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SubjectRe: [tip:x86/boot] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G
Hi Kirill,

Something is wrong in this patch. We regularly run CRIU tests on
linux-next, and yesterday I found that a kernel didn't boot. We run this
tests in Travis-CI, and we don't have access to kernel logs. I tried to
reproduce the problem localy, but I failed.

In Travis-CI, we build kernel, then dump a travis deamon, boot the
kernel with help of kexec and restore the travis daemon back.

Here is logs without this patch:
https://travis-ci.org/avagin/linux/jobs/340820418

Here is logs with this patch:
https://travis-ci.org/avagin/linux/jobs/340820584

Thanks,
Andrei

On Sun, Feb 11, 2018 at 04:20:04AM -0800, tip-bot for Jacob Shin wrote:
> Commit-ID: b4b56015ed1c98cbc9469e35ebbc4373a2844030
> Gitweb: https://git.kernel.org/tip/b4b56015ed1c98cbc9469e35ebbc4373a2844030
> Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> AuthorDate: Fri, 9 Feb 2018 17:22:28 +0300
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Sun, 11 Feb 2018 12:36:19 +0100
>
> x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G
>
> This patch addresses a shortcoming in current boot process on machines
> that supports 5-level paging.
>
> If a bootloader enables 64-bit mode with 4-level paging, we might need to
> switch over to 5-level paging. The switching requires the disabling
> paging. It works fine if kernel itself is loaded below 4G.
>
> But if the bootloader put the kernel above 4G (not sure if anybody does
> this), we would lose control as soon as paging is disabled, because the
> code becomes unreachable to the CPU.
>
> This patch implements a trampoline in lower memory to handle this
> situation.
>
> We only need the memory for a very short time, until the main kernel
> image sets up own page tables.
>
> We go through the trampoline even if we don't have to: if we're already
> in 5-level paging mode or if we don't need to switch to it. This way the
> trampoline gets tested on every boot.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: linux-mm@kvack.org
> Link: http://lkml.kernel.org/r/20180209142228.21231-5-kirill.shutemov@linux.intel.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> arch/x86/boot/compressed/head_64.S | 127 ++++++++++++++++++++++++++-----------
> 1 file changed, 89 insertions(+), 38 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index af9ffbd..70b30f2 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -307,13 +307,34 @@ ENTRY(startup_64)
>
> /*
> * At this point we are in long mode with 4-level paging enabled,
> - * but we want to enable 5-level paging.
> + * but we might want to enable 5-level paging or vice versa.
> *
> - * The problem is that we cannot do it directly. Setting LA57 in
> - * long mode would trigger #GP. So we need to switch off long mode
> - * first.
> + * The problem is that we cannot do it directly. Setting or clearing
> + * CR4.LA57 in long mode would trigger #GP. So we need to switch off
> + * long mode and paging first.
> + *
> + * We also need a trampoline in lower memory to switch over from
> + * 4- to 5-level paging for cases when the bootloader puts the kernel
> + * above 4G, but didn't enable 5-level paging for us.
> + *
> + * The same trampoline can be used to switch from 5- to 4-level paging
> + * mode, like when starting 4-level paging kernel via kexec() when
> + * original kernel worked in 5-level paging mode.
> + *
> + * For the trampoline, we need the top page table to reside in lower
> + * memory as we don't have a way to load 64-bit values into CR3 in
> + * 32-bit mode.
> + *
> + * We go though the trampoline even if we don't have to: if we're
> + * already in a desired paging mode. This way the trampoline code gets
> + * tested on every boot.
> */
>
> + /* Make sure we have GDT with 32-bit code segment */
> + leaq gdt(%rip), %rax
> + movl %eax, gdt64+2(%rip)
> + lgdt gdt64(%rip)
> +
> /*
> * paging_prepare() sets up the trampoline and checks if we need to
> * enable 5-level paging.
> @@ -331,30 +352,20 @@ ENTRY(startup_64)
> /* Save the trampoline address in RCX */
> movq %rax, %rcx
>
> - /* Check if we need to enable 5-level paging */
> - cmpq $0, %rdx
> - jz lvl5
> -
> - /* Clear additional page table */
> - leaq lvl5_pgtable(%rbx), %rdi
> - xorq %rax, %rax
> - movq $(PAGE_SIZE/8), %rcx
> - rep stosq
> -
> /*
> - * Setup current CR3 as the first and only entry in a new top level
> - * page table.
> + * Load the address of trampoline_return() into RDI.
> + * It will be used by the trampoline to return to the main code.
> */
> - movq %cr3, %rdi
> - leaq 0x7 (%rdi), %rax
> - movq %rax, lvl5_pgtable(%rbx)
> + leaq trampoline_return(%rip), %rdi
>
> /* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
> pushq $__KERNEL32_CS
> - leaq compatible_mode(%rip), %rax
> + leaq TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax
> pushq %rax
> lretq
> -lvl5:
> +trampoline_return:
> + /* Restore the stack, the 32-bit trampoline uses its own stack */
> + leaq boot_stack_end(%rbx), %rsp
>
> /*
> * cleanup_trampoline() would restore trampoline memory.
> @@ -503,45 +514,82 @@ relocated:
> jmp *%rax
>
> .code32
> +/*
> + * This is the 32-bit trampoline that will be copied over to low memory.
> + *
> + * RDI contains the return address (might be above 4G).
> + * ECX contains the base address of the trampoline memory.
> + * Non zero RDX on return means we need to enable 5-level paging.
> + */
> ENTRY(trampoline_32bit_src)
> -compatible_mode:
> /* Set up data and stack segments */
> movl $__KERNEL_DS, %eax
> movl %eax, %ds
> movl %eax, %ss
>
> + /* Setup new stack */
> + leal TRAMPOLINE_32BIT_STACK_END(%ecx), %esp
> +
> /* Disable paging */
> movl %cr0, %eax
> btrl $X86_CR0_PG_BIT, %eax
> movl %eax, %cr0
>
> - /* Point CR3 to 5-level paging */
> - leal lvl5_pgtable(%ebx), %eax
> - movl %eax, %cr3
> + /* Check what paging mode we want to be in after the trampoline */
> + cmpl $0, %edx
> + jz 1f
>
> - /* Enable PAE and LA57 mode */
> + /* We want 5-level paging: don't touch CR3 if it already points to 5-level page tables */
> movl %cr4, %eax
> - orl $(X86_CR4_PAE | X86_CR4_LA57), %eax
> + testl $X86_CR4_LA57, %eax
> + jnz 3f
> + jmp 2f
> +1:
> + /* We want 4-level paging: don't touch CR3 if it already points to 4-level page tables */
> + movl %cr4, %eax
> + testl $X86_CR4_LA57, %eax
> + jz 3f
> +2:
> + /* Point CR3 to the trampoline's new top level page table */
> + leal TRAMPOLINE_32BIT_PGTABLE_OFFSET(%ecx), %eax
> + movl %eax, %cr3
> +3:
> + /* Enable PAE and LA57 (if required) paging modes */
> + movl $X86_CR4_PAE, %eax
> + cmpl $0, %edx
> + jz 1f
> + orl $X86_CR4_LA57, %eax
> +1:
> movl %eax, %cr4
>
> - /* Calculate address we are running at */
> - call 1f
> -1: popl %edi
> - subl $1b, %edi
> + /* Calculate address of paging_enabled() once we are executing in the trampoline */
> + leal paging_enabled - trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_OFFSET(%ecx), %eax
>
> - /* Prepare stack for far return to Long Mode */
> + /* Prepare the stack for far return to Long Mode */
> pushl $__KERNEL_CS
> - leal lvl5(%edi), %eax
> - push %eax
> + pushl %eax
>
> - /* Enable paging back */
> + /* Enable paging again */
> movl $(X86_CR0_PG | X86_CR0_PE), %eax
> movl %eax, %cr0
>
> lret
>
> + .code64
> +paging_enabled:
> + /* Return from the trampoline */
> + jmp *%rdi
> +
> + /*
> + * The trampoline code has a size limit.
> + * Make sure we fail to compile if the trampoline code grows
> + * beyond TRAMPOLINE_32BIT_CODE_SIZE bytes.
> + */
> + .org trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_SIZE
> +
> + .code32
> no_longmode:
> - /* This isn't an x86-64 CPU so hang */
> + /* This isn't an x86-64 CPU, so hang intentionally, we cannot continue */
> 1:
> hlt
> jmp 1b
> @@ -549,6 +597,11 @@ no_longmode:
> #include "../../kernel/verify_cpu.S"
>
> .data
> +gdt64:
> + .word gdt_end - gdt
> + .long 0
> + .word 0
> + .quad 0
> gdt:
> .word gdt_end - gdt
> .long gdt
> @@ -602,8 +655,6 @@ trampoline_save:
> .balign 4096
> pgtable:
> .fill BOOT_PGT_SIZE, 1, 0
> -lvl5_pgtable:
> - .fill PAGE_SIZE, 1, 0
>
> .global pgtable_trampoline
> pgtable_trampoline:

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