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

Borislav Petkov
2020-04-02T11:49:48Z

On Thu, Mar 19, 2020 at 10:13:11AM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.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@suse.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.