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

[Nadav Amit <nadav.amit@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