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 [thread overview]
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.
next prev parent reply other threads:[~2018-07-13 23:31 UTC|newest]
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
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