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

Nadav Amit
2019-02-12T18:23:45Z

> 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