linux-kernel - Re: [PATCH v2 2/2] powerpc/rtas: block error injection when locked down

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Message-ID: <591a3e016605181e119496992027ae21700a2c3b.camel@linux.ibm.com>
Date:   Wed, 28 Sep 2022 20:02:40 +1000
From:   Andrew Donnellan <ajd@...ux.ibm.com>
To:     Nathan Lynch <nathanl@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     jmorris@...ei.org, mpe@...erman.id.au, paul@...l-moore.com,
        serge@...lyn.com, gcwilson@...ux.ibm.com, nayna@...ux.ibm.com
Subject: Re: [PATCH v2 2/2] powerpc/rtas: block error injection when locked
 down

On Mon, 2022-09-26 at 08:16 -0500, Nathan Lynch wrote:
> The error injection facility on pseries VMs allows corruption of
> arbitrary guest memory, potentially enabling a sufficiently
> privileged
> user to disable lockdown or perform other modifications of the
> running
> kernel via the rtas syscall.
> 
> Block the PAPR error injection facility from being opened or called
> when locked down.
> 
> Signed-off-by: Nathan Lynch <nathanl@...ux.ibm.com>

Is there any circumstance (short of arbitrary code execution etc) where
the rtas_call() check will actually trigger rather than the sys_rtas()
check? (Not that it matters, defence in depth is good.)

Reviewed-by: Andrew Donnellan <ajd@...ux.ibm.com>

> ---
>  arch/powerpc/kernel/rtas.c | 25 ++++++++++++++++++++++++-
>  include/linux/security.h   |  1 +
>  security/security.c        |  1 +
>  3 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
> index 693133972294..c2540d393f1c 100644
> --- a/arch/powerpc/kernel/rtas.c
> +++ b/arch/powerpc/kernel/rtas.c
> @@ -23,6 +23,7 @@
>  #include <linux/memblock.h>
>  #include <linux/slab.h>
>  #include <linux/reboot.h>
> +#include <linux/security.h>
>  #include <linux/syscalls.h>
>  #include <linux/of.h>
>  #include <linux/of_fdt.h>
> @@ -464,6 +465,9 @@ void rtas_call_unlocked(struct rtas_args *args,
> int token, int nargs, int nret,
>         va_end(list);
>  }
>  
> +static int ibm_open_errinjct_token;
> +static int ibm_errinjct_token;
> +
>  int rtas_call(int token, int nargs, int nret, int *outputs, ...)
>  {
>         va_list list;
> @@ -476,6 +480,16 @@ int rtas_call(int token, int nargs, int nret,
> int *outputs, ...)
>         if (!rtas.entry || token == RTAS_UNKNOWN_SERVICE)
>                 return -1;
>  
> +       if (token == ibm_open_errinjct_token || token ==
> ibm_errinjct_token) {
> +               /*
> +                * It would be nicer to not discard the error value
> +                * from security_locked_down(), but callers expect an
> +                * RTAS status, not an errno.
> +                */
> +               if
> (security_locked_down(LOCKDOWN_RTAS_ERROR_INJECTION))
> +                       return -1;
> +       }
> +
>         if ((mfmsr() & (MSR_IR|MSR_DR)) != (MSR_IR|MSR_DR)) {
>                 WARN_ON_ONCE(1);
>                 return -1;
> @@ -1227,6 +1241,14 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user
> *, uargs)
>         if (block_rtas_call(token, nargs, &args))
>                 return -EINVAL;
>  
> +       if (token == ibm_open_errinjct_token || token ==
> ibm_errinjct_token) {
> +               int err;
> +
> +               err =
> security_locked_down(LOCKDOWN_RTAS_ERROR_INJECTION);
> +               if (err)
> +                       return err;
> +       }
> +
>         /* Need to handle ibm,suspend_me call specially */
>         if (token == rtas_token("ibm,suspend-me")) {
>  
> @@ -1325,7 +1347,8 @@ void __init rtas_initialize(void)
>  #ifdef CONFIG_RTAS_ERROR_LOGGING
>         rtas_last_error_token = rtas_token("rtas-last-error");
>  #endif
> -
> +       ibm_open_errinjct_token = rtas_token("ibm,open-errinjct");
> +       ibm_errinjct_token = rtas_token("ibm,errinjct");
>         rtas_syscall_filter_init();
>  }
>  
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 39e7c0e403d9..70f89dc3a712 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -123,6 +123,7 @@ enum lockdown_reason {
>         LOCKDOWN_XMON_WR,
>         LOCKDOWN_BPF_WRITE_USER,
>         LOCKDOWN_DBG_WRITE_KERNEL,
> +       LOCKDOWN_RTAS_ERROR_INJECTION,
>         LOCKDOWN_INTEGRITY_MAX,
>         LOCKDOWN_KCORE,
>         LOCKDOWN_KPROBES,
> diff --git a/security/security.c b/security/security.c
> index 51bf66d4f472..eabe3ce7e74e 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -61,6 +61,7 @@ const char *const
> lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>         [LOCKDOWN_XMON_WR] = "xmon write access",
>         [LOCKDOWN_BPF_WRITE_USER] = "use of bpf to write user RAM",
>         [LOCKDOWN_DBG_WRITE_KERNEL] = "use of kgdb/kdb to write
> kernel RAM",
> +       [LOCKDOWN_RTAS_ERROR_INJECTION] = "RTAS error injection",
>         [LOCKDOWN_INTEGRITY_MAX] = "integrity",
>         [LOCKDOWN_KCORE] = "/proc/kcore access",
>         [LOCKDOWN_KPROBES] = "use of kprobes",

-- 
Andrew Donnellan    OzLabs, ADL Canberra
ajd@...ux.ibm.com   IBM Australia Limited