[PATCH v2 4/6] fuse: Kill suid/sgid using ATTR_MODE if it is not truncate

[Vivek Goyal <vgoyal@xxxxxxxxxx>]

If a truncate is happening with ->handle_killpriv_v2 is enabled, then
we don't have to send ATTR_MODE to kill suid/sgid as server will
kill it as part of the protocol.

But if this is non-truncate setattr then server will not kill suid/sgid.
So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr,
even if ->handle_killpriv_v2 is enabled.

This path is taken when client does a write on a file which has suid/
sgid is set. VFS will first kill suid/sgid and then proceed with WRITE.

One can argue that why not simply ignore ATTR_MODE because a WRITE
will follow and ->handle_killpriv_v2 will kill suid/sgid that time.
I feel this is a safer approach for following reasons.

- With ->writeback_cache enabled, WRITE will not go to server. I feel
  that for this reason ->writeback_cache mode is not fully compatible
  with ->handle_killpriv_v2. But if we kill suid/sgid now, this will
  solve this particular issue for ->writeback_cache mode too.

  Again, I will not solve all the issues around ->writeback_cache but
  makes things better.

- If we rely on WRITE killing suid/sgid, then after cache becomes
  out of sync w.r.t host. Client will still have suid/sgid set but
  subsequent WRITE will clear suid/sgid. Well WRITE will also invalidate
  client cache so further access to inode->i_mode should result in
  a ->getattr. Hmm..., for the case of ->writeback_cache, I am
  kind of inclined to send ATTR_MODE.

- We are sending setattr(ATTR_FORCE) anyway (even if we clear ATTR_MODE).
  So if we are not saving on setattr(), why not kill suid/sgid now.

Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
---
 fs/fuse/dir.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index ecdb7895c156..4b0fe0828e36 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1655,6 +1655,21 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 		return -EACCES;
 
 	if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
+		bool kill_sugid = true;
+		bool is_truncate = !!(attr->ia_valid & ATTR_SIZE);
+
+		if (fc->handle_killpriv ||
+		    (fc->handle_killpriv_v2 && is_truncate)) {
+			/*
+			 * If this is truncate and ->handle_killpriv_v2 is
+			 * enabled, we don't have to send ATTR_MODE to
+			 * kill suid/sgid as server will do it anyway as
+			 * part of truncate. But if this is not truncate
+			 * then kill suid/sgid by sending ATTR_MODE.
+			 */
+			kill_sugid = false;
+		}
+
 		attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
 				    ATTR_MODE);
 
@@ -1664,7 +1679,7 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr)
 		 *
 		 * This should be done on write(), truncate() and chown().
 		 */
-		if (!fc->handle_killpriv) {
+		if (kill_sugid) {
 			/*
 			 * ia_mode calculation may have used stale i_mode.
 			 * Refresh and recalculate.
-- 
2.25.4