linux-kernel - Re: AW: [PATCH] amd64: Fix csum_partial_copy

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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: