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.