Re: [PATCH] NFSv4.2: request mode after CLONE

[Chuck Lever]

On Wed, Aug 12, 2026, at 2:25 AM, Cedric Blancher wrote:
> On Sun, 2 Aug 2026 at 21:58, Felix Hoffmann <f3lix.dev@xxxxxx> 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@xxxxxxxxxxxxxxx
>> Assisted-by: Codex:gpt-5
>> Signed-off-by: Felix Hoffmann <f3lix.dev@xxxxxx>
>> ---
>> 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?

Responding since you asked me directly, but I don't have a
considered opinion about this. Just some advice: the OP
provided a reproducer, you could try that and see. Naively
I would expect it might apply to COPY.

-- 
Chuck Lever