Re: [PATCH 3/4] fix setuid sometimes wouldn't

[Alexey Dobriyan]

On Sat, Mar 28, 2009 at 11:21:27PM +0000, Hugh Dickins wrote:
> check_unsafe_exec() also notes whether the fs_struct is being
> shared by more threads than will get killed by the exec, and if so
> sets LSM_UNSAFE_SHARE to make bprm_set_creds() careful about euid.
> But /proc/<pid>/cwd and /proc/<pid>/root lookups make transient
> use of get_fs_struct(), which also raises that sharing count.
> 
> This might occasionally cause a setuid program not to change euid,
> in the same way as happened with files->count (check_unsafe_exec
> also looks at sighand->count, but /proc doesn't raise that one).
> 
> We'd prefer exec not to unshare fs_struct: so fix this in procfs,
> replacing get_fs_struct() by get_fs_path(), which does path_get
> while still holding task_lock, instead of raising fs->count.

> --- 2.6.29/fs/proc/base.c
> +++ linux/fs/proc/base.c
> @@ -146,15 +146,22 @@ static unsigned int pid_entry_count_dirs
>  	return count;
>  }
>  
> -static struct fs_struct *get_fs_struct(struct task_struct *task)
> +static int get_fs_path(struct task_struct *task, struct path *path, bool root)
>  {
>  	struct fs_struct *fs;
> +	int result = -ENOENT;
> +
>  	task_lock(task);
>  	fs = task->fs;
> -	if(fs)
> -		atomic_inc(&fs->count);
> +	if (fs) {
> +		read_lock(&fs->lock);
> +		*path = root ? fs->root : fs->pwd;
> +		path_get(path);
> +		read_unlock(&fs->lock);
> +		result = 0;
> +	}
>  	task_unlock(task);
> -	return fs;
> +	return result;
>  }

I think it's better to open-code. "root" parameter is unnatural.
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