-----邮件原件-----
发件人: Will Deacon [mailto:will@kernel.org]
发送时间: Thursday, July 30, 2020 16:44
收件人: Guodeqing (A) [off-list ref]
抄送: Robin Murphy [off-list ref]; catalin.marinas@arm.com;
kernel-team@android.com; linux-arm-kernel@lists.infradead.org;
luke.starrett@broadcom.com
主题: Re: 答复: [PATCH,v2] arm64: fix the illegal address access in some cases

On Wed, Jul 29, 2020 at 07:05:09AM +0000, Guodeqing (A) wrote:

quoted

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On Tue, Jul 28, 2020 at 03:30:50PM +0100, Robin Murphy wrote:

quoted

On 2020-07-28 14:03, Will Deacon wrote:

quoted

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?

[  388.873996] Call trace:
[  388.874280]  ip_send_check+0x40/0x78
[  388.874657]  __ip_local_out+0x78/0x160
[  388.875060]  ip_local_out+0x34/0x68
[  388.875432]  ipvlan_queue_xmit+0x608/0x7f0
[  388.875870]  ipvlan_start_xmit+0x2c/0x90
[  388.876293]  dev_hard_start_xmit+0xac/0x258
[  388.876734]  sch_direct_xmit+0x1a8/0x4c0
[  388.877149]  __qdisc_run+0x88/0xe0
[  388.877508]  __dev_queue_xmit+0x630/0x950
[  388.877942]  dev_queue_xmit+0x24/0x30
[  388.878356]  ip_finish_output2+0x26c/0x420
[  388.878791]  ip_finish_output+0x1c8/0x2a0

The midify action is in the qdisc module, because netem is a kernel module,
the kernel config is CONFIG_NET_SCH_NETEM.

quoted

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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.

Maybe ipvlan has to drop the corrupted packet, but ipvlan also has the choice to let the corrupted
packet to be dropped by the network stack, the network stack will call the ip_fast_csum. so ip_fast_csum
must be robust.

quoted

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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.

https://www.spinics.net/lists/netdev/msg671867.html

quoted

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.

netem is a kernel module, the kernel config is CONFIG_NET_SCH_NETEM.

https://www.mail-archive.com/netdev@vger.kernel.org/msg342331.html

Will

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