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Date: Tue, 12 Feb 2019 10:23:35 -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 2:47 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> On Mon, Feb 11, 2019 at 11:18 AM Nadav Amit <nadav.amit@...il.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@...are.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@...nel.org>
Signed-off-by: Nadav Amit <namit@...are.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
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