Re: [Xen-devel] [RFC 2/3] pc: Read PVH entry point from ELF note in kernel binary

[Stefan Hajnoczi <stefanha@xxxxxxxxxx>]

On Wed, Dec 05, 2018 at 10:37:25PM +0000, Liam Merwick wrote:
> From: Liam Merwick <Liam.Merwick@xxxxxxxxxx>
> 
> Add support to read the PVH Entry address from an ELF note in the
> uncompressed kernel binary (as defined by the x86/HVM direct boot ABI).
> This 32-bit entry point will be used by QEMU to load the kernel in the
> guest and jump into the kernel entry point.
> 
> For now, a call to this function is added in pc_memory_init() to read the
> address - a future patch will use the entry point.
> 
> Signed-off-by: Liam Merwick <Liam.Merwick@xxxxxxxxxx>
> ---
>  hw/i386/pc.c  | 272 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  include/elf.h |  10 +++
>  2 files changed, 281 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index f095725dbab2..056aa46d99b9 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -109,6 +109,9 @@ static struct e820_entry *e820_table;
>  static unsigned e820_entries;
>  struct hpet_fw_config hpet_cfg = {.count = UINT8_MAX};
>  
> +/* Physical Address of PVH entry point read from kernel ELF NOTE */
> +static size_t pvh_start_addr;
> +
>  void gsi_handler(void *opaque, int n, int level)
>  {
>      GSIState *s = opaque;
> @@ -834,6 +837,267 @@ struct setup_data {
>      uint8_t data[0];
>  } __attribute__((packed));
>  
> +/*
> + * Search through the ELF Notes for an entry with the given
> + * ELF Note type
> + */
> +static void *get_elf_note_type(void *ehdr, void *phdr, bool elf_is64,
> +    size_t elf_note_type)

Generic ELF code.  Can you put it in hw/core/loader.c?

> +{
> +    void *nhdr = NULL;
> +    size_t nhdr_size = elf_is64 ? sizeof(Elf64_Nhdr) : sizeof(Elf32_Nhdr);
> +    size_t elf_note_entry_sz = 0;
> +    size_t phdr_off;
> +    size_t phdr_align;
> +    size_t phdr_memsz;
> +    size_t nhdr_namesz;
> +    size_t nhdr_descsz;
> +    size_t note_type;

The macro tricks used by hw/core/loader.c are nasty, but I think they
get the types right.  Here the Elf64 on 32-bit host case is definitely
broken due to using size_t.  Perhaps 64-on-32 isn't supported, but
getting the types right is worth discussing.

> +
> +    phdr_off = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_offset : ((Elf32_Phdr *)phdr)->p_offset;
> +    phdr_align = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_align : ((Elf32_Phdr *)phdr)->p_align;
> +    phdr_memsz = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_memsz : ((Elf32_Phdr *)phdr)->p_memsz;
> +
> +    nhdr = ehdr + phdr_off;

The ELF file is untrusted.  All inputs must be validated.  phdr_off
could be an bogus/malicious value.

> +    note_type = elf_is64 ?
> +        ((Elf64_Nhdr *)nhdr)->n_type : ((Elf32_Nhdr *)nhdr)->n_type;
> +    nhdr_namesz = elf_is64 ?
> +        ((Elf64_Nhdr *)nhdr)->n_namesz : ((Elf32_Nhdr *)nhdr)->n_namesz;
> +    nhdr_descsz = elf_is64 ?
> +        ((Elf64_Nhdr *)nhdr)->n_descsz : ((Elf32_Nhdr *)nhdr)->n_descsz;
> +
> +    while (note_type != elf_note_type) {
> +        elf_note_entry_sz = nhdr_size +
> +            QEMU_ALIGN_UP(nhdr_namesz, phdr_align) +
> +            QEMU_ALIGN_UP(nhdr_descsz, phdr_align);
> +
> +        /*
> +         * Verify that we haven't exceeded the end of the ELF Note section.
> +         * If we have, then there is no note of the given type present
> +         * in the ELF Notes.
> +         */
> +        if (phdr_off + phdr_memsz < ((nhdr - ehdr) + elf_note_entry_sz)) {
> +            error_report("Note type (0x%lx) not found in ELF Note section",
> +                elf_note_type);
> +            return NULL;
> +        }
> +
> +        /* skip to the next ELF Note entry */
> +        nhdr += elf_note_entry_sz;
> +        note_type = elf_is64 ?
> +            ((Elf64_Nhdr *)nhdr)->n_type : ((Elf32_Nhdr *)nhdr)->n_type;
> +        nhdr_namesz = elf_is64 ?
> +            ((Elf64_Nhdr *)nhdr)->n_namesz : ((Elf32_Nhdr *)nhdr)->n_namesz;
> +        nhdr_descsz = elf_is64 ?
> +            ((Elf64_Nhdr *)nhdr)->n_descsz : ((Elf32_Nhdr *)nhdr)->n_descsz;
> +    }
> +
> +    return nhdr;
> +}
> +
> +/*
> + * The entry point into the kernel for PVH boot is different from
> + * the native entry point.  The PVH entry is defined by the x86/HVM
> + * direct boot ABI and is available in an ELFNOTE in the kernel binary.
> + * This function reads the ELF headers of the binary specified on the
> + * command line by -kernel (path contained in 'filename') and discovers
> + * the PVH entry address from the appropriate ELF Note.
> + *
> + * The address of the PVH entry point is saved to the 'pvh_start_addr'
> + * global variable. The ELF class of the binary is returned via 'elfclass'
> + * (although the entry point is 32-bit, the kernel binary can be either
> + * 32-bit or 64-bit).
> + */
> +static bool read_pvh_start_addr_elf_note(const char *filename,
> +    unsigned char *elfclass)
> +{

Can this be integrated into ELF loading?  For example, could the elf
loader take a function pointer to perform additional logic (e.g.
extracting the PVH entry point)?  That avoids reparsing the input file.

> +    void *ehdr = NULL; /* Cast to Elf64_Ehdr or Elf32_Ehdr */
> +    void *phdr = NULL; /* Cast to Elf64_Phdr or Elf32_Phdr */
> +    void *nhdr = NULL; /* Cast to Elf64_Nhdr or Elf32_Nhdr */
> +    struct stat statbuf;
> +    size_t ehdr_size;
> +    size_t phdr_size;
> +    size_t nhdr_size;
> +    size_t elf_note_data_addr;
> +    /* Ehdr fields */
> +    size_t ehdr_poff;
> +    /* Phdr fields */
> +    size_t phdr_off;
> +    size_t phdr_align;
> +    size_t phdr_memsz;
> +    size_t phdr_type;
> +    /* Nhdr fields */
> +    size_t nhdr_namesz;
> +    size_t nhdr_descsz;
> +    bool elf_is64;
> +    FILE *file;
> +    union {
> +        Elf32_Ehdr h32;
> +        Elf64_Ehdr h64;
> +    } elf_header;
> +    Error *err = NULL;
> +
> +    pvh_start_addr = 0;
> +
> +    if (filename == NULL) {
> +        return false;
> +    }
> +
> +    file = fopen(filename, "rb");
> +    if (file == NULL) {
> +        error_report("fopen(%s) failed", filename);
> +        return false;
> +    }
> +
> +    if (fstat(fileno(file), &statbuf) < 0) {
> +        error_report("fstat() failed on file (%s)", filename);
> +        return false;
> +    }
> +
> +    load_elf_hdr(filename, &elf_header, &elf_is64, &err);
> +    if (err) {
> +        error_free(err);
> +        fclose(file);
> +        return false;
> +    }
> +
> +    *elfclass = elf_is64 ?
> +        elf_header.h64.e_ident[EI_CLASS] : elf_header.h32.e_ident[EI_CLASS];
> +    if (*elfclass == ELFCLASSNONE) {
> +        error_report("kernel binary (%s) is ELFCLASSNONE", filename);
> +        fclose(file);
> +        return false;
> +    }
> +
> +    ehdr_size = elf_is64 ? sizeof(Elf64_Ehdr) : sizeof(Elf32_Ehdr);
> +    phdr_size = elf_is64 ? sizeof(Elf64_Phdr) : sizeof(Elf32_Phdr);
> +    nhdr_size = elf_is64 ? sizeof(Elf64_Nhdr) : sizeof(Elf32_Nhdr);
> +
> +    /* We have already validated the ELF header when calling elf_load_hdr() 
> */
> +
> +    ehdr = mmap(0, statbuf.st_size,
> +        PROT_READ | PROT_WRITE, MAP_PRIVATE, fileno(file), 0);
> +    if (ehdr == MAP_FAILED) {
> +        error_report("Failed to mmap kernel binary (%s)", filename);
> +        goto done;
> +    }
> +
> +    /*
> +     * Search through the program execution header for the
> +     * ELF Note section.
> +     */
> +
> +    ehdr_poff = elf_is64 ?
> +        ((Elf64_Ehdr *)(ehdr))->e_phoff : ((Elf32_Ehdr *)(ehdr))->e_phoff;
> +    if (statbuf.st_size < (ehdr_size + ehdr_poff)) {
> +        error_report("ELF NOTE section exceeds file (%s) size",
> +            filename);
> +        goto done;
> +    }
> +
> +    phdr = ehdr + ehdr_poff;
> +    phdr_type = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_type : ((Elf32_Phdr *)phdr)->p_type;
> +    while (phdr != NULL && phdr_type != PT_NOTE) {
> +        if (statbuf.st_size < ((phdr - ehdr) + phdr_size)) {
> +            error_report("ELF Program headers in file (%s) too short",
> +                filename);
> +            goto done;
> +        }
> +        phdr += phdr_size;
> +        phdr_type = elf_is64 ?
> +            ((Elf64_Phdr *)phdr)->p_type : ((Elf32_Phdr *)phdr)->p_type;
> +    }
> +
> +    phdr_off = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_offset : ((Elf32_Phdr *)phdr)->p_offset;
> +    phdr_align = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_align : ((Elf32_Phdr *)phdr)->p_align;
> +    phdr_memsz = elf_is64 ?
> +        ((Elf64_Phdr *)phdr)->p_memsz : ((Elf32_Phdr *)phdr)->p_memsz;
> +
> +    /*
> +     * check that the start of the ELF Note section is within the bounds
> +     * of the kernel ELF binary
> +     */
> +    if (statbuf.st_size < (ehdr_poff + phdr_size + phdr_off)) {
> +        error_report("Start of ELF note section outside of file (%s) bounds",
> +            filename);
> +        goto done;
> +    }
> +    /*
> +     * check that the end of the ELF Note section is within the bounds
> +     * of the kernel ELF binary
> +     */
> +    if (statbuf.st_size < (phdr_off + phdr_memsz)) {
> +        error_report("End of ELF note section outside of file (%s) bounds",
> +            filename);
> +        goto done;
> +    }
> +
> +    /*
> +     * Search through the ELF Notes for an entry with the
> +     * Physical Address (PA) of the PVH entry point.
> +     */
> +    nhdr = get_elf_note_type(ehdr, phdr, elf_is64, XEN_ELFNOTE_PHYS32_ENTRY);
> +    if (nhdr == NULL) {
> +        error_report("No PVH Entry details in kernel (%s) ELF Note section",
> +            filename);
> +        goto done;
> +    }
> +
> +    /*
> +     * Verify that the returned ELF Note header doesn't exceed the
> +     * end of the kernel file
> +     */
> +    if (statbuf.st_size < ((nhdr - ehdr))) {
> +        error_report("ELF Nhdr offset (0x%lx) exceeds file (%s) bounds 
> (%ld)",
> +            (nhdr - ehdr), filename, statbuf.st_size);
> +        goto done;
> +    }
> +
> +    nhdr_namesz = elf_is64 ?
> +        ((Elf64_Nhdr *)nhdr)->n_namesz : ((Elf32_Nhdr *)nhdr)->n_namesz;
> +    nhdr_descsz = elf_is64 ?
> +        ((Elf64_Nhdr *)nhdr)->n_descsz : ((Elf32_Nhdr *)nhdr)->n_descsz;
> +
> +    /*
> +     * Verify that the ELF Note contents don't exceed the end of the
> +     * kernel file
> +     */
> +    if (statbuf.st_size < ((nhdr - ehdr)) + nhdr_size +
> +        QEMU_ALIGN_UP(nhdr_namesz, phdr_align) +
> +        QEMU_ALIGN_UP(nhdr_descsz, phdr_align)) {
> +        error_report("ELF Nhdr contents (0x%lx) exceeds file bounds (%ld)",
> +            (nhdr - ehdr) + nhdr_size + QEMU_ALIGN_UP(nhdr_namesz, 
> phdr_align) +
> +            QEMU_ALIGN_UP(nhdr_descsz, phdr_align), statbuf.st_size);
> +        goto done;
> +    }
> +
> +    elf_note_data_addr =
> +        (size_t)nhdr + nhdr_size + QEMU_ALIGN_UP(nhdr_namesz, phdr_align);
> +
> +    pvh_start_addr = *(size_t *)elf_note_data_addr;
> +
> +    /*
> +     * Verify that the PVH Entry point address does not exceed the
> +     * bounds of the kernel file.
> +     */
> +    if (statbuf.st_size < pvh_start_addr) {
> +        error_report("PVH ELF note addr (0x%lx) exceeds file (%s) bounds 
> (%ld)",
> +            (elf_note_data_addr - (size_t)ehdr), filename, statbuf.st_size);
> +        pvh_start_addr = 0;
> +        goto done;
> +    }
> +
> +done:
> +    (void) munmap(ehdr, statbuf.st_size);
> +    return pvh_start_addr != 0;
> +}
> +
>  static void load_linux(PCMachineState *pcms,
>                         FWCfgState *fw_cfg)
>  {
> @@ -1334,9 +1598,11 @@ void pc_memory_init(PCMachineState *pcms,
>      int linux_boot, i;
>      MemoryRegion *ram, *option_rom_mr;
>      MemoryRegion *ram_below_4g, *ram_above_4g;
> -    FWCfgState *fw_cfg;
> +    FWCfgState *fw_cfg = NULL;
> +    unsigned char class = ELFCLASSNONE;
>      MachineState *machine = MACHINE(pcms);
>      PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
> +    const char *kernel_filename = machine->kernel_filename;
>  
>      assert(machine->ram_size == pcms->below_4g_mem_size +
>                                  pcms->above_4g_mem_size);
> @@ -1418,6 +1684,10 @@ void pc_memory_init(PCMachineState *pcms,
>                                      &machine->device_memory->mr);
>      }
>  
> +    if (linux_boot) {
> +        read_pvh_start_addr_elf_note(kernel_filename, &class);
> +    }
> +
>      /* Initialize PC system firmware */
>      pc_system_firmware_init(rom_memory, !pcmc->pci_enabled);
>  
> diff --git a/include/elf.h b/include/elf.h
> index c151164b63da..1f82c7a7124b 100644
> --- a/include/elf.h
> +++ b/include/elf.h
> @@ -1585,6 +1585,16 @@ typedef struct elf64_shdr {
>  #define NT_ARM_HW_WATCH 0x403           /* ARM hardware watchpoint registers 
> */
>  #define NT_ARM_SYSTEM_CALL      0x404   /* ARM system call number */
>  
> +/*
> + * Physical entry point into the kernel.
> + *
> + * 32bit entry point into the kernel. When requested to launch the
> + * guest kernel, use this entry point to launch the guest in 32-bit
> + * protected mode with paging disabled.
> + *
> + * [ Corresponding definition in Linux kernel: 
> include/xen/interface/elfnote.h ]
> + */
> +#define XEN_ELFNOTE_PHYS32_ENTRY    18  /* 0x12 */
>  
>  /* Note header in a PT_NOTE section */
>  typedef struct elf32_note {
> -- 
> 1.8.3.1
>

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