linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Andy Lutomirski <luto@amacapital.net>
To: Joerg Roedel <joro@8bytes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Juergen Gross <jgross@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Brian Gerst <brgerst@gmail.com>,
	David Laight <David.Laight@aculab.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Eduardo Valentin <eduval@amazon.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Will Deacon <will.deacon@arm.com>,
	"Liguori, Anthony" <aliguori@amazon.com>,
	Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
	Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	"David H . Gutteridge" <dhgutteridge@sympatico.ca>,
	Joerg Roedel <jroedel@suse.de>
Subject: Re: [PATCH 07/39] x86/entry/32: Enter the kernel via trampoline stack
Date: Fri, 13 Jul 2018 10:21:39 -0700	[thread overview]
Message-ID: <CALCETrW4XMD9TSTxK3h-3p5ZE5Z=DupiUBtiXnMmSprbXtJr3g@mail.gmail.com> (raw)
In-Reply-To: <20180713105620.z6bjhqzfez2hll6r@8bytes.org>

On Fri, Jul 13, 2018 at 3:56 AM, Joerg Roedel <joro@8bytes.org> wrote:
> Hi Andy,
>
> thanks for you valuable feedback.
>
> On Thu, Jul 12, 2018 at 02:09:45PM -0700, Andy Lutomirski wrote:
>> > On Jul 11, 2018, at 4:29 AM, Joerg Roedel <joro@8bytes.org> wrote:
>> > -.macro SAVE_ALL pt_regs_ax=%eax
>> > +.macro SAVE_ALL pt_regs_ax=%eax switch_stacks=0
>> >    cld
>> > +    /* Push segment registers and %eax */
>> >    PUSH_GS
>> >    pushl    %fs
>> >    pushl    %es
>> >    pushl    %ds
>> >    pushl    \pt_regs_ax
>> > +
>> > +    /* Load kernel segments */
>> > +    movl    $(__USER_DS), %eax
>>
>> If \pt_regs_ax != %eax, then this will behave oddly. Maybe it’s okay.
>> But I don’t see why this change was needed at all.
>
> This is a left-over from a previous approach I tried and then abandoned
> later. You are right, it is not needed.
>
>> > +/*
>> > + * Called with pt_regs fully populated and kernel segments loaded,
>> > + * so we can access PER_CPU and use the integer registers.
>> > + *
>> > + * We need to be very careful here with the %esp switch, because an NMI
>> > + * can happen everywhere. If the NMI handler finds itself on the
>> > + * entry-stack, it will overwrite the task-stack and everything we
>> > + * copied there. So allocate the stack-frame on the task-stack and
>> > + * switch to it before we do any copying.
>>
>> Ick, right. Same with machine check, though. You could alternatively
>> fix it by running NMIs on an irq stack if the irq count is zero.  How
>> confident are you that you got #MC right?
>
> Pretty confident, #MC uses the exception entry path which also handles
> entry-stack and user-cr3 correctly. It might go through through the slow
> paranoid exit path, but that's okay for #MC I guess.
>
> And when the #MC happens while we switch to the task stack and do the
> copying the same precautions as for NMI apply.
>
>> > + */
>> > +.macro SWITCH_TO_KERNEL_STACK
>> > +
>> > +    ALTERNATIVE     "", "jmp .Lend_\@", X86_FEATURE_XENPV
>> > +
>> > +    /* Are we on the entry stack? Bail out if not! */
>> > +    movl    PER_CPU_VAR(cpu_entry_area), %edi
>> > +    addl    $CPU_ENTRY_AREA_entry_stack, %edi
>> > +    cmpl    %esp, %edi
>> > +    jae    .Lend_\@
>>
>> That’s an alarming assumption about the address space layout. How
>> about an xor and an and instead of cmpl?  As it stands, if the address
>> layout ever changes, the failure may be rather subtle.
>
> Right, I implement a more restrictive check.

But the check needs to be correct or we'll mess up, right?  I think
the code will be much more robust and easier to review if you check
"on the entry stack" instead of ">= the entry stack".  (Or <= -- I can
never remember how this works in AT&T syntax.)

>
>> Anyway, wouldn’t it be easier to solve this by just not switching
>> stacks on entries from kernel mode and making the entry stack bigger?
>> Stick an assertion in the scheduling code that we’re not on an entry
>> stack, perhaps.
>
> That'll save us the check whether we are on the entry stack and replace
> it with a check whether we are coming from user/vm86 mode. I don't think
> that this will simplify things much and I am a bit afraid that it'll
> break unwritten assumptions elsewhere. It is probably something we can
> look into later separatly from the basic pti-x32 enablement.
>

Fair enough.  There's also the issue that NMI still has to switch CR3
if it hits with the wrong CR3.

I personally much prefer checking whether you came from user mode
rather than the stack address, but I'm okay with either approach here.

  reply	other threads:[~2018-07-13 17:22 UTC|newest]

Thread overview: 83+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-11 11:29 [PATCH 00/39 v7] PTI support for x86-32 Joerg Roedel
2018-07-11 11:29 ` [PATCH 01/39] x86/asm-offsets: Move TSS_sp0 and TSS_sp1 to asm-offsets.c Joerg Roedel
2018-07-12 20:44   ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 02/39] x86/entry/32: Rename TSS_sysenter_sp0 to TSS_entry_stack Joerg Roedel
2018-07-12 20:44   ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 03/39] x86/entry/32: Load task stack from x86_tss.sp1 in SYSENTER handler Joerg Roedel
2018-07-12 20:49   ` Andy Lutomirski
2018-07-13  9:48     ` Joerg Roedel
2018-07-13 17:19       ` Andy Lutomirski
2018-07-13 23:17         ` Andy Lutomirski
2018-07-17  7:05           ` Joerg Roedel
2018-07-17 20:04             ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 04/39] x86/entry/32: Put ESPFIX code into a macro Joerg Roedel
2018-07-11 11:29 ` [PATCH 05/39] x86/entry/32: Unshare NMI return path Joerg Roedel
2018-07-12 20:53   ` Andy Lutomirski
2018-07-13 10:05     ` Joerg Roedel
2018-07-13 17:26       ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 06/39] x86/entry/32: Split off return-to-kernel path Joerg Roedel
2018-07-11 11:29 ` [PATCH 07/39] x86/entry/32: Enter the kernel via trampoline stack Joerg Roedel
2018-07-12 21:09   ` Andy Lutomirski
2018-07-13 10:56     ` Joerg Roedel
2018-07-13 17:21       ` Andy Lutomirski [this message]
2018-07-17  7:07         ` Joerg Roedel
2018-07-11 11:29 ` [PATCH 08/39] x86/entry/32: Leave " Joerg Roedel
2018-07-11 11:29 ` [PATCH 09/39] x86/entry/32: Introduce SAVE_ALL_NMI and RESTORE_ALL_NMI Joerg Roedel
2018-07-11 11:29 ` [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack Joerg Roedel
2018-07-13 23:31   ` Andy Lutomirski
2018-07-14  5:21     ` Joerg Roedel
2018-07-14  6:26       ` Andy Lutomirski
2018-07-14  8:01         ` Joerg Roedel
2018-07-14 14:36           ` Andy Lutomirski
2018-07-17  7:15             ` Joerg Roedel
2018-07-17 20:06               ` Andy Lutomirski
2018-07-18 11:59                 ` Joerg Roedel
2018-07-11 11:29 ` [PATCH 11/39] x86/entry/32: Simplify debug entry point Joerg Roedel
2018-07-11 11:29 ` [PATCH 12/39] x86/32: Use tss.sp1 as cpu_current_top_of_stack Joerg Roedel
2018-07-11 11:29 ` [PATCH 13/39] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points Joerg Roedel
2018-07-11 11:29 ` [PATCH 14/39] x86/entry/32: Add PTI cr3 switches to NMI handler code Joerg Roedel
2018-07-11 11:29 ` [PATCH 15/39] x86/pgtable: Rename pti_set_user_pgd to pti_set_user_pgtbl Joerg Roedel
2018-07-11 11:29 ` [PATCH 16/39] x86/pgtable/pae: Unshare kernel PMDs when PTI is enabled Joerg Roedel
2018-07-11 11:29 ` [PATCH 17/39] x86/pgtable/32: Allocate 8k page-tables " Joerg Roedel
2018-07-11 11:29 ` [PATCH 18/39] x86/pgtable: Move pgdp kernel/user conversion functions to pgtable.h Joerg Roedel
2018-07-11 11:29 ` [PATCH 19/39] x86/pgtable: Move pti_set_user_pgtbl() " Joerg Roedel
2018-07-11 11:29 ` [PATCH 20/39] x86/pgtable: Move two more functions from pgtable_64.h " Joerg Roedel
2018-07-11 11:29 ` [PATCH 21/39] x86/mm/pae: Populate valid user PGD entries Joerg Roedel
2018-07-11 11:29 ` [PATCH 22/39] x86/mm/pae: Populate the user page-table with user pgd's Joerg Roedel
2018-07-11 11:29 ` [PATCH 23/39] x86/mm/legacy: " Joerg Roedel
2018-07-11 11:29 ` [PATCH 24/39] x86/mm/pti: Add an overflow check to pti_clone_pmds() Joerg Roedel
2018-07-11 11:29 ` [PATCH 25/39] x86/mm/pti: Define X86_CR3_PTI_PCID_USER_BIT on x86_32 Joerg Roedel
2018-07-11 11:29 ` [PATCH 26/39] x86/mm/pti: Clone CPU_ENTRY_AREA on PMD level " Joerg Roedel
2018-07-11 11:29 ` [PATCH 27/39] x86/mm/pti: Make pti_clone_kernel_text() compile on 32 bit Joerg Roedel
2018-07-11 11:29 ` [PATCH 28/39] x86/mm/pti: Keep permissions when cloning kernel text in pti_clone_kernel_text() Joerg Roedel
2018-07-13 23:25   ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 29/39] x86/mm/pti: Introduce pti_finalize() Joerg Roedel
2018-07-11 11:29 ` [PATCH 30/39] x86/mm/pti: Clone entry-text again in pti_finalize() Joerg Roedel
2018-07-13 23:21   ` Andy Lutomirski
2018-07-14  5:04     ` Joerg Roedel
2018-07-11 11:29 ` [PATCH 31/39] x86/mm/dump_pagetables: Define INIT_PGD Joerg Roedel
2018-07-11 11:29 ` [PATCH 32/39] x86/pgtable/pae: Use separate kernel PMDs for user page-table Joerg Roedel
2018-07-11 11:29 ` [PATCH 33/39] x86/ldt: Reserve address-space range on 32 bit for the LDT Joerg Roedel
2018-07-11 11:29 ` [PATCH 34/39] x86/ldt: Define LDT_END_ADDR Joerg Roedel
2018-07-13 17:29   ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 35/39] x86/ldt: Split out sanity check in map_ldt_struct() Joerg Roedel
2018-07-13 23:18   ` Andy Lutomirski
2018-07-11 11:29 ` [PATCH 36/39] x86/ldt: Enable LDT user-mapping for PAE Joerg Roedel
2018-07-11 11:29 ` [PATCH 37/39] x86/pti: Allow CONFIG_PAGE_TABLE_ISOLATION for x86_32 Joerg Roedel
2018-07-11 11:29 ` [PATCH 38/39] x86/mm/pti: Add Warning when booting on a PCID capable CPU Joerg Roedel
2018-07-13 18:59   ` Andy Lutomirski
2018-07-14  5:08     ` Joerg Roedel
2018-07-11 11:29 ` [PATCH 39/39] x86/entry/32: Add debug code to check entry/exit cr3 Joerg Roedel
2018-07-13 17:28   ` Andy Lutomirski
2018-07-14  5:09     ` Joerg Roedel
2018-07-11 16:28 ` [PATCH 00/39 v7] PTI support for x86-32 Linus Torvalds
2018-07-11 17:28   ` Jiri Kosina
2018-07-11 19:57     ` Thomas Backlund
2018-07-12 13:59       ` Boris Ostrovsky
2018-07-11 21:07   ` Pavel Machek
2018-07-16  7:51 ` Pavel Machek
2018-07-17  2:07 ` David H. Gutteridge
2018-07-17  6:16   ` Joerg Roedel
2018-07-18  9:40 [PATCH 00/39 v8] " Joerg Roedel
2018-07-18  9:40 ` [PATCH 07/39] x86/entry/32: Enter the kernel via trampoline stack Joerg Roedel
2018-07-18 18:09   ` Brian Gerst
2018-07-19 20:52     ` Thomas Gleixner

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='CALCETrW4XMD9TSTxK3h-3p5ZE5Z=DupiUBtiXnMmSprbXtJr3g@mail.gmail.com' \
    --to=luto@amacapital.net \
    --cc=David.Laight@aculab.com \
    --cc=aarcange@redhat.com \
    --cc=aliguori@amazon.com \
    --cc=boris.ostrovsky@oracle.com \
    --cc=bp@alien8.de \
    --cc=brgerst@gmail.com \
    --cc=daniel.gruss@iaik.tugraz.at \
    --cc=dave.hansen@intel.com \
    --cc=dhgutteridge@sympatico.ca \
    --cc=dvlasenk@redhat.com \
    --cc=eduval@amazon.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=hpa@zytor.com \
    --cc=hughd@google.com \
    --cc=jgross@suse.com \
    --cc=jkosina@suse.cz \
    --cc=joro@8bytes.org \
    --cc=jpoimboe@redhat.com \
    --cc=jroedel@suse.de \
    --cc=keescook@google.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=llong@redhat.com \
    --cc=luto@kernel.org \
    --cc=mingo@kernel.org \
    --cc=pavel@ucw.cz \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --cc=will.deacon@arm.com \
    --cc=x86@kernel.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: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).