Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

[Daniel Colascione]

On Sun, Nov 11, 2018 at 12:09 AM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
> [...]
>> >>>>>>> I see two reasonable solutions:
>> >>>>>>>
>> >>>>>>> 1. Donât 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. Thatâs 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 wonât need the wait_for_pins and mapping_deny_write logic.
>> >>
>> >> That really should be all thatâs 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 itâs 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)" <joel@xxxxxxxxxxxxxxxxx>
> Subject: [PATCH] mm/memfd: implement future write seal using shmem ops
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  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.