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Message-ID: <87wmvi3ker.ffs@tglx>
Date: Thu, 19 Oct 2023 13:45:48 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Al Viro <viro@...iv.linux.org.uk>,
gus Gusenleitner Klaus <gus@...a.com>
Cc: Al Viro <viro@....linux.org.uk>,
lkml <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
"x86@...nel.org" <x86@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
"dsahern@...nel.org" <dsahern@...nel.org>,
"edumazet@...gle.com" <edumazet@...gle.com>,
"kuba@...nel.org" <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Subject: Re: AW: [PATCH] amd64: Fix csum_partial_copy_generic()
On Thu, Oct 19 2023 at 07:39, Al Viro wrote:
> On Thu, Oct 19, 2023 at 07:14:27AM +0100, Al Viro wrote:
>> > > Here's our situation:
>> > > Our device gets pinged by a third party manufacturer robot controller.
>> > > We have updated the kernel in our device to 5.15 from 4.9, the robot
>> > > controller is kept unchanged. At 4.9, our device's ping reply is accepted
>> > > by the robot controller, at 5.15 it's not.
>> > >
>> > > Wireshark shows a bad checksum warning:
>> > > 'Checksum: 0x0000 incorrect, should be 0xffff'
>> > >
>> >
>> > Lovely. I think I see what's going on, give me a few to think about it...
>>
>> The real source of trouble was switching csum_and_copy_{to,from}_user()
>> to reporting faults as 0. And yes, it's broken. Bugger...
>
> I really hate the idea of bringing back the old horrors and splitting
> _nocheck and _user variants ;-/ Especially since we don't care (and
> never had, really) where in the EFAULT case had the damn thing faulted
> and what csum had it managed to accumulate prior to that point.
>
> The only callers are csum_and_copy_..._iter() and they discard
> the entire iovec segment if fault happens; all users of
> csum_and_copy_from_iter() are actually discarding everything in
> that case (reverting iterator to the point where it had been
> prior to the call).
>
> And they are all thread-synchronous. Hell, it's tempting to steal
> a thread flag, clear it before the call of those suckers, set it in
> exception handlers in those and check in csum_and_copy_..._iter()
> after the call... Let me see how ugly something of that sort would
> be...
Eew. That's horrible.
The checksum is strictly 16bit. __wsum is 32bit (for whatever
reason). So you can differentiate between error and valid checksum
easily by using bit 16-31 as indicator for fail or success, no?
Something like the incomplete below.
Thanks,
tglx
---
--- a/arch/x86/lib/csum-copy_64.S
+++ b/arch/x86/lib/csum-copy_64.S
@@ -194,6 +194,9 @@ SYM_FUNC_START(csum_partial_copy_generic
.Lende:
testq %r10, %r10
js .Lwas_odd
+
+.Lsuccess:
+ orl $0xFF00, %eax
.Lout:
movq 0*8(%rsp), %rbx
movq 1*8(%rsp), %r12
@@ -247,7 +250,7 @@ SYM_FUNC_START(csum_partial_copy_generic
.Lwas_odd:
roll $8, %eax
- jmp .Lout
+ jmp .Lsuccess
/* Exception: just return 0 */
.Lfault:
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