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Message-ID: <F84CA28B-8BAA-468E-BA85-442E245871E5@zytor.com> Date: Thu, 17 Jan 2019 14:59:09 -0800 From: hpa@...or.com To: Nadav Amit <namit@...are.com> CC: Masami Hiramatsu <mhiramat@...nel.org>, Rick Edgecombe <rick.p.edgecombe@...el.com>, Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>, LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>, Damian Tometzki <linux_dti@...oud.com>, linux-integrity <linux-integrity@...r.kernel.org>, LSM List <linux-security-module@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux-MM <linux-mm@...ck.org>, Will Deacon <will.deacon@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Kristen Carlson Accardi <kristen@...ux.intel.com>, "Dock, Deneen T" <deneen.t.dock@...el.com>, Kees Cook <keescook@...omium.org>, Dave Hansen <dave.hansen@...el.com> Subject: Re: [PATCH 01/17] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()" On January 17, 2019 2:39:15 PM PST, Nadav Amit <namit@...are.com> wrote: >> On Jan 17, 2019, at 1:15 PM, hpa@...or.com wrote: >> >> On January 16, 2019 10:47:01 PM PST, Masami Hiramatsu ><mhiramat@...nel.org> wrote: >>> On Wed, 16 Jan 2019 16:32:43 -0800 >>> Rick Edgecombe <rick.p.edgecombe@...el.com> wrote: >>> >>>> From: Nadav Amit <namit@...are.com> >>>> >>>> text_mutex is currently expected to be held before text_poke() is >>>> called, but we kgdb does not take the mutex, and instead >*supposedly* >>>> ensures the lock is not taken and will not be acquired by any other >>> core >>>> while text_poke() is running. >>>> >>>> The reason for the "supposedly" comment is that it is not entirely >>> clear >>>> that this would be the case if gdb_do_roundup is zero. >>>> >>>> This patch creates two wrapper functions, text_poke() and >>>> text_poke_kgdb() which do or do not run the lockdep assertion >>>> respectively. >>>> >>>> While we are at it, change the return code of text_poke() to >>> something >>>> meaningful. One day, callers might actually respect it and the >>> existing >>>> BUG_ON() when patching fails could be removed. For kgdb, the return >>>> value can actually be used. >>> >>> Looks good to me. >>> >>> Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org> >>> >>> Thank you, >>> >>>> Cc: Andy Lutomirski <luto@...nel.org> >>>> Cc: Kees Cook <keescook@...omium.org> >>>> Cc: Dave Hansen <dave.hansen@...el.com> >>>> Cc: Masami Hiramatsu <mhiramat@...nel.org> >>>> Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex >in >>> text_poke*()") >>>> Suggested-by: Peter Zijlstra <peterz@...radead.org> >>>> Acked-by: Jiri Kosina <jkosina@...e.cz> >>>> Signed-off-by: Nadav Amit <namit@...are.com> >>>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com> >>>> --- >>>> arch/x86/include/asm/text-patching.h | 1 + >>>> arch/x86/kernel/alternative.c | 52 >>> ++++++++++++++++++++-------- >>>> arch/x86/kernel/kgdb.c | 11 +++--- >>>> 3 files changed, 45 insertions(+), 19 deletions(-) >>>> >>>> diff --git a/arch/x86/include/asm/text-patching.h >>> b/arch/x86/include/asm/text-patching.h >>>> index e85ff65c43c3..f8fc8e86cf01 100644 >>>> --- a/arch/x86/include/asm/text-patching.h >>>> +++ b/arch/x86/include/asm/text-patching.h >>>> @@ -35,6 +35,7 @@ extern void *text_poke_early(void *addr, const >void >>> *opcode, size_t len); >>>> * inconsistent instruction while you patch. >>>> */ >>>> extern void *text_poke(void *addr, const void *opcode, size_t len); >>>> +extern void *text_poke_kgdb(void *addr, const void *opcode, size_t >>> len); >>>> extern int poke_int3_handler(struct pt_regs *regs); >>>> extern void *text_poke_bp(void *addr, const void *opcode, size_t >>> len, void *handler); >>>> extern int after_bootmem; >>>> diff --git a/arch/x86/kernel/alternative.c >>> b/arch/x86/kernel/alternative.c >>>> index ebeac487a20c..c6a3a10a2fd5 100644 >>>> --- a/arch/x86/kernel/alternative.c >>>> +++ b/arch/x86/kernel/alternative.c >>>> @@ -678,18 +678,7 @@ void *__init_or_module text_poke_early(void >>> *addr, const void *opcode, >>>> return addr; >>>> } >>>> >>>> -/** >>>> - * text_poke - Update instructions on a live kernel >>>> - * @addr: address to modify >>>> - * @opcode: source of the copy >>>> - * @len: length to copy >>>> - * >>>> - * Only atomic text poke/set should be allowed when not doing >early >>> patching. >>>> - * It means the size must be writable atomically and the address >>> must be aligned >>>> - * in a way that permits an atomic write. It also makes sure we >fit >>> on a single >>>> - * page. >>>> - */ >>>> -void *text_poke(void *addr, const void *opcode, size_t len) >>>> +static void *__text_poke(void *addr, const void *opcode, size_t >len) >>>> { >>>> unsigned long flags; >>>> char *vaddr; >>>> @@ -702,8 +691,6 @@ void *text_poke(void *addr, const void *opcode, >>> size_t len) >>>> */ >>>> BUG_ON(!after_bootmem); >>>> >>>> - lockdep_assert_held(&text_mutex); >>>> - >>>> if (!core_kernel_text((unsigned long)addr)) { >>>> pages[0] = vmalloc_to_page(addr); >>>> pages[1] = vmalloc_to_page(addr + PAGE_SIZE); >>>> @@ -732,6 +719,43 @@ void *text_poke(void *addr, const void >*opcode, >>> size_t len) >>>> return addr; >>>> } >>>> >>>> +/** >>>> + * text_poke - Update instructions on a live kernel >>>> + * @addr: address to modify >>>> + * @opcode: source of the copy >>>> + * @len: length to copy >>>> + * >>>> + * Only atomic text poke/set should be allowed when not doing >early >>> patching. >>>> + * It means the size must be writable atomically and the address >>> must be aligned >>>> + * in a way that permits an atomic write. It also makes sure we >fit >>> on a single >>>> + * page. >>>> + */ >>>> +void *text_poke(void *addr, const void *opcode, size_t len) >>>> +{ >>>> + lockdep_assert_held(&text_mutex); >>>> + >>>> + return __text_poke(addr, opcode, len); >>>> +} >>>> + >>>> +/** >>>> + * text_poke_kgdb - Update instructions on a live kernel by kgdb >>>> + * @addr: address to modify >>>> + * @opcode: source of the copy >>>> + * @len: length to copy >>>> + * >>>> + * Only atomic text poke/set should be allowed when not doing >early >>> patching. >>>> + * It means the size must be writable atomically and the address >>> must be aligned >>>> + * in a way that permits an atomic write. It also makes sure we >fit >>> on a single >>>> + * page. >>>> + * >>>> + * Context: should only be used by kgdb, which ensures no other >core >>> is running, >>>> + * despite the fact it does not hold the text_mutex. >>>> + */ >>>> +void *text_poke_kgdb(void *addr, const void *opcode, size_t len) >>>> +{ >>>> + return __text_poke(addr, opcode, len); >>>> +} >>>> + >>>> static void do_sync_core(void *info) >>>> { >>>> sync_core(); >>>> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c >>>> index 5db08425063e..1461544cba8b 100644 >>>> --- a/arch/x86/kernel/kgdb.c >>>> +++ b/arch/x86/kernel/kgdb.c >>>> @@ -758,13 +758,13 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt >>> *bpt) >>>> if (!err) >>>> return err; >>>> /* >>>> - * It is safe to call text_poke() because normal kernel execution >>>> + * It is safe to call text_poke_kgdb() because normal kernel >>> execution >>>> * is stopped on all cores, so long as the text_mutex is not >>> locked. >>>> */ >>>> if (mutex_is_locked(&text_mutex)) >>>> return -EBUSY; >>>> - text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr, >>>> - BREAK_INSTR_SIZE); >>>> + text_poke_kgdb((void *)bpt->bpt_addr, >arch_kgdb_ops.gdb_bpt_instr, >>>> + BREAK_INSTR_SIZE); >>>> err = probe_kernel_read(opc, (char *)bpt->bpt_addr, >>> BREAK_INSTR_SIZE); >>>> if (err) >>>> return err; >>>> @@ -783,12 +783,13 @@ int kgdb_arch_remove_breakpoint(struct >>> kgdb_bkpt *bpt) >>>> if (bpt->type != BP_POKE_BREAKPOINT) >>>> goto knl_write; >>>> /* >>>> - * It is safe to call text_poke() because normal kernel execution >>>> + * It is safe to call text_poke_kgdb() because normal kernel >>> execution >>>> * is stopped on all cores, so long as the text_mutex is not >>> locked. >>>> */ >>>> if (mutex_is_locked(&text_mutex)) >>>> goto knl_write; >>>> - text_poke((void *)bpt->bpt_addr, bpt->saved_instr, >>> BREAK_INSTR_SIZE); >>>> + text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr, >>>> + BREAK_INSTR_SIZE); >>>> err = probe_kernel_read(opc, (char *)bpt->bpt_addr, >>> BREAK_INSTR_SIZE); >>>> if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE)) >>>> goto knl_write; >>>> -- >>>> 2.17.1 >> >> If you are reorganizing this code, please do so so that the caller >doesn’t >> have to worry about if it should call text_poke_bp() or >text_poke_early(). >> Right now the caller had to know that, which makes no sense. > >Did you look at "[11/17] x86/jump-label: remove support for custom >poker”? > >https://lore.kernel.org/patchwork/patch/1032857/ > >If this is not what you regard, please be more concrete. >text_poke_early() >is still used directly on init and while modules are loaded, which >might not >be great, but is outside of the scope of this patch-set. I don't think it is out of scope, although that patch is a huge step in the right direction. text_poke_{early,bp,...}, however, should be fully internal, that is, static functions, and we should present a single interface, preferably called text_poke(), to the outside world. I think we have three subcases: 1. Early, UP, or under stop_machine(); 2. Atomic and aligned; 3. Breakpoint. My proposed algorithm should remove the need for a fixup which should help this interface, too. The specific alignment needed for #2 is started by the hardware people to be not crossing 16 bytes (NOT a cache line) on any CPU we support SMP on and, of course, being possible to do atomically do on the specific CPU (note that we *can* do a redundantly large store of existing bytes, which adds flexibility.) To the best of my knowledge any CPU supporting SSE can do an atomic (for our purposes) aligned 16-byte store via MOVAPS; of course any CPU with cx16 can do it without SSE registers. For older CPUs we may be limited to 8-byte stores (cx8) or even 4-byte stores before we need to use the breakpoint algorithm. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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