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From: Nadav Amit <nadav.amit@gmail.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Damian Tometzki <linux_dti@icloud.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux-MM <linux-mm@kvack.org>, Will Deacon <will.deacon@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Kristen Carlson Accardi <kristen@linux.intel.com>,
	"Dock, Deneen T" <deneen.t.dock@intel.com>,
	Kees Cook <keescook@chromium.org>,
	Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching
Date: Tue, 12 Feb 2019 10:23:35 -0800
Message-ID: <570C76C6-DDD2-49C4-8DAF-E8CFEAA21081@gmail.com> (raw)
In-Reply-To: <CALCETrXE0n4BrkX2ZLDJjdLqD-N_WwSZHt=S2KKBrTV6Zt5Teg@mail.gmail.com>

> On Feb 11, 2019, at 2:47 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Mon, Feb 11, 2019 at 11:18 AM Nadav Amit <nadav.amit@gmail.com> wrote:
>> 
>> +
>> +       /*
>> +        * If breakpoints are enabled, disable them while the temporary mm is
>> +        * used - they do not belong and might cause wrong signals or crashes.
>> +        */
> 
> Maybe clarify this?  Add some mention that the specific problem is
> that user code could set a watchpoint on an address that is also used
> in the temporary mm.
> 
> Arguably we should not disable *kernel* breakpoints a la perf, but
> that seems like quite a minor issue, at least as long as
> use_temporary_mm() doesn't get wider use.  But a comment that this
> also disables perf breakpoints and that this could be undesirable
> might be in order as well.

I think that in the future there may also be security benefits for disabling
breakpoints when you are in a sensitive code-block, for instance when you
poke text, to prevent the control flow from being hijacked (by exploiting a
bug in the debug exception handler). Some additional steps need to be taken
for that to be beneficial so I leave it out of the comment for now.

Anyhow, how about this:

-- >8 --

From: Nadav Amit <namit@vmware.com>
Date: Mon, 11 Feb 2019 03:07:08 -0800
Subject: [PATCH] x86/mm: Save DRs when loading a temporary mm

Prevent user watchpoints from mistakenly firing while the temporary mm
is being used. As the addresses that of the temporary mm might overlap
those of the user-process, this is necessary to prevent wrong signals
or worse things from happening.

Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/include/asm/mmu_context.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index d684b954f3c0..0d6c72ece750 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 +382,22 @@ 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. Userspace might set up watchpoints on addresses that are used
+	 * in the temporary mm, which would lead to wrong signals being sent or
+	 * crashes.
+	 *
+	 * Note that breakpoints are not disabled selectively, which also causes
+	 * kernel breakpoints (e.g., perf's) to be disabled. This might be
+	 * undesirable, but still seems reasonable as the code that runs in the
+	 * temporary mm should be short.
+	 */
+	state.bp_enabled = hw_breakpoint_active();
+	if (state.bp_enabled)
+		hw_breakpoint_disable();
+
 	return state;
 }
 
@@ -387,6 +405,13 @@ 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

  reply index

Thread overview: 71+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-29  0:34 [PATCH v2 00/20] Merge text_poke fixes and executable lockdowns Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 01/20] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()" Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 02/20] x86/jump_label: Use text_poke_early() during early init Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 03/20] x86/mm: temporary mm struct Rick Edgecombe
2019-01-31 11:29   ` Borislav Petkov
2019-01-31 22:19     ` Nadav Amit
2019-02-01  0:08       ` Borislav Petkov
2019-02-01  0:25         ` Nadav Amit
2019-02-04 14:28       ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 04/20] fork: provide a function for copying init_mm Rick Edgecombe
2019-02-05  8:53   ` Borislav Petkov
2019-02-05  9:03     ` Nadav Amit
2019-01-29  0:34 ` [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching Rick Edgecombe
2019-02-05  9:18   ` Borislav Petkov
2019-02-11  0:39   ` Nadav Amit
2019-02-11  5:18     ` Andy Lutomirski
2019-02-11 18:04       ` Nadav Amit
2019-02-11 19:07         ` Andy Lutomirski
2019-02-11 19:18           ` Nadav Amit
2019-02-11 22:47             ` Andy Lutomirski
2019-02-12 18:23               ` Nadav Amit [this message]
2019-01-29  0:34 ` [PATCH v2 06/20] x86/alternative: use temporary mm for text poking Rick Edgecombe
2019-02-05  9:58   ` Borislav Petkov
2019-02-05 11:31     ` Peter Zijlstra
2019-02-05 12:35       ` Borislav Petkov
2019-02-05 13:25         ` Peter Zijlstra
2019-02-05 17:54         ` Nadav Amit
2019-02-05 13:29       ` Peter Zijlstra
2019-01-29  0:34 ` [PATCH v2 07/20] x86/kgdb: avoid redundant comparison of patched code Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 08/20] x86/ftrace: set trampoline pages as executable Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements Rick Edgecombe
2019-02-11 18:22   ` Borislav Petkov
2019-02-11 19:36     ` Nadav Amit
2019-01-29  0:34 ` [PATCH v2 10/20] x86: avoid W^X being broken during modules loading Rick Edgecombe
2019-02-11 18:29   ` Borislav Petkov
2019-02-11 18:45     ` Nadav Amit
2019-02-11 19:01       ` Borislav Petkov
2019-02-11 19:09         ` Nadav Amit
2019-02-11 19:10           ` Borislav Petkov
2019-02-11 19:27             ` Nadav Amit
2019-02-11 19:42               ` Borislav Petkov
2019-02-11 20:32                 ` Nadav Amit
2019-03-07 15:10                   ` [PATCH] x86/cpufeature: Remove __pure attribute to _static_cpu_has() Borislav Petkov
2019-03-07 16:43                     ` hpa
2019-03-07 17:02                       ` Borislav Petkov
2019-03-07  7:29                 ` [PATCH v2 10/20] x86: avoid W^X being broken during modules loading Borislav Petkov
2019-03-07 16:53                   ` hpa
2019-03-07 17:06                     ` Borislav Petkov
2019-03-07 20:02                       ` Andy Lutomirski
2019-03-07 20:25                         ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 11/20] x86/jump-label: remove support for custom poker Rick Edgecombe
2019-02-11 18:37   ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 12/20] x86/alternative: Remove the return value of text_poke_*() Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 13/20] Add set_alias_ function and x86 implementation Rick Edgecombe
2019-02-11 19:09   ` Borislav Petkov
2019-02-11 19:27     ` Edgecombe, Rick P
2019-02-11 22:59     ` Andy Lutomirski
2019-02-12  0:01       ` Edgecombe, Rick P
2019-01-29  0:34 ` [PATCH v2 14/20] mm: Make hibernate handle unmapped pages Rick Edgecombe
2019-02-19 11:04   ` Borislav Petkov
2019-02-19 21:28     ` Edgecombe, Rick P
2019-02-20 16:07       ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 15/20] vmalloc: New flags for safe vfree on special perms Rick Edgecombe
2019-02-19 12:48   ` Borislav Petkov
2019-02-19 19:42     ` Edgecombe, Rick P
2019-02-20 16:14       ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 16/20] modules: Use vmalloc special flag Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 17/20] bpf: " Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 18/20] x86/ftrace: " Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 19/20] x86/kprobes: " Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 20/20] x86/alternative: comment about module removal races Rick Edgecombe

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