Re: [PATCH v2 1/2] exec: add PR_HIDE_SELF_EXE prctl

Brian Masney
2023-01-19T17:50:40Z

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@redhat.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