On Tue, Jan 31, 2023, 15:55 H. Peter Anvin <h...@zytor.com> wrote:
> On January 30, 2023 12:19:14 PM PST, "Michael S. Tsirkin" <m...@redhat.com>
> wrote:
> >From: "Jason A. Donenfeld" <ja...@zx2c4.com>
> >
> >The setup_data links are appended to the compressed kernel image. Since
> >the kernel image is typically loaded at 0x100000, setup_data lives at
> >`0x100000 + compressed_size`, which does not get relocated during the
> >kernel's boot process.
> >
> >The kernel typically decompresses the image starting at address
> >0x1000000 (note: there's one more zero there than the compressed image
> >above). This usually is fine for most kernels.
> >
> >However, if the compressed image is actually quite large, then
> >setup_data will live at a `0x100000 + compressed_size` that extends into
> >the decompressed zone at 0x1000000. In other words, if compressed_size
> >is larger than `0x1000000 - 0x100000`, then the decompression step will
> >clobber setup_data, resulting in crashes.
> >
> >Visually, what happens now is that QEMU appends setup_data to the kernel
> >image:
> >
> > kernel image setup_data
> > |--------------------------||----------------|
> >0x100000 0x100000+l1 0x100000+l1+l2
> >
> >The problem is that this decompresses to 0x1000000 (one more zero). So
> >if l1 is > (0x1000000-0x100000), then this winds up looking like:
> >
> > kernel image setup_data
> > |--------------------------||----------------|
> >0x100000 0x100000+l1 0x100000+l1+l2
> >
> > d e c o m p r e s s e d k e r n e l
> >
> |-------------------------------------------------------------|
> > 0x1000000
> 0x1000000+l3
> >
> >The decompressed kernel seemingly overwriting the compressed kernel
> >image isn't a problem, because that gets relocated to a higher address
> >early on in the boot process, at the end of startup_64. setup_data,
> >however, stays in the same place, since those links are self referential
> >and nothing fixes them up. So the decompressed kernel clobbers it.
> >
> >Fix this by appending setup_data to the cmdline blob rather than the
> >kernel image blob, which remains at a lower address that won't get
> >clobbered.
> >
> >This could have been done by overwriting the initrd blob instead, but
> >that poses big difficulties, such as no longer being able to use memory
> >mapped files for initrd, hurting performance, and, more importantly, the
> >initrd address calculation is hard coded in qboot, and it always grows
> >down rather than up, which means lots of brittle semantics would have to
> >be changed around, incurring more complexity. In contrast, using cmdline
> >is simple and doesn't interfere with anything.
> >
> >The microvm machine has a gross hack where it fiddles with fw_cfg data
> >after the fact. So this hack is updated to account for this appending,
> >by reserving some bytes.
> >
> >Fixup-by: Michael S. Tsirkin <m...@redhat.com>
> >Cc: x...@kernel.org
> >Cc: Philippe Mathieu-Daudé <phi...@linaro.org>
> >Cc: H. Peter Anvin <h...@zytor.com>
> >Cc: Borislav Petkov <b...@alien8.de>
> >Cc: Eric Biggers <ebigg...@kernel.org>
> >Signed-off-by: Jason A. Donenfeld <ja...@zx2c4.com>
> >Message-Id: <20221230220725.618763-1-ja...@zx2c4.com>
> >Message-ID: <20230128061015-mutt-send-email-...@kernel.org>
> >Reviewed-by: Michael S. Tsirkin <m...@redhat.com>
> >Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
> >Tested-by: Eric Biggers <ebigg...@google.com>
> >Tested-by: Mathias Krause <mini...@grsecurity.net>
> >---
> > include/hw/i386/microvm.h | 5 ++--
> > include/hw/nvram/fw_cfg.h | 9 +++++++
> > hw/i386/microvm.c | 15 +++++++----
> > hw/i386/x86.c | 52 +++++++++++++++++++++------------------
> > hw/nvram/fw_cfg.c | 9 +++++++
> > 5 files changed, 59 insertions(+), 31 deletions(-)
> >
> >diff --git a/include/hw/i386/microvm.h b/include/hw/i386/microvm.h
> >index fad97a891d..e8af61f194 100644
> >--- a/include/hw/i386/microvm.h
> >+++ b/include/hw/i386/microvm.h
> >@@ -50,8 +50,9 @@
> > */
> >
> > /* Platform virtio definitions */
> >-#define VIRTIO_MMIO_BASE 0xfeb00000
> >-#define VIRTIO_CMDLINE_MAXLEN 64
> >+#define VIRTIO_MMIO_BASE 0xfeb00000
> >+#define VIRTIO_CMDLINE_MAXLEN 64
> >+#define VIRTIO_CMDLINE_TOTAL_MAX_LEN ((VIRTIO_CMDLINE_MAXLEN + 1) *
> 16)
> >
> > #define GED_MMIO_BASE 0xfea00000
> > #define GED_MMIO_BASE_MEMHP (GED_MMIO_BASE + 0x100)
> >diff --git a/include/hw/nvram/fw_cfg.h b/include/hw/nvram/fw_cfg.h
> >index 2e503904dc..990dcdbb2e 100644
> >--- a/include/hw/nvram/fw_cfg.h
> >+++ b/include/hw/nvram/fw_cfg.h
> >@@ -139,6 +139,15 @@ void fw_cfg_add_bytes_callback(FWCfgState *s,
> uint16_t key,
> > void *data, size_t len,
> > bool read_only);
> >
> >+/**
> >+ * fw_cfg_read_bytes_ptr:
> >+ * @s: fw_cfg device being modified
> >+ * @key: selector key value for new fw_cfg item
> >+ *
> >+ * Reads an existing fw_cfg data pointer.
> >+ */
> >+void *fw_cfg_read_bytes_ptr(FWCfgState *s, uint16_t key);
> >+
> > /**
> > * fw_cfg_add_string:
> > * @s: fw_cfg device being modified
> >diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
> >index 170a331e3f..29f30dd6d3 100644
> >--- a/hw/i386/microvm.c
> >+++ b/hw/i386/microvm.c
> >@@ -378,7 +378,8 @@ static void microvm_fix_kernel_cmdline(MachineState
> *machine)
> > MicrovmMachineState *mms = MICROVM_MACHINE(machine);
> > BusState *bus;
> > BusChild *kid;
> >- char *cmdline;
> >+ char *cmdline, *existing_cmdline;
> >+ size_t len;
> >
> > /*
> > * Find MMIO transports with attached devices, and add them to the
> kernel
> >@@ -387,7 +388,8 @@ static void microvm_fix_kernel_cmdline(MachineState
> *machine)
> > * Yes, this is a hack, but one that heavily improves the UX without
> > * introducing any significant issues.
> > */
> >- cmdline = g_strdup(machine->kernel_cmdline);
> >+ existing_cmdline = fw_cfg_read_bytes_ptr(x86ms->fw_cfg,
> FW_CFG_CMDLINE_DATA);
> >+ cmdline = g_strdup(existing_cmdline);
> > bus = sysbus_get_default();
> > QTAILQ_FOREACH(kid, &bus->children, sibling) {
> > DeviceState *dev = kid->child;
> >@@ -411,9 +413,12 @@ static void microvm_fix_kernel_cmdline(MachineState
> *machine)
> > }
> > }
> >
> >- fw_cfg_modify_i32(x86ms->fw_cfg, FW_CFG_CMDLINE_SIZE,
> strlen(cmdline) + 1);
> >- fw_cfg_modify_string(x86ms->fw_cfg, FW_CFG_CMDLINE_DATA, cmdline);
> >-
> >+ len = strlen(cmdline);
> >+ if (len > VIRTIO_CMDLINE_TOTAL_MAX_LEN + strlen(existing_cmdline)) {
> >+ fprintf(stderr, "qemu: virtio mmio cmdline too large,
> skipping\n");
> >+ } else {
> >+ memcpy(existing_cmdline, cmdline, len + 1);
> >+ }
> > g_free(cmdline);
> > }
> >
> >diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> >index 78cc131926..eaff4227bd 100644
> >--- a/hw/i386/x86.c
> >+++ b/hw/i386/x86.c
> >@@ -50,6 +50,7 @@
> > #include "hw/intc/i8259.h"
> > #include "hw/rtc/mc146818rtc.h"
> > #include "target/i386/sev.h"
> >+#include "hw/i386/microvm.h"
> >
> > #include "hw/acpi/cpu_hotplug.h"
> > #include "hw/irq.h"
> >@@ -813,12 +814,18 @@ void x86_load_linux(X86MachineState *x86ms,
> > const char *kernel_filename = machine->kernel_filename;
> > const char *initrd_filename = machine->initrd_filename;
> > const char *dtb_filename = machine->dtb;
> >- const char *kernel_cmdline = machine->kernel_cmdline;
> >+ char *kernel_cmdline;
> > SevKernelLoaderContext sev_load_ctx = {};
> > enum { RNG_SEED_LENGTH = 32 };
> >
> >- /* Align to 16 bytes as a paranoia measure */
> >- cmdline_size = (strlen(kernel_cmdline) + 16) & ~15;
> >+ /*
> >+ * Add the NUL terminator, some padding for the microvm cmdline
> fiddling
> >+ * hack, and then align to 16 bytes as a paranoia measure
> >+ */
> >+ cmdline_size = (strlen(machine->kernel_cmdline) + 1 +
> >+ VIRTIO_CMDLINE_TOTAL_MAX_LEN + 16) & ~15;
> >+ /* Make a copy, since we might append arbitrary bytes to it later. */
> >+ kernel_cmdline = g_strndup(machine->kernel_cmdline, cmdline_size);
> >
> > /* load the kernel header */
> > f = fopen(kernel_filename, "rb");
> >@@ -959,12 +966,6 @@ void x86_load_linux(X86MachineState *x86ms,
> > initrd_max = x86ms->below_4g_mem_size - acpi_data_size - 1;
> > }
> >
> >- fw_cfg_add_i32(fw_cfg, FW_CFG_CMDLINE_ADDR, cmdline_addr);
> >- fw_cfg_add_i32(fw_cfg, FW_CFG_CMDLINE_SIZE, strlen(kernel_cmdline) +
> 1);
> >- fw_cfg_add_string(fw_cfg, FW_CFG_CMDLINE_DATA, kernel_cmdline);
> >- sev_load_ctx.cmdline_data = (char *)kernel_cmdline;
> >- sev_load_ctx.cmdline_size = strlen(kernel_cmdline) + 1;
> >-
> > if (protocol >= 0x202) {
> > stl_p(header + 0x228, cmdline_addr);
> > } else {
> >@@ -1091,27 +1092,24 @@ void x86_load_linux(X86MachineState *x86ms,
> > exit(1);
> > }
> >
> >- setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
> >- kernel_size = setup_data_offset + sizeof(SetupData) + dtb_size;
> >- kernel = g_realloc(kernel, kernel_size);
> >-
> >-
> >- setup_data = (SetupData *)(kernel + setup_data_offset);
> >+ setup_data_offset = cmdline_size;
> >+ cmdline_size += sizeof(SetupData) + dtb_size;
> >+ kernel_cmdline = g_realloc(kernel_cmdline, cmdline_size);
> >+ setup_data = (void *)kernel_cmdline + setup_data_offset;
> > setup_data->next = cpu_to_le64(first_setup_data);
> >- first_setup_data = prot_addr + setup_data_offset;
> >+ first_setup_data = cmdline_addr + setup_data_offset;
> > setup_data->type = cpu_to_le32(SETUP_DTB);
> > setup_data->len = cpu_to_le32(dtb_size);
> >-
> > load_image_size(dtb_filename, setup_data->data, dtb_size);
> > }
> >
> >- if (!legacy_no_rng_seed) {
> >- setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
> >- kernel_size = setup_data_offset + sizeof(SetupData) +
> RNG_SEED_LENGTH;
> >- kernel = g_realloc(kernel, kernel_size);
> >- setup_data = (SetupData *)(kernel + setup_data_offset);
> >+ if (!legacy_no_rng_seed && protocol >= 0x209) {
> >+ setup_data_offset = cmdline_size;
> >+ cmdline_size += sizeof(SetupData) + RNG_SEED_LENGTH;
> >+ kernel_cmdline = g_realloc(kernel_cmdline, cmdline_size);
> >+ setup_data = (void *)kernel_cmdline + setup_data_offset;
> > setup_data->next = cpu_to_le64(first_setup_data);
> >- first_setup_data = prot_addr + setup_data_offset;
> >+ first_setup_data = cmdline_addr + setup_data_offset;
> > setup_data->type = cpu_to_le32(SETUP_RNG_SEED);
> > setup_data->len = cpu_to_le32(RNG_SEED_LENGTH);
> > qemu_guest_getrandom_nofail(setup_data->data, RNG_SEED_LENGTH);
> >@@ -1122,6 +1120,12 @@ void x86_load_linux(X86MachineState *x86ms,
> > fw_cfg_add_bytes(fw_cfg, FW_CFG_KERNEL_DATA, kernel,
> kernel_size);
> > }
> >
> >+ fw_cfg_add_i32(fw_cfg, FW_CFG_CMDLINE_ADDR, cmdline_addr);
> >+ fw_cfg_add_i32(fw_cfg, FW_CFG_CMDLINE_SIZE, cmdline_size);
> >+ fw_cfg_add_bytes(fw_cfg, FW_CFG_CMDLINE_DATA, kernel_cmdline,
> cmdline_size);
> >+ sev_load_ctx.cmdline_data = (char *)kernel_cmdline;
> >+ sev_load_ctx.cmdline_size = cmdline_size;
> >+
> > fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_ADDR, prot_addr);
> > fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_SIZE, kernel_size);
> > sev_load_ctx.kernel_data = (char *)kernel;
> >@@ -1134,7 +1138,7 @@ void x86_load_linux(X86MachineState *x86ms,
> > * kernel on the other side of the fw_cfg interface matches the hash
> of the
> > * file the user passed in.
> > */
> >- if (!sev_enabled()) {
> >+ if (!sev_enabled() && first_setup_data) {
> > SetupDataFixup *fixup = g_malloc(sizeof(*fixup));
> >
> > memcpy(setup, header, MIN(sizeof(header), setup_size));
> >diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> >index a00881bc64..432754eda4 100644
> >--- a/hw/nvram/fw_cfg.c
> >+++ b/hw/nvram/fw_cfg.c
> >@@ -741,6 +741,15 @@ void fw_cfg_add_bytes(FWCfgState *s, uint16_t key,
> void *data, size_t len)
> > fw_cfg_add_bytes_callback(s, key, NULL, NULL, NULL, data, len, true);
> > }
> >
> >+void *fw_cfg_read_bytes_ptr(FWCfgState *s, uint16_t key)
> >+{
> >+ int arch = !!(key & FW_CFG_ARCH_LOCAL);
> >+
> >+ key &= FW_CFG_ENTRY_MASK;
> >+ assert(key < fw_cfg_max_entry(s));
> >+ return s->entries[arch][key].data;
> >+}
> >+
> > void fw_cfg_add_string(FWCfgState *s, uint16_t key, const char *value)
> > {
> > size_t sz = strlen(value) + 1;
>
> Saying they are "appended to" is wrong; the loader is free to put them
> anywhere in usable RAM that is not covered by the kernel image, the kernel
> keepout area, the command line or initrd.
>
No. That sentence describes what QEMU does currently, without this patch,
and it is accurate. This is not a kernel commit. It's a QEMU one.
(Also, this patch sat on the list for over a month, during which time you
could have chimed in. The ship for nitpicking has sailed now.)
Sorry for the HTML crud; I'll be mobile for another 10 days, without a real
computer.
Jason
>
