Re: [PULL 10/56] x86: don't let decompressed kernel image clobber setup_data

[H. Peter Anvin]

On January 30, 2023 12:19:14 PM PST, "Michael S. Tsirkin" <mst@redhat.com> 
wrote:
>From: "Jason A. Donenfeld" <Jason@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 <mst@redhat.com>
>Cc: x86@kernel.org
>Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
>Cc: H. Peter Anvin <hpa@zytor.com>
>Cc: Borislav Petkov <bp@alien8.de>
>Cc: Eric Biggers <ebiggers@kernel.org>
>Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
>Message-Id: <20221230220725.618763-1-Jason@zx2c4.com>
>Message-ID: <20230128061015-mutt-send-email-mst@kernel.org>
>Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
>Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>Tested-by: Eric Biggers <ebiggers@google.com>
>Tested-by: Mathias Krause <minipli@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.