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

[Joel Fernandes <joel@joelfernandes.org>]

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.
> 
> > 
> >> - 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;
		}

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

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

Signed-off-by: Joel Fernandes (Google) <joel@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;
 		}
diff --git a/mm/memfd.c b/mm/memfd.c
index 5ba9804e9515..a9ece5fab439 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -220,25 +220,6 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	if ((seals & F_SEAL_FUTURE_WRITE) &&
-	    !(*file_seals & F_SEAL_FUTURE_WRITE)) {
-		/*
-		 * The FUTURE_WRITE seal also prevents growing and shrinking
-		 * so we need them to be already set, or requested now.
-		 */
-		int test_seals = (seals | *file_seals) &
-				 (F_SEAL_GROW | F_SEAL_SHRINK);
-
-		if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
-			error = -EINVAL;
-			goto unlock;
-		}
-
-		spin_lock(&file->f_lock);
-		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
-		spin_unlock(&file->f_lock);
-	}
-
 	*file_seals |= seals;
 	error = 0;
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 446942677cd4..7dad7efd8b99 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2163,6 +2163,12 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user)
 
 static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct shmem_inode_info *info = SHMEM_I(file_inode(file));
+
+	/* New shared mmaps are not allowed when "future write" seal active. */
+	if ((vma->vm_flags & VM_SHARED) && (info->seals & F_SEAL_FUTURE_WRITE))
+		return -EPERM;
+
 	file_accessed(file);
 	vma->vm_ops = &shmem_vm_ops;
 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2391,8 +2397,9 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
-		if (info->seals & F_SEAL_WRITE)
+	if (unlikely(info->seals & (F_SEAL_GROW |
+				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
@@ -2657,7 +2664,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_mutex */
-		if (info->seals & F_SEAL_WRITE) {
+		if (info->seals & F_SEAL_WRITE || info->seals & F_SEAL_FUTURE_WRITE) {
 			error = -EPERM;
 			goto out;
 		}
-- 
2.19.1.930.g4563a0d9d0-goog