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Date: Tue, 24 Jan 2023 12:53:48 +1100
From: Aleksa Sarai <cyphar@...har.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,
viro@...iv.linux.org.uk, alexl@...hat.com, peterz@...radead.org,
bmasney@...hat.com
Subject: Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl
On 2023-01-20, Giuseppe Scrivano <gscrivan@...hat.com> 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.
I suspect this doesn't protect against the execve("/proc/self/exe")
tactic (because it clears the bit on execve), so I'm not sure this is
much safer than PR_SET_DUMPABLE (yeah, it stops root in the source
userns from accessing /proc/$pid/exe but the above attack makes that no
longer that important).
I think the only way to fix this properly is by blocking re-opens of
magic links that have more permissions than they originally did. I just
got back from vacation, but I'm working on fixing up [1] so it's ready
to be an RFC so we can close this hole once and for all.
[1]: https://github.com/cyphar/linux/tree/magiclink/open_how-reopen
>
> Signed-off-by: Giuseppe Scrivano <gscrivan@...hat.com>
> ---
> v2: https://lkml.org/lkml/2023/1/19/849
>
> Differences from v2:
>
> - fixed the test to check PR_SET_HIDE_SELF_EXE after fork
>
> 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.
>
> 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(-)
>
> 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);
> 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);
> put_task_struct(task);
> - if (exe_file) {
> + if (exe_file && !hide_self_exe) {
> *exe_path = exe_file->f_path;
> path_get(&exe_file->f_path);
> fput(exe_file);
> return 0;
> - } else
> - return -ENOENT;
> + }
> + return -ENOENT;
> }
>
> static const char *proc_pid_get_link(struct dentry *dentry,
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 853d08f7562b..8db32d5fc285 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1790,6 +1790,7 @@ static __always_inline bool is_percpu_thread(void)
> #define PFA_SPEC_IB_DISABLE 5 /* Indirect branch speculation restricted */
> #define PFA_SPEC_IB_FORCE_DISABLE 6 /* Indirect branch speculation permanently restricted */
> #define PFA_SPEC_SSB_NOEXEC 7 /* Speculative Store Bypass clear on execve() */
> +#define PFA_HIDE_SELF_EXE 8 /* Hide /proc/self/exe for the process */
>
> #define TASK_PFA_TEST(name, func) \
> static inline bool task_##func(struct task_struct *p) \
> @@ -1832,6 +1833,10 @@ TASK_PFA_CLEAR(SPEC_IB_DISABLE, spec_ib_disable)
> TASK_PFA_TEST(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable)
> TASK_PFA_SET(SPEC_IB_FORCE_DISABLE, spec_ib_force_disable)
>
> +TASK_PFA_TEST(HIDE_SELF_EXE, hide_self_exe)
> +TASK_PFA_SET(HIDE_SELF_EXE, hide_self_exe)
> +TASK_PFA_CLEAR(HIDE_SELF_EXE, hide_self_exe)
> +
> static inline void
> current_restore_flags(unsigned long orig_flags, unsigned long flags)
> {
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index a5e06dcbba13..f12f3df12468 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -284,4 +284,7 @@ struct prctl_mm_map {
> #define PR_SET_VMA 0x53564d41
> # define PR_SET_VMA_ANON_NAME 0
>
> +#define PR_SET_HIDE_SELF_EXE 65
> +#define PR_GET_HIDE_SELF_EXE 66
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 5fd54bf0e886..e992f1b72973 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2626,6 +2626,15 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> case PR_SET_VMA:
> error = prctl_set_vma(arg2, arg3, arg4, arg5);
> break;
> + case PR_SET_HIDE_SELF_EXE:
> + if (arg2 != 1 || arg3 || arg4 || arg5)
> + return -EINVAL;
> + task_set_hide_self_exe(current);
> + break;
> + case PR_GET_HIDE_SELF_EXE:
> + if (arg2 || arg3 || arg4 || arg5)
> + return -EINVAL;
> + return task_hide_self_exe(current) ? 1 : 0;
> default:
> error = -EINVAL;
> break;
> diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
> index a5e06dcbba13..f12f3df12468 100644
> --- a/tools/include/uapi/linux/prctl.h
> +++ b/tools/include/uapi/linux/prctl.h
> @@ -284,4 +284,7 @@ struct prctl_mm_map {
> #define PR_SET_VMA 0x53564d41
> # define PR_SET_VMA_ANON_NAME 0
>
> +#define PR_SET_HIDE_SELF_EXE 65
> +#define PR_GET_HIDE_SELF_EXE 66
> +
> #endif /* _LINUX_PRCTL_H */
> --
> 2.38.1
>
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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