linux-kernel - Re: [PATCH v2 1/2] exec: add PR_HIDE_SELF

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Message-ID: <Y8mCqn3fIaqk9N0Q@x1>
Date:   Thu, 19 Jan 2023 12:49:30 -0500
From:   Brian Masney <bmasney@...hat.com>
To:     Giuseppe Scrivano <gscrivan@...hat.com>
Cc:     linux-kernel@...r.kernel.org, keescook@...omium.org,
        bristot@...hat.com, ebiederm@...ssion.com, brauner@...nel.org,
        cyphar@...har.com, viro@...iv.linux.org.uk, alexl@...hat.com,
        peterz@...radead.org
Subject: Re: [PATCH v2 1/2] exec: add PR_HIDE_SELF_EXE prctl

On Thu, Jan 19, 2023 at 06:07:17PM +0100, Giuseppe Scrivano wrote:
> This patch adds a new prctl called PR_HIDE_SELF_EXE which allows
> processes to hide their own /proc/*/exe file. When this prctl is
> used, every access to /proc/*/exe for the calling process will
> fail with ENOENT.
> 
> This is useful for preventing issues like CVE-2019-5736, where an
> attacker can gain host root access by overwriting the binary
> in OCI runtimes through file-descriptor mishandling in containers.
> 
> The current fix for CVE-2019-5736 is to create a read-only copy or
> a bind-mount of the current executable, and then re-exec the current
> process.  With the new prctl, the read-only copy or bind-mount copy is
> not needed anymore.
> 
> While map_files/ also might contain symlinks to files in host,
> proc_map_files_get_link() permissions checks are already sufficient.
> 
> Signed-off-by: Giuseppe Scrivano <gscrivan@...hat.com>
> ---
>  fs/exec.c                        | 1 +
>  fs/proc/base.c                   | 8 +++++---
>  include/linux/sched.h            | 5 +++++
>  include/uapi/linux/prctl.h       | 3 +++
>  kernel/sys.c                     | 9 +++++++++
>  tools/include/uapi/linux/prctl.h | 3 +++
>  6 files changed, 26 insertions(+), 3 deletions(-)
> 
> v1: https://lkml.org/lkml/2023/1/4/334
> 
> Differences from v1:
> 
> - amended more information in the commit message wrt map_files not
>   requiring the same protection.
> - changed the test to verify PR_HIDE_SELF_EXE cannot be unset after
>   a fork.
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index ab913243a367..5a5dd964c3a3 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1855,6 +1855,7 @@ static int bprm_execve(struct linux_binprm *bprm,
>  	/* execve succeeded */
>  	current->fs->in_exec = 0;
>  	current->in_execve = 0;
> +	task_clear_hide_self_exe(current);

[snip]

>  	rseq_execve(current);
>  	acct_update_integrals(current);
>  	task_numa_free(current, false);
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 9e479d7d202b..959968e2da0d 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1723,19 +1723,21 @@ static int proc_exe_link(struct dentry *dentry, struct path *exe_path)
>  {
>  	struct task_struct *task;
>  	struct file *exe_file;
> +	long hide_self_exe;
>  
>  	task = get_proc_task(d_inode(dentry));
>  	if (!task)
>  		return -ENOENT;
>  	exe_file = get_task_exe_file(task);
> +	hide_self_exe = task_hide_self_exe(task);

Perhaps I am missing something, but where is task_clear_hide_self_exe()
and task_hide_self_exe() defined?

Brian