Re: [PATCH] riscv: Sync efi page table's kernel mappings before switching

[Emil Renner Berthing]

Hi Alexandre,

On Mon, 21 Nov 2022 at 14:33, Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> The EFI page table is initially created as a copy of the kernel page table.
> With VMAP_STACK enabled, kernel stacks are allocated in the vmalloc area:
> if the stack is allocated in a new PGD (one that was not present at the
> moment of the efi page table creation or not synced in a previous vmalloc
> fault), the kernel will take a trap when switching to the efi page table
> when the vmalloc kernel stack is accessed, resulting in a kernel panic.
>
> Fix that by updating the efi kernel mappings before switching to the efi
> page table.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Thanks for the patch! With this applied on the Ubuntu 5.19 kernel I
can enable CONFIG_VMAP_STACK and cat /sys/firmware/efi/efivars/* on
the Unmatched without locking up. So

Tested-by: Emil Renner Berthing <emil.renner.berthing@xxxxxxxxxxxxx>

> ---
>  arch/riscv/include/asm/efi.h     |  6 +++++-
>  arch/riscv/include/asm/pgalloc.h | 11 ++++++++---
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/efi.h b/arch/riscv/include/asm/efi.h
> index f74879a8f1ea..e229d7be4b66 100644
> --- a/arch/riscv/include/asm/efi.h
> +++ b/arch/riscv/include/asm/efi.h
> @@ -10,6 +10,7 @@
>  #include <asm/mmu_context.h>
>  #include <asm/ptrace.h>
>  #include <asm/tlbflush.h>
> +#include <asm/pgalloc.h>
>
>  #ifdef CONFIG_EFI
>  extern void efi_init(void);
> @@ -20,7 +21,10 @@ extern void efi_init(void);
>  int efi_create_mapping(struct mm_struct *mm, efi_memory_desc_t *md);
>  int efi_set_mapping_permissions(struct mm_struct *mm, efi_memory_desc_t *md);
>
> -#define arch_efi_call_virt_setup()      efi_virtmap_load()
> +#define arch_efi_call_virt_setup()      ({             \
> +               sync_kernel_mappings(efi_mm.pgd);       \
> +               efi_virtmap_load();                     \
> +       })
>  #define arch_efi_call_virt_teardown()   efi_virtmap_unload()
>
>  #define ARCH_EFI_IRQ_FLAGS_MASK (SR_IE | SR_SPIE)
> diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> index 947f23d7b6af..59dc12b5b7e8 100644
> --- a/arch/riscv/include/asm/pgalloc.h
> +++ b/arch/riscv/include/asm/pgalloc.h
> @@ -127,6 +127,13 @@ static inline void p4d_free(struct mm_struct *mm, p4d_t *p4d)
>  #define __p4d_free_tlb(tlb, p4d, addr)  p4d_free((tlb)->mm, p4d)
>  #endif /* __PAGETABLE_PMD_FOLDED */
>
> +static inline void sync_kernel_mappings(pgd_t *pgd)
> +{
> +       memcpy(pgd + USER_PTRS_PER_PGD,
> +              init_mm.pgd + USER_PTRS_PER_PGD,
> +              (PTRS_PER_PGD - USER_PTRS_PER_PGD) * sizeof(pgd_t));
> +}
> +
>  static inline pgd_t *pgd_alloc(struct mm_struct *mm)
>  {
>         pgd_t *pgd;
> @@ -135,9 +142,7 @@ static inline pgd_t *pgd_alloc(struct mm_struct *mm)
>         if (likely(pgd != NULL)) {
>                 memset(pgd, 0, USER_PTRS_PER_PGD * sizeof(pgd_t));
>                 /* Copy kernel mappings */
> -               memcpy(pgd + USER_PTRS_PER_PGD,
> -                       init_mm.pgd + USER_PTRS_PER_PGD,
> -                       (PTRS_PER_PGD - USER_PTRS_PER_PGD) * sizeof(pgd_t));
> +               sync_kernel_mappings(pgd);
>         }
>         return pgd;
>  }
> --
> 2.37.2
>