LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Andy Lutomirski <luto@kernel.org>
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 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack
Date: Fri, 13 Jul 2018 16:31:02 -0700
Message-ID: <CALCETrUg_4q8a2Tt_Z+GtVuBwj3Ct3=j7M-YhiK06=XjxOG82A@mail.gmail.com> (raw)
In-Reply-To: <1531308586-29340-11-git-send-email-joro@8bytes.org>

On Wed, Jul 11, 2018 at 4:29 AM, Joerg Roedel <joro@8bytes.org> wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> It can happen that we enter the kernel from kernel-mode and
> on the entry-stack. The most common way this happens is when
> we get an exception while loading the user-space segment
> registers on the kernel-to-userspace exit path.
>
> The segment loading needs to be done after the entry-stack
> switch, because the stack-switch needs kernel %fs for
> per_cpu access.
>
> When this happens, we need to make sure that we leave the
> kernel with the entry-stack again, so that the interrupted
> code-path runs on the right stack when switching to the
> user-cr3.
>
> We do this by detecting this condition on kernel-entry by
> checking CS.RPL and %esp, and if it happens, we copy over
> the complete content of the entry stack to the task-stack.
> This needs to be done because once we enter the exception
> handlers we might be scheduled out or even migrated to a
> different CPU, so that we can't rely on the entry-stack
> contents. We also leave a marker in the stack-frame to
> detect this condition on the exit path.
>
> On the exit path the copy is reversed, we copy all of the
> remaining task-stack back to the entry-stack and switch
> to it.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/entry/entry_32.S | 116 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 115 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 3d1a114..b3af76e 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -299,6 +299,9 @@
>   * copied there. So allocate the stack-frame on the task-stack and
>   * switch to it before we do any copying.
>   */
> +
> +#define CS_FROM_ENTRY_STACK    (1 << 31)
> +
>  .macro SWITCH_TO_KERNEL_STACK
>
>         ALTERNATIVE     "", "jmp .Lend_\@", X86_FEATURE_XENPV
> @@ -320,6 +323,16 @@
>         /* Load top of task-stack into %edi */
>         movl    TSS_entry_stack(%edi), %edi
>
> +       /*
> +        * Clear upper bits of the CS slot in pt_regs in case hardware
> +        * didn't clear it for us
> +        */
> +       andl    $(0x0000ffff), PT_CS(%esp)

The comment is highly confusing, give that the upper bits aren't part
of the slot any more:

commit 385eca8f277c4c34f361a4c3a088fd876d29ae21
Author: Andy Lutomirski <luto@kernel.org>
Date:   Fri Jul 28 06:00:30 2017 -0700

    x86/asm/32: Make pt_regs's segment registers be 16 bits

What you're really doing is keeping it available for an extra flag.
Please update the comment as such.  But see below.

> +
> +       /* Special case - entry from kernel mode via entry stack */
> +       testl   $SEGMENT_RPL_MASK, PT_CS(%esp)
> +       jz      .Lentry_from_kernel_\@
> +
>         /* Bytes to copy */
>         movl    $PTREGS_SIZE, %ecx
>
> @@ -333,8 +346,8 @@
>          */
>         addl    $(4 * 4), %ecx
>
> -.Lcopy_pt_regs_\@:
>  #endif
> +.Lcopy_pt_regs_\@:
>
>         /* Allocate frame on task-stack */
>         subl    %ecx, %edi
> @@ -350,6 +363,56 @@
>         cld
>         rep movsl
>
> +       jmp .Lend_\@
> +
> +.Lentry_from_kernel_\@:
> +
> +       /*
> +        * This handles the case when we enter the kernel from
> +        * kernel-mode and %esp points to the entry-stack. When this
> +        * happens we need to switch to the task-stack to run C code,
> +        * but switch back to the entry-stack again when we approach
> +        * iret and return to the interrupted code-path. This usually
> +        * happens when we hit an exception while restoring user-space
> +        * segment registers on the way back to user-space.
> +        *
> +        * When we switch to the task-stack here, we can't trust the
> +        * contents of the entry-stack anymore, as the exception handler
> +        * might be scheduled out or moved to another CPU. Therefore we
> +        * copy the complete entry-stack to the task-stack and set a
> +        * marker in the iret-frame (bit 31 of the CS dword) to detect
> +        * what we've done on the iret path.
> +        *
> +        * On the iret path we copy everything back and switch to the
> +        * entry-stack, so that the interrupted kernel code-path
> +        * continues on the same stack it was interrupted with.
> +        *
> +        * Be aware that an NMI can happen anytime in this code.
> +        *
> +        * %esi: Entry-Stack pointer (same as %esp)
> +        * %edi: Top of the task stack
> +        */
> +
> +       /* Calculate number of bytes on the entry stack in %ecx */
> +       movl    %esi, %ecx
> +
> +       /* %ecx to the top of entry-stack */
> +       andl    $(MASK_entry_stack), %ecx
> +       addl    $(SIZEOF_entry_stack), %ecx
> +
> +       /* Number of bytes on the entry stack to %ecx */
> +       sub     %esi, %ecx
> +
> +       /* Mark stackframe as coming from entry stack */
> +       orl     $CS_FROM_ENTRY_STACK, PT_CS(%esp)
> +
> +       /*
> +        * %esi and %edi are unchanged, %ecx contains the number of
> +        * bytes to copy. The code at .Lcopy_pt_regs_\@ will allocate
> +        * the stack-frame on task-stack and copy everything over
> +        */
> +       jmp .Lcopy_pt_regs_\@
> +
>  .Lend_\@:
>  .endm
>
> @@ -408,6 +471,56 @@
>  .endm
>
>  /*
> + * This macro handles the case when we return to kernel-mode on the iret
> + * path and have to switch back to the entry stack.
> + *
> + * See the comments below the .Lentry_from_kernel_\@ label in the
> + * SWITCH_TO_KERNEL_STACK macro for more details.
> + */
> +.macro PARANOID_EXIT_TO_KERNEL_MODE
> +
> +       /*
> +        * Test if we entered the kernel with the entry-stack. Most
> +        * likely we did not, because this code only runs on the
> +        * return-to-kernel path.
> +        */
> +       testl   $CS_FROM_ENTRY_STACK, PT_CS(%esp)
> +       jz      .Lend_\@
> +
> +       /* Unlikely slow-path */
> +
> +       /* Clear marker from stack-frame */
> +       andl    $(~CS_FROM_ENTRY_STACK), PT_CS(%esp)
> +
> +       /* Copy the remaining task-stack contents to entry-stack */
> +       movl    %esp, %esi
> +       movl    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi

I'm confused.  Why do we need any special handling here at all?  How
could we end up with the contents of the stack frame we interrupted in
a corrupt state?

I guess I don't understand why this patch is needed.

  reply index

Thread overview: 82+ 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
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 [this message]
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 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack Joerg Roedel
2018-10-12 18:29   ` Jan Kiszka

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='CALCETrUg_4q8a2Tt_Z+GtVuBwj3Ct3=j7M-YhiK06=XjxOG82A@mail.gmail.com' \
    --to=luto@kernel.org \
    --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=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

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git
	git clone --mirror https://lore.kernel.org/lkml/10 lkml/git/10.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git