Re: [PATCH v2 09/10] x86/entry/32: Simplify and fix up the SYSENTER stack #DB/NMI fixup

[Borislav Petkov <bp@alien8.de>]

On Sat, Mar 05, 2016 at 09:52:22PM -0800, Andy Lutomirski wrote:
> Right after SYSENTER, we can get a #DB or NMI.  On x86_32, there's no IST,
> so the exception handler is invoked on the temporary SYSENTER stack.
> 
> Because the SYSENTER stack is very small, we have a fixup to switch
> off the stack quickly when this happens.  The old fixup had several issues:
> 
> 1. It checked the interrupt frame's CS and EIP.  This wasn't
>    obviously correct on Xen or if vm86 mode was in use [1].
> 
> 2. In the NMI handler, it did some frightening digging into the
>    stack frame.  I'm not convinced this digging was correct.
> 
> 3. The fixup didn't switch stacks and then switch back.  Instead, it
>    synthesized a brand new stack frame that would redirect the IRET
>    back to the SYSENTER code.  That frame was highly questionable.
>    For one thing, if NMI nested inside #DB, we would effectively
>    abort the #DB prologue, which was probably safe but was
>    frightening.  For another, the code used PUSHFL to write the
>    FLAGS portion of the frame, which was simply bogus -- by the time
>    PUSHFL was called, at least TF, NT, VM, and all of the arithmetic
>    flags were clobbered.
> 
> Simplify this considerably.  Instead of looking at the saved frame
> to see where we came from, check the hardware ESP register against
> the SYSENTER stack directly.  Malicious user code cannot spoof the
> kernel ESP register, and by moving the check after SAVE_ALL, we can
> use normal PER_CPU accesses to find all the relevant addresses.
> 
> With this patch applied, the improved syscall_nt_32 test finally
> passes on 32-bit kernels.
> 
> [1] It isn't obviously correct, but it is nonetheless safe from vm86
>     shenanigans as far as I can tell.  A user can't point EIP at
>     entry_SYSENTER_32 while in vm86 mode because entry_SYSENTER_32,
>     like all kernel addresses, is greater than 0xffff and would thus
>     violate the CS segment limit.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/entry/entry_32.S        | 114 ++++++++++++++++++---------------------
>  arch/x86/kernel/asm-offsets_32.c |   5 ++
>  2 files changed, 56 insertions(+), 63 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 7700cf695d8c..ad9a85e62128 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -987,51 +987,48 @@ error_code:
>  	jmp	ret_from_exception
>  END(page_fault)
>  
> -/*
> - * Debug traps and NMI can happen at the one SYSENTER instruction
> - * that sets up the real kernel stack. Check here, since we can't
> - * allow the wrong stack to be used.
> - *
> - * "TSS_sysenter_sp0+12" is because the NMI/debug handler will have
> - * already pushed 3 words if it hits on the sysenter instruction:
> - * eflags, cs and eip.
> - *
> - * We just load the right stack, and push the three (known) values
> - * by hand onto the new stack - while updating the return eip past
> - * the instruction that would have done it for sysenter.
> - */
> -.macro FIX_STACK offset ok label
> -	cmpw	$__KERNEL_CS, 4(%esp)
> -	jne	\ok
> -\label:
> -	movl	TSS_sysenter_sp0 + \offset(%esp), %esp
> -	pushfl
> -	pushl	$__KERNEL_CS
> -	pushl	$sysenter_past_esp
> -.endm
> -
>  ENTRY(debug)
> +	/*
> +	 * #DB can happen at the first instruction of
> +	 * entry_SYSENTER_32 or in Xen's SYSENTER prologue.  If this
> +	 * happens, then we will be running on a very small stack.  We
> +	 * need to detect this condition and switch to the thread
> +	 * stack before calling any C code at all.
> +	 *
> +	 * If you edit this code, keep in mind that NMIs can happen in here.
> +	 */

Btw, I think it is a bit more readable if this comment is over ENTRY(debug) like
the one over ENTRY(nmi) below.

>  	ASM_CLAC
> -	cmpl	$entry_SYSENTER_32, (%esp)
> -	jne	debug_stack_correct
> -	FIX_STACK 12, debug_stack_correct, debug_esp_fix_insn
> -debug_stack_correct:
>  	pushl	$-1				# mark this as an int
>  	SAVE_ALL
> -	TRACE_IRQS_OFF
>  	xorl	%edx, %edx			# error code 0
>  	movl	%esp, %eax			# pt_regs pointer
> +
> +	/* Are we currently on the SYSENTER stack? */
> +	PER_CPU(cpu_tss + CPU_TSS_SYSENTER_stack + SIZEOF_SYSENTER_stack, %ecx)
> +	subl	%eax, %ecx		/* ecx = (end of SYENTER_stack) - esp */
							 ^^^^^^^
Just a typo for the committer of this patch to fix:	SYSENTER_stack

> +	cmpl	$SIZEOF_SYSENTER_stack, %ecx
> +	jb	.Ldebug_from_sysenter_stack
> +
> +	TRACE_IRQS_OFF
> +	call	do_debug
> +	jmp	ret_from_exception
> +
> +.Ldebug_from_sysenter_stack:
> +	/* We're on the SYSENTER stack.  Switch off. */
> +	movl	%esp, %ebp
> +	movl	PER_CPU_VAR(cpu_current_top_of_stack), %esp
> +	TRACE_IRQS_OFF
>  	call	do_debug
> +	movl	%ebp, %esp
>  	jmp	ret_from_exception
>  END(debug)
>  
>  /*
> - * NMI is doubly nasty. It can happen _while_ we're handling
> - * a debug fault, and the debug fault hasn't yet been able to
> - * clear up the stack. So we first check whether we got  an
> - * NMI on the sysenter entry path, but after that we need to
> - * check whether we got an NMI on the debug path where the debug
> - * fault happened on the sysenter path.
> + * NMI is doubly nasty.  It can happen on the first instruction of
> + * entry_SYSENTER_32 (just like #DB), but it can also interrupt the beginning
> + * of the #DB handler even if that #DB in turn hit before entry_SYSENTER_32
> + * switched stacks.  We handle both conditions by simply checking whether we
> + * interrupted kernel code running on the SYSENTER stack.
>   */
>  ENTRY(nmi)
>  	ASM_CLAC
> @@ -1042,41 +1039,32 @@ ENTRY(nmi)
>  	popl	%eax
>  	je	nmi_espfix_stack
>  #endif
> -	cmpl	$entry_SYSENTER_32, (%esp)
> -	je	nmi_stack_fixup
> -	pushl	%eax
> -	movl	%esp, %eax
> -	/*
> -	 * Do not access memory above the end of our stack page,
> -	 * it might not exist.
> -	 */
> -	andl	$(THREAD_SIZE-1), %eax
> -	cmpl	$(THREAD_SIZE-20), %eax
> -	popl	%eax
> -	jae	nmi_stack_correct
> -	cmpl	$entry_SYSENTER_32, 12(%esp)
> -	je	nmi_debug_stack_check
> -nmi_stack_correct:
> -	pushl	%eax
> +
> +	pushl	%eax				# pt_regs->orig_ax
>  	SAVE_ALL
>  	xorl	%edx, %edx			# zero error code
>  	movl	%esp, %eax			# pt_regs pointer
> +
> +	/* Are we currently on the SYSENTER stack? */
> +	PER_CPU(cpu_tss + CPU_TSS_SYSENTER_stack + SIZEOF_SYSENTER_stack, %ecx)
> +	subl	%eax, %ecx		/* ecx = (end of SYENTER_stack) - esp */

Typo copied :)

> +	cmpl	$SIZEOF_SYSENTER_stack, %ecx
> +	jb	.Lnmi_from_sysenter_stack
> +
> +	/* Not on SYSENTER stack. */
>  	call	do_nmi
>  	jmp	restore_all_notrace
>  
> -nmi_stack_fixup:
> -	FIX_STACK 12, nmi_stack_correct, 1
> -	jmp	nmi_stack_correct
> -
> -nmi_debug_stack_check:
> -	cmpw	$__KERNEL_CS, 16(%esp)
> -	jne	nmi_stack_correct
> -	cmpl	$debug, (%esp)
> -	jb	nmi_stack_correct
> -	cmpl	$debug_esp_fix_insn, (%esp)
> -	ja	nmi_stack_correct
> -	FIX_STACK 24, nmi_stack_correct, 1
> -	jmp	nmi_stack_correct
> +.Lnmi_from_sysenter_stack:
> +	/*
> +	 * We're on the SYSENTER stack.  Switch off.  No one (not even debug)
> +	 * is using the thread stack right now, so it's safe for us to use it.
> +	 */
> +	movl	%esp, %ebp
> +	movl	PER_CPU_VAR(cpu_current_top_of_stack), %esp
> +	call	do_nmi
> +	movl	%ebp, %esp
> +	jmp	restore_all_notrace
>  
>  #ifdef CONFIG_X86_ESPFIX32
>  nmi_espfix_stack:

-- 
Regards/Gruss,
    Boris.

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