[PATCH v6 0/2] vfs: fix dedupe permission check

[Mark Fasheh <mfasheh@suse.de>]

Hi Andrew/Al,

Could I please have these patches put in a tree for more public testing?
They've hit fsdevel a few times now, I have links to the discussions in the
change log below.


The following patches fix a couple of issues with the permission check
we do in vfs_dedupe_file_range().

The first patch expands our check to allow dedupe of a file if the
user owns it or otherwise would be allowed to write to it.

Current behavior is that we'll allow dedupe only if:

- the user is an admin (root)
- the user has the file open for write

This makes it impossible for a user to dedupe their own file set
unless they do it as root, or ensure that all files have write
permission. There's a couple of duperemove bugs open for this:

https://github.com/markfasheh/duperemove/issues/129
https://github.com/markfasheh/duperemove/issues/86

The other problem we have is also related to forcing the user to open
target files for write - A process trying to exec a file currently
being deduped gets ETXTBUSY. The answer (as above) is to allow them to
open the targets ro - root can already do this. There was a patch from
Adam Borowski to fix this back in 2016:

https://lkml.org/lkml/2016/7/17/130

which I have incorporated into my changes.


The 2nd patch fixes our return code for permission denied to be
EPERM. For some reason we're returning EINVAL - I think that's
probably my fault. At any rate, we need to be returning something
descriptive of the actual problem, otherwise callers see EINVAL and
can't really make a valid determination of what's gone wrong.

This has also popped up in duperemove, mostly in the form of cryptic
error messages. Because this is a code returned to userspace, I did
check the other users of extent-same that I could find. Both 'bees'
and 'rust-btrfs' do the same as duperemove and simply report the error
(as they should).


One way I tested these patches was to make non-root owned files with
read-only permissions and see if I could dedupe them as the owning user. For
example, the following script fails on an unpatched kernel and succeeds with
the patches applied.

  TESTDIR=/btrfs
  USER=mfasheh

  rm -f $TESTDIR/file*

  dd if=/dev/zero of=$TESTDIR/file1 count=1024 bs=1024
  dd if=/dev/zero of=$TESTDIR/file2 count=1024 bs=1024

  chown $USER $TESTDIR/file*
  chmod 444 $TESTDIR/file*

  # open file* for read and dedupe
  sudo -u $USER duperemove -Ad $TESTDIR/file*


Lastly, I have an update to the fi_deduperange manpage to reflect these
changes. That patch is attached below.


git pull https://github.com/markfasheh/linux dedupe-perms

Thanks,
  --Mark

Changes from V5 to V6:
- Rebase and retest on 4.19-rc3
- Add a note on testing

Changes from V4 to V5:
- Rebase and retest on 4.18-rc8
- Place updated manpage patch below, CC linux-api
- V4 discussion: https://patchwork.kernel.org/patch/10530365/

Changes from V3 to V4:
- Add a patch (below) to ioctl_fideduperange.2 explaining our
  changes. I will send this patch once the kernel update is
  accepted. Thanks to Darrick Wong for this suggestion.
- V3 discussion: https://www.spinics.net/lists/linux-btrfs/msg79135.html

Changes from V2 to V3:
- Return bool from allow_file_dedupe
- V2 discussion: https://www.spinics.net/lists/linux-btrfs/msg78421.html

Changes from V1 to V2:
- Add inode_permission check as suggested by Adam Borowski
- V1 discussion: https://marc.info/?l=linux-xfs&m=152606684017965&w=2


From: Mark Fasheh <mfasheh@suse.de>

[PATCH] ioctl_fideduperange.2: clarify permission requirements

dedupe permission checks were recently relaxed - update our man page to
reflect those changes.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
---
 man2/ioctl_fideduperange.2 | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/man2/ioctl_fideduperange.2 b/man2/ioctl_fideduperange.2
index 84d20a276..4040ee064 100644
--- a/man2/ioctl_fideduperange.2
+++ b/man2/ioctl_fideduperange.2
@@ -105,9 +105,12 @@ The field
 must be zero.
 During the call,
 .IR src_fd
-must be open for reading and
+must be open for reading.
 .IR dest_fd
-must be open for writing.
+can be open for writing, or reading.
+If
+.IR dest_fd
+is open for reading, the user must have write access to the file.
 The combined size of the struct
 .IR file_dedupe_range
 and the struct
@@ -185,8 +188,8 @@ This can appear if the filesystem does not support deduplicating either file
 descriptor, or if either file descriptor refers to special inodes.
 .TP
 .B EPERM
-.IR dest_fd
-is immutable.
+This will be returned if the user lacks permission to dedupe the file referenced by
+.IR dest_fd .
 .TP
 .B ETXTBSY
 One of the files is a swap file.
-- 
2.15.1