linux-kernel - Re: [PATCH v2 3/3] pidfs: implement file handle support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAOQ4uxh2HWkVE_aMeYSTsYRO9_sKMPH7V2uksWFSo3ucWOoJ2g@mail.gmail.com>
Date: Thu, 14 Nov 2024 08:07:52 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Erin Shepherd <erin.shepherd@....eu>
Cc: Christian Brauner <brauner@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	Chuck Lever <chuck.lever@...cle.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>, 
	linux-nfs@...r.kernel.org
Subject: Re: [PATCH v2 3/3] pidfs: implement file handle support

On Wed, Nov 13, 2024 at 7:11 PM Erin Shepherd <erin.shepherd@....eu> wrote:
>
> On 64-bit platforms, userspace can read the pidfd's inode in order to
> get a never-repeated PID identifier. On 32-bit platforms this identifier
> is not exposed, as inodes are limited to 32 bits. Instead expose the
> identifier via export_fh, which makes it available to userspace via
> name_to_handle_at
>
> In addition we implement fh_to_dentry, which allows userspace to
> recover a pidfd from a PID file handle.

"In addition" is a good indication that a separate patch was a good idea..

>
> We stash the process' PID in the root pid namespace inside the handle,
> and use that to recover the pid (validating that pid->ino matches the
> value in the handle, i.e. that the pid has not been reused).
>
> We use the root namespace in order to ensure that file handles can be
> moved across namespaces; however, we validate that the PID exists in
> the current namespace before returning the inode.
>
> Reviewed-by: Amir Goldstein <amir73il@...il.com>

This patch has changed enough that you should not have kept my RVB.

BTW, please refrain from using the same subject for the cover letter and
a single patch. They are not the same thing, so if they get the same
name, one of them has an inaccurate description.

> Signed-off-by: Erin Shepherd <erin.shepherd@....eu>
> ---
>  fs/pidfs.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 61 insertions(+), 1 deletion(-)
>
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index 80675b6bf88459c22787edaa68db360bdc0d0782..0684a9b8fe71c5205fb153b2714bc9c672045fd5 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/anon_inodes.h>
> +#include <linux/exportfs.h>
>  #include <linux/file.h>
>  #include <linux/fs.h>
>  #include <linux/magic.h>
> @@ -347,11 +348,69 @@ static const struct dentry_operations pidfs_dentry_operations = {
>         .d_prune        = stashed_dentry_prune,
>  };
>
> +#define PIDFD_FID_LEN 3
> +
> +struct pidfd_fid {
> +       u64 ino;
> +       s32 pid;
> +} __packed;
> +
> +static int pidfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
> +                          struct inode *parent)
> +{
> +       struct pid *pid = inode->i_private;
> +       struct pidfd_fid *fid = (struct pidfd_fid *)fh;
> +
> +       if (*max_len < PIDFD_FID_LEN) {
> +               *max_len = PIDFD_FID_LEN;
> +               return FILEID_INVALID;
> +       }
> +
> +       fid->ino = pid->ino;
> +       fid->pid = pid_nr(pid);
> +       *max_len = PIDFD_FID_LEN;
> +       return FILEID_INO64_GEN;
> +}
> +
> +static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
> +                                        struct fid *gen_fid,
> +                                        int fh_len, int fh_type)
> +{
> +       int ret;
> +       struct path path;
> +       struct pidfd_fid *fid = (struct pidfd_fid *)gen_fid;
> +       struct pid *pid;
> +
> +       if (fh_type != FILEID_INO64_GEN || fh_len < PIDFD_FID_LEN)
> +               return NULL;
> +
> +       scoped_guard(rcu) {
> +               pid = find_pid_ns(fid->pid, &init_pid_ns);
> +               if (!pid || pid->ino != fid->ino || pid_vnr(pid) == 0)
> +                       return NULL;
> +
> +               pid = get_pid(pid);
> +       }
> +
> +       ret = path_from_stashed(&pid->stashed, pidfs_mnt, pid, &path);
> +       if (ret < 0)
> +               return ERR_PTR(ret);

How come no need to put_pid() in case of error?

> +
> +       mntput(path.mnt);
> +       return path.dentry;
> +}
> +
> +static const struct export_operations pidfs_export_operations = {
> +       .encode_fh = pidfs_encode_fh,
> +       .fh_to_dentry = pidfs_fh_to_dentry,
> +       .flags = EXPORT_OP_UNRESTRICTED_OPEN,
> +};
> +
>  static int pidfs_init_inode(struct inode *inode, void *data)
>  {
>         inode->i_private = data;
>         inode->i_flags |= S_PRIVATE;
> -       inode->i_mode |= S_IRWXU;
> +       inode->i_mode |= S_IRWXU | S_IRWXG | S_IRWXO;

This change is not explained.
Why is it here?

Thanks,
Amir.