linux-kernel - Re: [PATCH v4 15/16] module: Move where we mark modules RO,X

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Date:   Wed, 23 Oct 2019 17:16:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, jeyu@...nel.org
Subject: Re: [PATCH v4 15/16] module: Move where we mark modules RO,X

On Wed, Oct 23, 2019 at 01:48:35PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 21, 2019 at 04:14:02PM +0200, Peter Zijlstra wrote:
> > On Mon, Oct 21, 2019 at 08:53:12AM -0500, Josh Poimboeuf wrote:
> 
> > > Doesn't livepatch code also need to be modified?  We have:
> > 
> > Urgh bah.. I was too focussed on the other klp borkage :/ But yes,
> > arm64/ftrace and klp are the only two users of that function (outside of
> > module.c) and Mark was already writing a patch for arm64.
> > 
> > Means these last two patches need to wait a little until we've fixed
> > those.
> 
> So the other KLP issue:
> 
> <mbenes> peterz: ad klp, apply_relocate_add() and text_poke()... what
>          about apply_alternatives() and apply_paravirt()? They call
>          text_poke_early(), which was ok with module_disable/enable_ro(), but
> 	 now it is not, I suppose. See arch_klp_init_object_loaded() in
>          arch/x86/kernel/livepatch.c.
> 
> <peterz> mbenes: hurm, I don't see why we would need to do
>          apply_alternatives() / apply_paravirt() later, why can't we do that
> 	 the moment we load the module
> 
> <peterz> mbenes: that is, those things _never_ change after boot
> 
> <mbenes> peterz: as jpoimboe explained. See commit
>          d4c3e6e1b193497da3a2ce495fb1db0243e41e37 for detailed explanation.
> 
> Now sadly that commit missed all the useful information, luckily I could
> find the patch in my LKML folder, more sad, that thread still didn't
> contain the actual useful information, for that I was directed to
> github:
> 
>   https://github.com/dynup/kpatch/issues/580
> 
> Now, someone is owning me a beer for having to look at github for this.
> 
> That finally explained that what happens is that the RELA was trying to
> fix up the paravirt indirect call to 'local_irq_disable', which
> apply_paravirt() will have overwritten with 'CLI; NOP'. This then
> obviously goes *bang*.
> 
> This then raises a number of questions:
> 
>  1) why is that RELA (that obviously does not depend on any module)
>     applied so late?
> 
>  2) why can't we unconditionally skip RELA's to paravirt sites?
> 
>  3) Is there ever a possible module-dependent RELA to a paravirt /
>     alternative site?
> 
> 
> Now, for 1), I would propose '.klp.rela.${mod}' sections only contain
> RELAs that depend on symbols in ${mod} (or modules in general). We can
> fix up RELAs that depend on core kernel early without problems. Let them
> be in the normal .rela sections and be fixed up on loading the
> patch-module as per usual.
> 
> This should also deal with 2, paravirt should always have RELAs into the
> core kernel.
> 
> Then for 3) we only have alternatives left, and I _think_ it unlikely to
> be the case, but I'll have to have a hard look at that.
> 
> Hmmm ?

Something like so on top, I suppose.

---

--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -131,7 +131,8 @@ static int __apply_relocate_add(Elf64_Sh
 		   unsigned int symindex,
 		   unsigned int relsec,
 		   struct module *me,
-		   void *(*write)(void *addr, const void *val, size_t size))
+		   void *(*write)(void *addr, const void *val, size_t size),
+		   bool klp)
 {
 	unsigned int i;
 	Elf64_Rela *rel = (void *)sechdrs[relsec].sh_addr;
@@ -157,6 +158,14 @@ static int __apply_relocate_add(Elf64_Sh
 
 		val = sym->st_value + rel[i].r_addend;
 
+		/*
+		 * .klp.rela.* sections should only contain module
+		 * related RELAs. All core-kernel RELAs should be in
+		 * normal .rela.* sections and be applied when loading
+		 * the patch module itself.
+		 */
+		WARN_ON_ONCE(klp && core_kernel_text(val));
+
 		switch (ELF64_R_TYPE(rel[i].r_info)) {
 		case R_X86_64_NONE:
 			break;
@@ -223,7 +232,7 @@ int apply_relocate_add(Elf64_Shdr *sechd
 		   unsigned int relsec,
 		   struct module *me)
 {
-	return __apply_relocate_add(sechdrs, strtab, symindex, relsec, me, memcpy);
+	return __apply_relocate_add(sechdrs, strtab, symindex, relsec, me, memcpy, false);
 }
 
 int klp_apply_relocate_add(Elf64_Shdr *sechdrs,
@@ -234,7 +243,7 @@ int klp_apply_relocate_add(Elf64_Shdr *s
 {
 	int ret;
 
-	ret = __apply_relocate_add(sechdrs, strtab, symindex, relsec, me, text_poke);
+	ret = __apply_relocate_add(sechdrs, strtab, symindex, relsec, me, text_poke, true);
 	if (!ret)
 		text_poke_sync();