pretty well. I would say its cleaner than the old > approach for sure (and I also added a /proc/pid/fd/N reopen test to the > selftest and made sure that issue goes away). > > Side note: One subtelty I discovered from the existing selftests is once the > F_SEAL_WRITE are active, an mmap of PROT_READ and MAP_SHARED region is > expected to fail. This is also evident from this code in mmap_region: > if (vm_flags & VM_SHARED) { > error = mapping_map_writable(file->f_mapping); > if (error) > goto allow_write_and_free_vma; > } > This behavior seems like a bug. Why should MAP_SHARED writes be denied here? There's no semantic incompatibility between shared mappings and the seal. And I think this change would represent an ABI break using memfd seals for ashmem, since ashmem currently allows MAP_SHARED mappings after changing prot_mask. > ---8<----------------------- > > From: "Joel Fernandes (Google)" <j...@joelfernandes.org> > Subject: [PATCH] mm/memfd: implement future write seal using shmem ops > > Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org> > --- > fs/hugetlbfs/inode.c | 2 +- > mm/memfd.c | 19 ------------------- > mm/shmem.c | 13 ++++++++++--- > 3 files changed, 11 insertions(+), 23 deletions(-) > > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c > index 32920a10100e..1978581abfdf 100644 > --- a/fs/hugetlbfs/inode.c > +++ b/fs/hugetlbfs/inode.c > @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, > loff_t offset, loff_t len) > inode_lock(inode); > > /* protected by i_mutex */ > - if (info->seals & F_SEAL_WRITE) { > + if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) { > inode_unlock(inode); > return -EPERM; > } Maybe we can always set F_SEAL_FUTURE_WRITE when F_SEAL_WRITE so we can just test one bit except where the F_SEAL_WRITE behavior differs from F_SEAL_FUTURE_WRITE.