From: Oleg Nesterov <oleg@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Mark Mossberg <mark.mossberg@gmail.com>,
tglx@linutronix.de, mingo@redhat.com, x86@kernel.org,
linux-kernel@vger.kernel.org, hpa@zytor.com, jannh@google.com,
kyin@redhat.com
Subject: Re: [PATCH v2] x86/dumpstack: Fix misleading instruction pointer error message
Date: Tue, 3 Nov 2020 18:47:44 +0100 [thread overview]
Message-ID: <20201103174744.GB23992@redhat.com> (raw)
In-Reply-To: <20201103171537.GC4111@zn.tnic>
On 11/03, Borislav Petkov wrote:
>
> On Tue, Nov 03, 2020 at 01:50:34PM +0100, Oleg Nesterov wrote:
> > Another problem is that show_opcodes() makes no sense if user_mode(regs)
> > and tsk is not current.
>
> Because if not current, we would access *some* user address space but
> not the one to which regs belong to?
Yes, because if not current, copy_from_user() will access the current's
user address space at address = foreign process's regs->ip.
> > Try "echo t > /proc/sysrq-trigger".
>
> What am I looking for?
>
> I see a bunch of:
>
> [ 37.622896] Code: Unable to access opcode bytes at RIP <user address>
this means that foreign_regs->ip is not mmapped,
> and three Code: lines with opcode bytes, as expected:
>
> [ 37.148693] Code: 11 0d 00 48 89 c6 4c 89 ef e8 98 07 00 00 48 83 f8 ff 0f 84 3e 02 00 00 48 3b 05 b7 28 0d 00 48 89 c3 0f 83 b5 00 00 00 48 8b <0d> e7 10 0d 00 48 83 f8 0d 76 13 48 b8 28 75 6e 72 65 61 63 68 48
I'd say this is NOT expected and adds the unnecessary confusion.
./scripts/decodecode reports
...
Code starting with the faulting instruction
===========================================
0: 0d e7 10 0d 00 or $0xd10e7,%eax
5: 48 83 f8 0d cmp $0xd,%rax
9: 76 13 jbe 0x1e
b: 48 b8 28 75 6e 72 65 movabs $0x68636165726e7528,%rax
12: 61 63 68
15: 48 rex.W
and this is because foreign_regs->ip happens to be a valid address in current->mm.
> I'm thinking this should not use the atomic variant if it can get called
> in !atomic context too.
For what?
Oleg.
next prev parent reply other threads:[~2020-11-03 17:47 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-02 4:29 [PATCH v2] x86/dumpstack: Fix misleading instruction pointer error message Mark Mossberg
2020-10-02 10:26 ` [tip: x86/core] " tip-bot2 for Mark Mossberg
2020-11-03 12:50 ` [PATCH v2] " Oleg Nesterov
2020-11-03 17:15 ` Borislav Petkov
2020-11-03 17:47 ` Oleg Nesterov [this message]
2020-11-03 17:52 ` Borislav Petkov
2020-11-03 18:11 ` Oleg Nesterov
2020-11-03 18:20 ` Borislav Petkov
2020-11-16 22:01 ` Thomas Gleixner
2020-11-16 23:04 ` Andy Lutomirski
2020-11-16 23:37 ` Thomas Gleixner
2020-11-17 3:29 ` Andy Lutomirski
2020-11-17 9:54 ` Borislav Petkov
2020-11-17 17:31 ` Oleg Nesterov
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