Re: 答复: [PATCH,v2] arm64: fix the illegal address access in some cases

[Robin Murphy]

On 2020-07-30 09:44, Will Deacon wrote:
> On Wed, Jul 29, 2020 at 07:05:09AM +0000, Guodeqing (A) wrote:
>>> On Tue, Jul 28, 2020 at 03:30:50PM +0100, Robin Murphy wrote:
>>>> On 2020-07-28 14:03, Will Deacon wrote:
>>>>> Applied to arm64 (for-next/fixes), thanks!
>>>>>
>>>>> [1/1] arm64: csum: Reject IP headers with 'ihl' field smaller than five
>>>>>         https://git.kernel.org/arm64/c/09aaef1c5f50
>>>>
>>>> I'm not sure your commit message is entirely right there. AFAICS it's
>>>> not "the same way as x86" at all - x86 dereferences the first word of
>>>> iph and returns that as the sum if ihl <= 4 (and thus is still capable
>>>> of crashing given sufficiently bogus data). I'm not sure where "return
>>>> 1" came from - if we're going to return nonsense then the mildly more
>>>> efficient choice of 0 seems just as good.
>>>
>>> Argh, yes, that's %1 not $1, so I don't know where the 1 comes from either.
>>> Geffrey?
>>>
>>
>> The return 1 is just the report of ip checksum error, the return value 0 means the ip
>> checksum correct. x86 dereferences the first word of iph and returns that as the sum,
>> this may be just the report of ip checksum error too.
> 
> On the receive path, sure, but the crash was on the transmit path where
> we're computing the checksum to insert into the header, no?
> 
>>>> Otherwise it would seem reasonable to jump straight into the
>>>> word-at-a-time loop if ip_fast_csum() is really expected to cope with
>>>> more than just genuine IP headers (which should be backed by at least
>>>> 20 bytes of valid memory regardless of what ihl says).
>>>
>>> Either copying the x86 behaviour or WARN_ON_ONCE() and assuming and ihl of
>>> 5 would be my preference, because I agree with you that this feels like it
>>> shouldn't be happening to start with.
>>
>> How about modify the patch like this?
>>
>> static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
>> {
>> 	__uint128_t tmp;
>> 	u64 sum;
>>
>>      if (unlikely(ihl < 5))
>>          ihl = 5;
> 
> I'd probably do:
> 
> 	/* Callers should really be checking this first */
> 	if (WARN_ON_ONCE(ihl < 5))
> 		ihl = 5;
> 
> because I'd still like to understand what the vlan code is up to.
> 
>>>> I still think this smells of papering over some other bug that led to
>>>> a bogus skb getting that far into the transmit stack in the first
>>>> place - presumably it's all wasted effort anyway since a "header" with
>>>> no space for a destination address and a deliberately wrong checksum
>>>> seems unlikely to go very far...
>>>
>>> Looking at the ipvlan_start_xmit() path from the backtrace, it looks to me like
>>> ipvlan_get_L3_hdr() returns NULL if the header length is invalid, but then
>>> ipvlan_xmit_mode_l3() ends up calling ipvlan_process_outbound() anyway.
>>> Hmm. I really don't know enough about VLANs to know what the right
>>> behaviour is here and I guess just returning NET_XMIT_DROP will break
>>> something.
>>
>> The network maintainer has replied to me,
>> " ip_fast_csum() must be able to handle any value that could fit in the
>> ihl field of the ip protocol header. That's not only the most correct
>> logic, but also the most robust."
> 
> Is that on a public list somewhere? Would be a good link for the commit
> message.
> 
>> This is a fault injection test, the corrupt function of netem is the
>> emulation of random noise introducing an error in a random position for a
>> chosen percent of packets to test the network module.the netem will modify
>> the packet randomly,so the ihl value of ip header may be modified to 1.
> 
> Ok, but netem is running in userspace (right?) and so I still think the
> network layer can reject the invalid ihl before calling into the checksum
> code.

Oh, on second look I realise it's probably not that the fault emanates from
dereferencing the actual header itself, it'll be from the code just going
completely bonkers *after* that. I still agree that this case should be
avoidable entirely on the transmit path, but I accept that robustness for
the sake of receive does make good sense. How about this?

Robin.

----->8-----
Subject: [PATCH] arm64: csum: Fix handling of bad packets

Although iph is expected to point to at least 20 bytes of valid memory,
ihl may be bogus, for example on reception of a corrupt packet. If it
happens to be less than 5, we really don't want to run away and
dereference 16GB worth of memory until it wraps back to exactly zero...

Fixes: 0e455d8e80aa ("arm64: Implement optimised IP checksum helpers")
Reported-by: guodeqing <geffrey.guo at huawei.com>
Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---
 arch/arm64/include/asm/checksum.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/checksum.h b/arch/arm64/include/asm/checksum.h
index b6f7bc6da5fb..93a161b3bf3f 100644
--- a/arch/arm64/include/asm/checksum.h
+++ b/arch/arm64/include/asm/checksum.h
@@ -24,16 +24,17 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 {
 	__uint128_t tmp;
 	u64 sum;
+	int n = ihl; /* we want it signed */
 
 	tmp = *(const __uint128_t *)iph;
 	iph += 16;
-	ihl -= 4;
+	n -= 4;
 	tmp += ((tmp >> 64) | (tmp << 64));
 	sum = tmp >> 64;
 	do {
 		sum += *(const u32 *)iph;
 		iph += 4;
-	} while (--ihl);
+	} while (--n > 0);
 
 	sum += ((sum >> 32) | (sum << 32));
 	return csum_fold((__force u32)(sum >> 32));
-- 
2.28.0.dirty