The NFS server can clear setuid and setgid bits when CLONE modifies the
destination.  The client currently omits mode from the post-operation
attributes, so the cached mode can retain setid bits after the server has
removed them.  A local process can then execute the file using those stale
privilege bits.

Request mode in the CLONE compound's GETATTR.  This lets the ordered
post-operation update install the server's mode before CLONE returns.

Fixes: e5341f3a5762 ("nfs42: add CLONE proc functions")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5
Signed-off-by: Felix Hoffmann <f3lix.dev@gmx.de>
---
Tested on x86-64 using an NFSv4.2 loopback mount with Linux NFSD and a
Btrfs export:

  - before this change, the client retained mode 04775 while the server
    stored mode 0775, and the privilege transition reproduced in 10/10
    non-debug runs
  - with this change, both client and server reported mode 0775 and no
    privilege transition occurred in 10/10 non-debug runs or 3/3 KASAN
    runs

The reproducer requires write authorization to the destination; NFSD
rejected the same operation against a non-writable 04755 file with EACCES.
A working reproducer is available privately on request.

 fs/nfs/nfs42proc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/nfs42proc.c b/fs/nfs/nfs42proc.c
index ab86246fc364..35a4909d51b7 100644
--- a/fs/nfs/nfs42proc.c
+++ b/fs/nfs/nfs42proc.c
@@ -1302,7 +1302,8 @@ static int _nfs42_proc_clone(struct rpc_message *msg, struct file *src_f,
 		return -ENOMEM;
 
 	nfs4_bitmask_set(dst_bitmask, server->cache_consistency_bitmask,
-			 dst_inode, NFS_INO_INVALID_BLOCKS);
+			 dst_inode,
+			 NFS_INO_INVALID_BLOCKS | NFS_INO_INVALID_MODE);
 
 	status = nfs4_call_sync(server->client, server, msg,
 				&args.seq_args, &res.seq_res, 0);
-- 
2.43.0

On Sun, 2 Aug 2026 at 21:58, Felix Hoffmann <f3lix.dev@gmx.de> wrote:
>
> The NFS server can clear setuid and setgid bits when CLONE modifies the
> destination.  The client currently omits mode from the post-operation
> attributes, so the cached mode can retain setid bits after the server has
> removed them.  A local process can then execute the file using those stale
> privilege bits.
>
> Request mode in the CLONE compound's GETATTR.  This lets the ordered
> post-operation update install the server's mode before CLONE returns.
>
> Fixes: e5341f3a5762 ("nfs42: add CLONE proc functions")
> Cc: stable@vger.kernel.org
> Assisted-by: Codex:gpt-5
> Signed-off-by: Felix Hoffmann <f3lix.dev@gmx.de>
> ---
> Tested on x86-64 using an NFSv4.2 loopback mount with Linux NFSD and a
> Btrfs export:
>
>   - before this change, the client retained mode 04775 while the server
>     stored mode 0775, and the privilege transition reproduced in 10/10
>     non-debug runs
>   - with this change, both client and server reported mode 0775 and no
>     privilege transition occurred in 10/10 non-debug runs or 3/3 KASAN
>     runs
>
> The reproducer requires write authorization to the destination; NFSD
> rejected the same operation against a non-writable 04755 file with EACCES.
> A working reproducer is available privately on request.
>
>  fs/nfs/nfs42proc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/nfs/nfs42proc.c b/fs/nfs/nfs42proc.c
> index ab86246fc364..35a4909d51b7 100644
> --- a/fs/nfs/nfs42proc.c
> +++ b/fs/nfs/nfs42proc.c
> @@ -1302,7 +1302,8 @@ static int _nfs42_proc_clone(struct rpc_message *msg, struct file *src_f,
>                 return -ENOMEM;
>
>         nfs4_bitmask_set(dst_bitmask, server->cache_consistency_bitmask,
> -                        dst_inode, NFS_INO_INVALID_BLOCKS);
> +                        dst_inode,
> +                        NFS_INO_INVALID_BLOCKS | NFS_INO_INVALID_MODE);
>
>         status = nfs4_call_sync(server->client, server, msg,
>                                 &args.seq_args, &res.seq_res, 0);

Chuck, what do you think? Does this apply only to NFS CLONE, or to NFS COPY too?

Ced

--
Cedric Blancher <cedric.blancher@gmail.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur