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Re: [Xen-devel] [RFC 2/3] pc: Read PVH entry point from ELF note in kernel binary
- To: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
- From: Liam Merwick <liam.merwick@xxxxxxxxxx>
- Date: Fri, 21 Dec 2018 20:03:36 +0000
- Cc: liam.merwick@xxxxxxxxxx, ehabkost@xxxxxxxxxx, mst@xxxxxxxxxx, maran.wilson@xxxxxxxxxx, qemu-devel@xxxxxxxxxx, George Kennedy <george.kennedy@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, pbonzini@xxxxxxxxxx, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, rth@xxxxxxxxxxx, sgarzare@xxxxxxxxxx
- Delivery-date: Fri, 21 Dec 2018 20:03:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Thanks Stefan for the review - comments inline.
On 11/12/2018 14:17, Stefan Hajnoczi wrote:
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?
I've added a modified/slimmed down version to include/hw/elf_ops.h
(which now handles 32 and 64 bit as you mention below). I've put this
in a separate commit.
+{
+ 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.
Most of the parsing of the ELF binary goes away due to moving to parse
during elf_load() - more info below.
+ 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.
I have rewritten this considerably based on that suggestion. The
reading of the PVH entry point is now done in a single pass during
elf_load() - I added a commit that adds a new optional function pointer
to parse the ELF note type (which is passed in via the existing
translate_opaque arg - the function already had 11 args so I didn't want
to add more than one new arg). Another commit adds a function to
elf_ops.h to find an ELF note matching a specific type and then the 4th
patch to do the PVH boot is for the most part the same - just minor
load_elfboot() changes and the addition of a read_pvh_start_addr()
helper function for load_elf()
v2 will follow in a sec.
Regards,
Liam
+ 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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