From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B01DAC282CE for ; Wed, 10 Apr 2019 02:37:04 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 73F07217F4 for ; Wed, 10 Apr 2019 02:37:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 73F07217F4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=informatik.wtf Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 44f7bJ4xzszDqMG for ; Wed, 10 Apr 2019 12:37:00 +1000 (AEST) Received: from ozlabs.org (bilbo.ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 44f7YS6WHdzDqBt for ; Wed, 10 Apr 2019 12:35:24 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=informatik.wtf Received: from ozlabs.org (bilbo.ozlabs.org [IPv6:2401:3900:2:1::2]) by bilbo.ozlabs.org (Postfix) with ESMTP id 44f7YS2Vjcz8x2T for ; Wed, 10 Apr 2019 12:35:24 +1000 (AEST) Received: by ozlabs.org (Postfix) id 44f7YS1yjvz9sP3; Wed, 10 Apr 2019 12:35:24 +1000 (AEST) Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=informatik.wtf (client-ip=68.65.122.24; helo=new-02.privateemail.com; envelope-from=cmr@informatik.wtf; receiver=) Authentication-Results: ozlabs.org; dmarc=none (p=none dis=none) header.from=informatik.wtf Received: from NEW-02.privateemail.com (new-02.privateemail.com [68.65.122.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 44f7YR2HmPz9sNl for ; Wed, 10 Apr 2019 12:35:21 +1000 (AEST) Received: from MTA-10.privateemail.com (unknown [10.20.147.20]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by NEW-02.privateemail.com (Postfix) with ESMTPS id D0ABA608FE; Wed, 10 Apr 2019 02:35:18 +0000 (UTC) Received: from MTA-10.privateemail.com (localhost [127.0.0.1]) by MTA-10.privateemail.com (Postfix) with ESMTP id A3DBA60047; Tue, 9 Apr 2019 22:35:18 -0400 (EDT) Received: from APP-02 (unknown [10.20.147.152]) by MTA-10.privateemail.com (Postfix) with ESMTPA id 846B760045; Wed, 10 Apr 2019 02:35:18 +0000 (UTC) Date: Tue, 9 Apr 2019 22:35:18 -0400 (EDT) From: Christopher M Riedl To: Oliver Message-ID: <864072306.101924.1554863718473@privateemail.com> In-Reply-To: References: <20190408030804.5805-1-cmr@informatik.wtf> Subject: Re: [PATCH v2] powerpc/xmon: add read-only mode MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Priority: 3 Importance: Medium X-Mailer: Open-Xchange Mailer v7.8.4-Rev54 X-Originating-Client: open-xchange-appsuite X-Virus-Scanned: ClamAV using ClamSMTP X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@ozlabs.org, Andrew Donnellan Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" > On April 8, 2019 at 1:34 AM Oliver wrote: > > > On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl wrote: > > > > Operations which write to memory and special purpose registers should be > > restricted on systems with integrity guarantees (such as Secure Boot) > > and, optionally, to avoid self-destructive behaviors. > > > > Add a config option, XMON_RW, to control default xmon behavior along > > with kernel cmdline options xmon=ro and xmon=rw for explicit control. > > Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to > > allow xmon in read-only mode alongside write-protected kernel text. > > XMON_RW defaults to !STRICT_KERNEL_RWX. > > > > The following xmon operations are affected: > > memops: > > disable memmove > > disable memset > > disable memzcan > > memex: > > no-op'd mwrite > > super_regs: > > no-op'd write_spr > > bpt_cmds: > > disable > > proc_call: > > disable > > > > Signed-off-by: Christopher M. Riedl > > --- > > v1->v2: > > Use bool type for xmon_is_ro flag > > Replace XMON_RO with XMON_RW config option > > Make XMON_RW dependent on STRICT_KERNEL_RWX > Do you mean make it dependent on XMON? > Yeah that's really not clear at all -- XMON_RW is set based on the value of STRICT_KERNEL_RWX. > > > Use XMON_RW to control PAGE_KERNEL_TEXT > > Add printf in xmon read-only mode when dropping/skipping writes > > Disable memzcan (zero-fill memop) in xmon read-only mode > > > > arch/powerpc/Kconfig.debug | 10 +++++ > > arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- > > arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- > > arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- > > arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ > > 5 files changed, 61 insertions(+), 6 deletions(-) > > > > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > > index 4e00cb0a5464..0c7f21476018 100644 > > --- a/arch/powerpc/Kconfig.debug > > +++ b/arch/powerpc/Kconfig.debug > > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > > to say Y here, unless you're building for a memory-constrained > > system. > > > > > +config XMON_RW > > + bool "Allow xmon read and write operations" > > + depends on XMON > > + default !STRICT_KERNEL_RWX > > + help > > + Allow xmon to read and write to memory and special-purpose registers. > > + Conversely, prevent xmon write access when set to N. Read and write > > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. > > Maybe I am a dumb, but I found this *extremely* confusing. > Conventionally Kconfig options will control what code is and is not > included in the kernel (see XMON_DISASSEMBLY) rather than changing the > default behaviour of code. It's not wrong to do so and I'm going to > assume that you were following the pattern of XMON_DEFAULT, but I > think you need to be a little more clear about what option actually > does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the > description to indicate it's a only a mode change would help a lot. > > Sorry if this comes across as pointless bikeshedding since it's the > opposite of what Christophe said in the last patch, but this was a bit > of a head scratcher. > If anyone is dumb here it's me for making this confusing :) I chatted with Michael Ellerman about this, so let me try to explain this more clearly. There are two things I am trying to address with XMON_RW: 1) provide a default access mode for xmon based on system "security" 2) replace XMON in the decision to write-protect kernel text at compile-time I think a single Kconfig for both of those things is sensible as ultimately the point is to allow xmon to operate in read-only mode on "secure" systems -- without violating any integrity/security guarantees (such as write-protected kernel text). Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states: > If this is set, kernel text and rodata memory will be made read-only, > and non-text memory will be made non-executable. This provides > protection against certain security exploits (e.g. executing the heap > or modifying text) > > These features are considered standard security practice these days. > You should say Y here in almost all cases. Considering this, does XMON_DEFAULT_RO_MODE really make things more clear? With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work since kernel text is write-protected at compile time. 'xmon=ro' remains for those who would optionally like to prevent shooting themselves in the foot/feet while using xmon. Hopefully this makes a bit more sense now? > > > config DEBUGGER > > bool > > depends on KGDB || XMON > > diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h > > index aa8406b8f7ba..615144ad667d 100644 > > --- a/arch/powerpc/include/asm/book3s/32/pgtable.h > > +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h > > @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > > index 581f91be9dd4..bc4655122f6b 100644 > > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > > @@ -168,8 +168,9 @@ > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h > > index 1ca1c1864b32..c052931bd243 100644 > > --- a/arch/powerpc/include/asm/nohash/pgtable.h > > +++ b/arch/powerpc/include/asm/nohash/pgtable.h > > @@ -22,8 +22,9 @@ > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c > > index a0f44f992360..224ca0b3506b 100644 > > --- a/arch/powerpc/xmon/xmon.c > > +++ b/arch/powerpc/xmon/xmon.c > > @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; > > #endif > > static unsigned long in_xmon __read_mostly = 0; > > static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); > > +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); > > > > static unsigned long adrs; > > static int size = 1; > > @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); > > #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) > > #endif > > > > +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; > > + > > static char *help_string = "\ > > Commands:\n\ > > b show breakpoints\n\ > > @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) > > memlocate(); > > break; > > case 'z': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memzcan(); > > break; > > case 'i': > > @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) > > set_lpp_cmd(); > > break; > > case 'b': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > bpt_cmds(); > > break; > > case 'C': > > @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) > > bootcmds(); > > break; > > case 'p': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > proccall(); > > break; > > case 'P': > > @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) > > static void > > write_spr(int n, unsigned long val) > > { > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + return; > > + } > > + > > if (setjmp(bus_error_jmp) == 0) { > > catch_spr_faults = 1; > > sync(); > > @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) > > char *p, *q; > > > > n = 0; > > + > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + return n; > > + } > > + > > if (setjmp(bus_error_jmp) == 0) { > > catch_memory_errors = 1; > > sync(); > > @@ -2884,9 +2910,17 @@ memops(int cmd) > > scanhex((void *)&mcount); > > switch( cmd ){ > > case 'm': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memmove((void *)mdest, (void *)msrc, mcount); > > break; > > case 's': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memset((void *)mdest, mval, mcount); > > break; > > case 'd': > > @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) > > } else if (strncmp(p, "on", 2) == 0) { > > xmon_init(1); > > xmon_on = 1; > > + } else if (strncmp(p, "rw", 2) == 0) { > > + xmon_init(1); > > + xmon_on = 1; > > + xmon_is_ro = false; > > + } else if (strncmp(p, "ro", 2) == 0) { > > + xmon_init(1); > > + xmon_on = 1; > > + xmon_is_ro = true; > > } else if (strncmp(p, "off", 3) == 0) > > xmon_on = 0; > > else > > -- > > 2.21.0 > >