Re: [PATCH v5 13/20] x86/kexec: Mark relocate_kernel page as ROX instead of RWX

[David Woodhouse <dwmw2@infradead.org>]

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> 
> Here is the output that I see with that patch applied when rebooting via
> 'systemctl kexec':
> 
> RAX=000000010c070001 RBX=0000000000000000 RCX=0000000000000000 RDX=000000047fffb1a0
> RSI=000000011c444000 RDI=000000011c450002 RBP=ff1cd0424d6e8c00 RSP=ff4178d5c5aebc60
> R8 =0000000000000000 R9 =000000011c446000 R10=ffffffff909f3e00 R11=0000000000000003
> R12=0000000000000000 R13=0000000000000001 R14=00000000fee1dead R15=0000000000000000
> RIP=ff1cd0425c44401c RFL=00010002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ES =0018 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> CS =0010 0000000000000000 ffffffff 00a09b00 DPL=0 CS64 [-RA]
> SS =0018 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> DS =0018 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> FS =0018 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> GS =0018 0000000000000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> LDT=0000 0000000000000000 ffffffff 00c00000
> TR =0040 fffffe043fedb000 00004087 00008b00 DPL=0 TSS64-busy
> GDT=     0000000000000000 00000000
> IDT=     0000000000000000 00000000
> CR0=80050033 CR2=ff1cd0425c4441de CR3=000000011c446000 CR4=00771ef0
> DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000
> DR6=00000000fffe0ff0 DR7=0000000000000400
> EFER=0000000000000d01
> Code=41 57 9c 6a 00 9d 0f 20 d8 4c 8b 0d ee 01 00 00 41 0f 22 d9 <48> 89 25 bb 01 00 00 48 89 05 c4 01 00 00 0f 20 c0 48 89 05 b2 01 00 00 41 0f 20 e5 4c 89

...

> I have attached the output of 'objdump -S'. Please let me know if you
> would like any other information or testing.

	/* Switch to the identity mapped page tables */
	movq	%cr3, %rax
   e:	0f 20 d8             	mov    %cr3,%rax
	movq	kexec_pa_table_page(%rip), %r9
  11:	4c 8b 0d 00 00 00 00 	mov    0x0(%rip),%r9        # 18 <relocate_kernel+0x18>
	movq	%r9, %cr3
  18:	41 0f 22 d9          	mov    %r9,%cr3

	/* Save %rsp and CRs. */
	movq    %rsp, saved_rsp(%rip)
  1c:	48 89 25 00 00 00 00 	mov    %rsp,0x0(%rip)        # 23 <relocate_kernel+0x23>
	movq	%rax, saved_cr3(%rip)


So it's faulting when it tries to write to saved_rsp, which is the
first instruction after it loads the new page tables into %cr3.

Before loading %cr3, the CPU is running on the original kernel page
tables, It's running from the *virtual* address of the control page,
which in your case is ff1cd0425c444000. In those old page tables, this
control page is read-only.

But in the newly-loaded page tables, this control page should be read-
write. 

What CPU is this? Shouldn't loading %cr3 have caused the existing TLB
entry to be flushed?

Can you tell me what happens if we don't write through *that* virtual
mapping of the page, but instead do it a few instructions later through
the identity mapping of the same page (which surely *won't* have an
older, non-writeable TLB entry already...)?



diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 8bc86a1e056a..2763c509e513 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -70,6 +70,18 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	movq	kexec_pa_table_page(%rip), %r9
 	movq	%r9, %cr3
 
+	/* setup a new stack at the end of the physical control page */
+	lea	PAGE_SIZE(%rsi), %rsp
+
+	/* jump to identity mapped page */
+	addq	$(identity_mapped - relocate_kernel), %rsi
+	ANNOTATE_RETPOLINE_SAFE
+	jmp	*%rsi
+	int3
+SYM_CODE_END(relocate_kernel)
+
+SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
+	UNWIND_HINT_END_OF_STACK
 	/* Save %rsp and CRs. */
 	movq    %rsp, saved_rsp(%rip)
 	movq	%rax, saved_cr3(%rip)
@@ -85,19 +97,6 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
 	movq	%rcx, %r11
 
-	/* setup a new stack at the end of the physical control page */
-	lea	PAGE_SIZE(%rsi), %rsp
-
-	/* jump to identity mapped page */
-	addq	$(identity_mapped - relocate_kernel), %rsi
-	pushq	%rsi
-	ANNOTATE_UNRET_SAFE
-	ret
-	int3
-SYM_CODE_END(relocate_kernel)
-
-SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
-	UNWIND_HINT_END_OF_STACK
 	/*
 	 * %rdi	indirection page
 	 * %rdx start address


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