linux-kernel - Re: [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching

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Message-Id: <3EA322C6-5645-4900-AEC6-97FC05716F75@gmail.com>
Date:   Mon, 11 Feb 2019 11:18:54 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        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 v2 05/20] x86/alternative: initializing temporary mm for
 patching

> On Feb 11, 2019, at 11:07 AM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> I'm certainly amenable to other solutions, but this one does seem the
> least messy.  I looked at my old patch, and it doesn't do what you
> want.  I'd suggest you just add a percpu variable like cpu_dr7 and rig
> up some accessors so that it stays up to date.  Then you can skip the
> dr7 writes if there are no watchpoints set.
> 
> Also, EFI is probably a less interesting example than rare_write.
> With rare_write, especially the dynamically allocated variants that
> people keep coming up with, we'll need a swath of address space fully
> as large as the vmalloc area. and getting *that* right while still
> using the kernel address range might be more of a mess than we really
> want to deal with.

As long as you feel comfortable with this solution, I’m fine with it.

Here is what I have (untested). I prefer to save/restore all the DRs,
because IIRC DR6 indications are updated even if breakpoints are disabled
(in DR7). And anyhow, that is the standard interface.


-- >8 --

From: Nadav Amit <namit@...are.com>
Date: Mon, 11 Feb 2019 03:07:08 -0800
Subject: [PATCH] mm: save DRs when loading temporary mm

Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/include/asm/mmu_context.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index d684b954f3c0..4f92ec3df149 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -13,6 +13,7 @@
 #include <asm/tlbflush.h>
 #include <asm/paravirt.h>
 #include <asm/mpx.h>
+#include <asm/debugreg.h>
 
 extern atomic64_t last_mm_ctx_id;
 
@@ -358,6 +359,7 @@ static inline unsigned long __get_current_cr3_fast(void)
 
 typedef struct {
 	struct mm_struct *prev;
+	unsigned short bp_enabled : 1;
 } temp_mm_state_t;
 
 /*
@@ -380,6 +382,15 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
 	lockdep_assert_irqs_disabled();
 	state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
 	switch_mm_irqs_off(NULL, mm, current);
+
+	/*
+	 * If breakpoints are enabled, disable them while the temporary mm is
+	 * used - they do not belong and might cause wrong signals or crashes.
+	 */
+	state.bp_enabled = hw_breakpoint_active();
+	if (state.bp_enabled)
+		hw_breakpoint_disable();
+
 	return state;
 }
 
@@ -387,6 +398,13 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev)
 {
 	lockdep_assert_irqs_disabled();
 	switch_mm_irqs_off(NULL, prev.prev, current);
+
+	/*
+	 * Restore the breakpoints if they were disabled before the temporary mm
+	 * was loaded.
+	 */
+	if (prev.bp_enabled)
+		hw_breakpoint_restore();
 }
 
 #endif /* _ASM_X86_MMU_CONTEXT_H */
-- 
2.17.1