From: Peter Zijlstra <peterz@infradead.org> To: Andrew Cooper <andrew.cooper3@citrix.com> Cc: Masami Hiramatsu <mhiramat@kernel.org>, Ingo Molnar <mingo@kernel.org>, Juergen Gross <jgross@suse.com>, Stefano Stabellini <sstabellini@kernel.org>, Randy Dunlap <rdunlap@infradead.org>, x86@kernel.org, linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>, Josh Poimboeuf <jpoimboe@redhat.com>, xen-devel@lists.xenproject.org, Boris Ostrovsky <boris.ostrovsky@oracle.com> Subject: Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX Date: Thu, 5 Sep 2019 11:26:27 +0200 [thread overview] Message-ID: <20190905092627.GB2332@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <4de91a14-2051-197e-6ab0-beb2538c40f9@citrix.com> On Thu, Sep 05, 2019 at 09:53:32AM +0100, Andrew Cooper wrote: > On 05/09/2019 09:26, Peter Zijlstra wrote: > > On Thu, Sep 05, 2019 at 08:54:17AM +0100, Andrew Cooper wrote: > > > >> I don't know if you've spotted, but the prefix is a ud2a instruction > >> followed by 'xen' in ascii. > >> > >> The KVM version was added in c/s 6c86eedc206dd1f9d37a2796faa8e6f2278215d2 > > While the Xen one disassebles to valid instructions, that KVM one does > > not: > > > > .text > > xen: > > ud2; .ascii "xen" > > kvm: > > ud2; .ascii "kvm" > > > > disassembles like: > > > > 0000000000000000 <xen>: > > 0: 0f 0b ud2 > > 2: 78 65 js 69 <kvm+0x64> > > 4: 6e outsb %ds:(%rsi),(%dx) > > 0000000000000005 <kvm>: > > 5: 0f 0b ud2 > > 7: 6b .byte 0x6b > > 8: 76 6d jbe 77 <kvm+0x72> > > > > Which is a bit unfortunate I suppose. At least they don't appear to > > consume further bytes. > > It does when you give objdump one extra byte to look at. > > 0000000000000005 <kvm>: > 5: 0f 0b ud2 > 7: 6b 76 6d 00 imul $0x0,0x6d(%rsi),%esi > > I did try to point out that this property should have been checked > before settling on 'kvm' as the string. Bah you're right; when I write: ud2; .ascii "kvm"; cpuid The output is gibberish :/ > but we're 13 years too late to amend this. Yah, I figured :/ > > I know it is water under the bridge at this point; but you could've used > > UD1 with a displacement with some 'unlikely' values. That way it > > would've decoded to a single instruction. > > > > Something like: > > > > ud1 0x6e6578(%rax),%rax > > > > which spells out "xen\0" in the displacement: > > > > 48 0f b9 80 78 65 6e 00 > > :) > > I seem to recall UD0 and UD1 being very late to the documentation party. They were; and the documentation for still UD0 differs between vendors :/
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org> To: Andrew Cooper <andrew.cooper3@citrix.com> Cc: Juergen Gross <jgross@suse.com>, Stefano Stabellini <sstabellini@kernel.org>, x86@kernel.org, linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>, Randy Dunlap <rdunlap@infradead.org>, Josh Poimboeuf <jpoimboe@redhat.com>, xen-devel@lists.xenproject.org, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Ingo Molnar <mingo@kernel.org>, Masami Hiramatsu <mhiramat@kernel.org> Subject: Re: [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX Date: Thu, 5 Sep 2019 11:26:27 +0200 [thread overview] Message-ID: <20190905092627.GB2332@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <4de91a14-2051-197e-6ab0-beb2538c40f9@citrix.com> On Thu, Sep 05, 2019 at 09:53:32AM +0100, Andrew Cooper wrote: > On 05/09/2019 09:26, Peter Zijlstra wrote: > > On Thu, Sep 05, 2019 at 08:54:17AM +0100, Andrew Cooper wrote: > > > >> I don't know if you've spotted, but the prefix is a ud2a instruction > >> followed by 'xen' in ascii. > >> > >> The KVM version was added in c/s 6c86eedc206dd1f9d37a2796faa8e6f2278215d2 > > While the Xen one disassebles to valid instructions, that KVM one does > > not: > > > > .text > > xen: > > ud2; .ascii "xen" > > kvm: > > ud2; .ascii "kvm" > > > > disassembles like: > > > > 0000000000000000 <xen>: > > 0: 0f 0b ud2 > > 2: 78 65 js 69 <kvm+0x64> > > 4: 6e outsb %ds:(%rsi),(%dx) > > 0000000000000005 <kvm>: > > 5: 0f 0b ud2 > > 7: 6b .byte 0x6b > > 8: 76 6d jbe 77 <kvm+0x72> > > > > Which is a bit unfortunate I suppose. At least they don't appear to > > consume further bytes. > > It does when you give objdump one extra byte to look at. > > 0000000000000005 <kvm>: > 5: 0f 0b ud2 > 7: 6b 76 6d 00 imul $0x0,0x6d(%rsi),%esi > > I did try to point out that this property should have been checked > before settling on 'kvm' as the string. Bah you're right; when I write: ud2; .ascii "kvm"; cpuid The output is gibberish :/ > but we're 13 years too late to amend this. Yah, I figured :/ > > I know it is water under the bridge at this point; but you could've used > > UD1 with a displacement with some 'unlikely' values. That way it > > would've decoded to a single instruction. > > > > Something like: > > > > ud1 0x6e6578(%rax),%rax > > > > which spells out "xen\0" in the displacement: > > > > 48 0f b9 80 78 65 6e 00 > > :) > > I seem to recall UD0 and UD1 being very late to the documentation party. They were; and the documentation for still UD0 differs between vendors :/ _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
next prev parent reply other threads:[~2019-09-05 9:26 UTC|newest] Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-09-04 11:45 [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX Masami Hiramatsu 2019-09-04 11:45 ` [Xen-devel] " Masami Hiramatsu 2019-09-04 11:45 ` [PATCH -tip 1/2] x86: xen: insn: Decode XEN_EMULATE_PREFIX correctly Masami Hiramatsu 2019-09-04 11:45 ` [Xen-devel] " Masami Hiramatsu 2019-09-04 11:46 ` [PATCH -tip 2/2] x86: kprobes: Prohibit probing on instruction which has Xen prefix Masami Hiramatsu 2019-09-04 11:46 ` [Xen-devel] " Masami Hiramatsu 2019-09-04 11:54 ` [Xen-devel] [PATCH -tip 0/2] x86: Prohibit kprobes on XEN_EMULATE_PREFIX Andrew Cooper 2019-09-04 11:54 ` Andrew Cooper 2019-09-05 1:49 ` Masami Hiramatsu 2019-09-05 1:49 ` Masami Hiramatsu 2019-09-05 7:54 ` Andrew Cooper 2019-09-05 7:54 ` Andrew Cooper 2019-09-05 8:26 ` Peter Zijlstra 2019-09-05 8:26 ` Peter Zijlstra 2019-09-05 8:53 ` Andrew Cooper 2019-09-05 8:53 ` Andrew Cooper 2019-09-05 9:26 ` Peter Zijlstra [this message] 2019-09-05 9:26 ` Peter Zijlstra 2019-09-05 9:38 ` Andrew Cooper 2019-09-05 9:38 ` Andrew Cooper 2019-09-05 12:49 ` [OT] " Masami Hiramatsu 2019-09-05 12:49 ` [Xen-devel] [OT] " Masami Hiramatsu 2019-09-05 11:32 ` [Xen-devel] " Masami Hiramatsu 2019-09-05 11:32 ` Masami Hiramatsu 2019-09-05 13:09 ` Masami Hiramatsu 2019-09-05 13:09 ` Masami Hiramatsu 2019-09-05 13:31 ` Andrew Cooper 2019-09-05 13:31 ` Andrew Cooper 2019-09-05 16:12 ` Masami Hiramatsu 2019-09-05 16:12 ` Masami Hiramatsu 2019-09-04 12:02 ` Peter Zijlstra 2019-09-04 12:02 ` [Xen-devel] " Peter Zijlstra
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20190905092627.GB2332@hirez.programming.kicks-ass.net \ --to=peterz@infradead.org \ --cc=andrew.cooper3@citrix.com \ --cc=boris.ostrovsky@oracle.com \ --cc=bp@alien8.de \ --cc=jgross@suse.com \ --cc=jpoimboe@redhat.com \ --cc=linux-kernel@vger.kernel.org \ --cc=mhiramat@kernel.org \ --cc=mingo@kernel.org \ --cc=rdunlap@infradead.org \ --cc=sstabellini@kernel.org \ --cc=x86@kernel.org \ --cc=xen-devel@lists.xenproject.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.