On Nov 11, 2018, at 12:30 AM, Daniel Colascione [off-list ref] wrote:

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On Sun, Nov 11, 2018 at 12:09 AM, Joel Fernandes [off-list ref] wrote:
On Sat, Nov 10, 2018 at 07:40:10PM -0800, Andy Lutomirski wrote:
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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 :)

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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.

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- 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.

Hmm. I’m guessing the intent is that the mmap count should track writable mappings in addition to mappings that could be made writable using mprotect.  I think you could address this for SEAL_FUTURE in two ways:

1. In shmem_mmap, mask off VM_MAYWRITE if SEAL_FUTURE is set, or

2. Add a new vm operation that allows a vma to reject an mprotect attempt, like security_file_mprotect but per vma.  Then give it reasonable semantics for shmem.

(1) probably gives the semantics you want for SEAL_FUTURE: old maps can be mprotected, but new maps can’t.

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---8<-----------------------

From: "Joel Fernandes (Google)" <redacted>
Subject: [PATCH] mm/memfd: implement future write seal using shmem ops

Signed-off-by: Joel Fernandes (Google) <redacted>
---
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.

This could plausibly confuse existing users that read the seal mask.