Re: [PATCH] locking/atomics: don't alias ____ptr

[Andrey Ryabinin]

On 06/28/2017 01:16 PM, Dmitry Vyukov wrote:
> On Wed, Jun 28, 2017 at 12:02 PM, Sebastian Andrzej Siewior
> <bigeasy@xxxxxxxxxxxxx> wrote:
>> Trying to boot tip/master resulted in:
>> |DMAR: dmar0: Using Queued invalidation
>> |DMAR: dmar1: Using Queued invalidation
>> |DMAR: Setting RMRR:
>> |DMAR: Setting identity map for device 0000:00:1a.0 [0xbdcf9000 - 0xbdd1dfff]
>> |BUG: unable to handle kernel NULL pointer dereference at           (null)
>> |IP: __domain_mapping+0x10f/0x3d0
>> |PGD 0
>> |P4D 0
>> |
>> |Oops: 0002 [#1] PREEMPT SMP
>> |Modules linked in:
>> |CPU: 19 PID: 1 Comm: swapper/0 Not tainted 4.12.0-rc6-00117-g235a93822a21 #113
>> |task: ffff8805271c2c80 task.stack: ffffc90000058000
>> |RIP: 0010:__domain_mapping+0x10f/0x3d0
>> |RSP: 0000:ffffc9000005bca0 EFLAGS: 00010246
>> |RAX: 0000000000000000 RBX: 00000000bdcf9003 RCX: 0000000000000000
>> |RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000001
>> |RBP: ffffc9000005bd00 R08: ffff880a243e9780 R09: ffff8805259e67c8
>> |R10: 00000000000bdcf9 R11: 0000000000000000 R12: 0000000000000025
>> |R13: 0000000000000025 R14: 0000000000000000 R15: 00000000000bdcf9
>> |FS:  0000000000000000(0000) GS:ffff88052acc0000(0000) knlGS:0000000000000000
>> |CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> |CR2: 0000000000000000 CR3: 0000000001c0f000 CR4: 00000000000406e0
>> |Call Trace:
>> | iommu_domain_identity_map+0x5a/0x80
>> | domain_prepare_identity_map+0x9f/0x160
>> | iommu_prepare_identity_map+0x7e/0x9b
>>
>> bisect points to commit 235a93822a21 ("locking/atomics, asm-generic: Add KASAN
>> instrumentation to atomic operations"), RIP is at
>>          tmp = cmpxchg64_local(&pte->val, 0ULL, pteval);
>> in drivers/iommu/intel-iommu.c. The assembly for this inline assembly
>> is:
>>     xor    %edx,%edx
>>     xor    %eax,%eax
>>     cmpxchg %rbx,(%rdx)
>>
>> and as you see edx is set to zero and used later as a pointer via the
>> full register. This happens with gcc-6, 5 and 8 (snapshot from last
>> week).
>> After a longer while of searching and swearing I figured out that this
>> bug occures once cmpxchg64_local() and cmpxchg_local() uses the same
>> ____ptr macro and they are shadow somehow. What I don't know why edx is
>> set to zero.
>>
>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
>> ---
>>  include/asm-generic/atomic-instrumented.h | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/asm-generic/atomic-instrumented.h b/include/asm-generic/atomic-instrumented.h
>> index a0f5b7525bb2..ac6155362b39 100644
>> --- a/include/asm-generic/atomic-instrumented.h
>> +++ b/include/asm-generic/atomic-instrumented.h
>> @@ -359,16 +359,16 @@ static __always_inline bool atomic64_add_negative(s64 i, atomic64_t *v)
>>
>>  #define cmpxchg64(ptr, old, new)                       \
>>  ({                                                     \
>> -       __typeof__(ptr) ____ptr = (ptr);                \
>> -       kasan_check_write(____ptr, sizeof(*____ptr));   \
>> -       arch_cmpxchg64(____ptr, (old), (new));          \
>> +       __typeof__(ptr) ____ptr64 = (ptr);              \
>> +       kasan_check_write(____ptr64, sizeof(*____ptr64));\
>> +       arch_cmpxchg64(____ptr64, (old), (new));        \
>>  })
>>
>>  #define cmpxchg64_local(ptr, old, new)                 \
>>  ({                                                     \
>> -       __typeof__(ptr) ____ptr = (ptr);                \
>> -       kasan_check_write(____ptr, sizeof(*____ptr));   \
>> -       arch_cmpxchg64_local(____ptr, (old), (new));    \
>> +       __typeof__(ptr) ____ptr64 = (ptr);              \
>> +       kasan_check_write(____ptr64, sizeof(*____ptr64));\
>> +       arch_cmpxchg64_local(____ptr64, (old), (new));  \
>>  })
>>
>>  #define cmpxchg_double(p1, p2, o1, o2, n1, n2)                         \
> 
> 
> Doh! Thanks for fixing this. I think I've a similar crash in a
> different place when I developed the patch.
> The problem is that when we do:
> 
>        __typeof__(ptr) ____ptr = (ptr);                \
>        arch_cmpxchg64_local(____ptr, (old), (new));    \
> 
> We don't necessary pass value of our just declared ____ptr to
> arch_cmpxchg64_local(). We just pass a symbolic identifier. So if
> arch_cmpxchg64_local() declares own ____ptr and then tries to use what
> we passed ("____ptr") it will actually refer to own variable declared
> rather than to what we wanted to pass in.
> 
> In my case I ended up with something like:
> 
> __typeof__(foo) __ptr = __ptr;
> 
> which compiler decided to turn into 0.
> 
> Thank you, macros.
> 
> We can add more underscores, but the problem can happen again. Should
> we prefix current function/macro name to all local vars?..
> 


The main problem here is that arch_cmpxchg64_local() calls cmpxhg_local() instead of using arch_cmpxchg_local().

So, the patch bellow should fix the problem, also this will fix double instrumentation of cmpcxchg64[_local]().
But I haven't tested this patch yet.


---
 arch/x86/include/asm/cmpxchg_64.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cmpxchg_64.h b/arch/x86/include/asm/cmpxchg_64.h
index fafaebacca2d..7046a3cc2493 100644
--- a/arch/x86/include/asm/cmpxchg_64.h
+++ b/arch/x86/include/asm/cmpxchg_64.h
@@ -9,13 +9,13 @@ static inline void set_64bit(volatile u64 *ptr, u64 val)
 #define arch_cmpxchg64(ptr, o, n)					\
 ({									\
 	BUILD_BUG_ON(sizeof(*(ptr)) != 8);				\
-	cmpxchg((ptr), (o), (n));					\
+	arch_cmpxchg((ptr), (o), (n));					\
 })
 
 #define arch_cmpxchg64_local(ptr, o, n)					\
 ({									\
 	BUILD_BUG_ON(sizeof(*(ptr)) != 8);				\
-	cmpxchg_local((ptr), (o), (n));					\
+	arch_cmpxchg_local((ptr), (o), (n));					\
 })
 
 #define system_has_cmpxchg_double() boot_cpu_has(X86_FEATURE_CX16)
-- 
2.13.0