linux-kernel - Re: [PATCH 14/70] x86/boot/compressed/64: Add page-fault handler

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Date:   Thu, 2 Apr 2020 13:49:41 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 14/70] x86/boot/compressed/64: Add page-fault handler

On Thu, Mar 19, 2020 at 10:13:11AM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Install a page-fault handler to add an identity mapping to addresses
> not yet mapped. Also do some checking whether the error code is sane.
> 
> This makes non SEV-ES machines use the exception handling
> infrastructure in the pre-decompressions boot code too, making it less
> likely to break in the future.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/boot/compressed/ident_map_64.c    | 38 ++++++++++++++++++++++
>  arch/x86/boot/compressed/idt_64.c          |  2 ++
>  arch/x86/boot/compressed/idt_handlers_64.S |  2 ++
>  arch/x86/boot/compressed/misc.h            |  6 ++++
>  4 files changed, 48 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index 3a2115582920..0865d181b85d 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -19,11 +19,13 @@
>  /* No PAGE_TABLE_ISOLATION support needed either: */
>  #undef CONFIG_PAGE_TABLE_ISOLATION
>  
> +#include "error.h"
>  #include "misc.h"
>  
>  /* These actually do the work of building the kernel identity maps. */
>  #include <asm/init.h>
>  #include <asm/pgtable.h>
> +#include <asm/trap_defs.h>
>  /* Use the static base for this part of the boot process */
>  #undef __PAGE_OFFSET
>  #define __PAGE_OFFSET __PAGE_OFFSET_BASE
> @@ -163,3 +165,39 @@ void finalize_identity_maps(void)
>  {
>  	write_cr3(top_level_pgt);
>  }
> +
> +static void pf_error(unsigned long error_code, unsigned long address,
> +		     struct pt_regs *regs)

AFAICT, that function is called below only so just merge its body into
the call site instead...

> +{
> +	error_putstr("Unexpected page-fault:");
> +	error_putstr("\nError Code: ");
> +	error_puthex(error_code);
> +	error_putstr("\nCR2: 0x");
> +	error_puthex(address);
> +	error_putstr("\nRIP relative to _head: 0x");
> +	error_puthex(regs->ip - (unsigned long)_head);
> +	error_putstr("\n");
> +
> +	error("Stopping.\n");
> +}
> +
> +void do_boot_page_fault(struct pt_regs *regs)
> +{
> +	unsigned long address = native_read_cr2();
> +	unsigned long error_code = regs->orig_ax;
> +
> +	/*
> +	 * Check for unexpected error codes. Unexpected are:
> +	 *	- Faults on present pages
> +	 *	- User faults
> +	 *	- Reserved bits set
> +	 */
> +	if (error_code & (X86_PF_PROT | X86_PF_USER | X86_PF_RSVD))
> +		pf_error(error_code, address, regs);
> +
> +	/*
> +	 * Error code is sane - now identity map the 2M region around
> +	 * the faulting address.
> +	 */
> +	add_identity_map(address & PMD_MASK, PMD_SIZE);
> +}
> diff --git a/arch/x86/boot/compressed/idt_64.c b/arch/x86/boot/compressed/idt_64.c
> index 46ecea671b90..84ba57d9d436 100644
> --- a/arch/x86/boot/compressed/idt_64.c
> +++ b/arch/x86/boot/compressed/idt_64.c
> @@ -39,5 +39,7 @@ void load_stage2_idt(void)
>  {
>  	boot_idt_desc.address = (unsigned long)boot_idt;
>  
> +	set_idt_entry(X86_TRAP_PF, boot_pf_handler);
> +
>  	load_boot_idt(&boot_idt_desc);
>  }
> diff --git a/arch/x86/boot/compressed/idt_handlers_64.S b/arch/x86/boot/compressed/idt_handlers_64.S
> index 3d86ab35ef52..bfb3fc5aa144 100644
> --- a/arch/x86/boot/compressed/idt_handlers_64.S
> +++ b/arch/x86/boot/compressed/idt_handlers_64.S
> @@ -73,3 +73,5 @@ SYM_FUNC_END(\name)
>  
>  	.text
>  	.code64
> +
> +EXCEPTION_HANDLER	boot_pf_handler do_boot_page_fault error_code=1

			boot_page_fault do_boot_page_fault

equivalent to the PF handler proper naming pls. Grepping "page_fault"
would give you all then.

> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 3a030a878d53..eff4ed0b1cea 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -37,6 +37,9 @@
>  #define memptr unsigned
>  #endif
>  
> +/* boot/compressed/vmlinux start and end markers */
> +extern char _head[], _end[];
> +
>  /* misc.c */
>  extern memptr free_mem_ptr;
>  extern memptr free_mem_end_ptr;
> @@ -146,4 +149,7 @@ extern pteval_t __default_kernel_pte_mask;
>  extern gate_desc boot_idt[BOOT_IDT_ENTRIES];
>  extern struct desc_ptr boot_idt_desc;
>  
> +/* IDT Entry Points */
> +void boot_pf_handler(void);
> +
>  #endif /* BOOT_COMPRESSED_MISC_H */
> -- 
> 2.17.1
> 

-- 
Regards/Gruss,
    Boris.

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