hw/i386/fw_cfg: Add etc/e820 to fw_cfg late

In e820_add_entry() the e820_table is reallocated with g_renew() to make
space for a new entry. However, fw_cfg_arch_create() just uses the
existing e820_table pointer. This leads to a use-after-free if anything
adds a new entry after fw_cfg is set up.

Shift the addition of the etc/e820 file to the machine done notifier, via
a new fw_cfg_add_e820() function.

Also make e820_table private and use an e820_get_table() accessor function
for it, which sets a flag that will trigger an assert() for any *later*
attempts to add to the table.

Make e820_add_entry() return void, as most callers don't check for error
anyway.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <a2708734f004b224f33d3b4824e9a5a262431568.camel@infradead.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/i386/e820_memory_layout.c

@@ -11,22 +11,29 @@
 #include "e820_memory_layout.h"
 
 static size_t e820_entries;
-struct e820_entry *e820_table;
+static struct e820_entry *e820_table;
+static gboolean e820_done;
 
-int e820_add_entry(uint64_t address, uint64_t length, uint32_t type)
+void e820_add_entry(uint64_t address, uint64_t length, uint32_t type)
 {
+    assert(!e820_done);
+
     /* new "etc/e820" file -- include ram and reserved entries */
     e820_table = g_renew(struct e820_entry, e820_table, e820_entries + 1);
     e820_table[e820_entries].address = cpu_to_le64(address);
     e820_table[e820_entries].length = cpu_to_le64(length);
     e820_table[e820_entries].type = cpu_to_le32(type);
     e820_entries++;
-
-    return e820_entries;
 }
 
-int e820_get_num_entries(void)
+int e820_get_table(struct e820_entry **table)
 {
+    e820_done = true;
+
+    if (table) {
+        *table = e820_table;
+    }
+
     return e820_entries;
 }
 

hw/i386/e820_memory_layout.h

@@ -22,13 +22,9 @@ struct e820_entry {
     uint32_t type;
 } QEMU_PACKED __attribute((__aligned__(4)));
 
-extern struct e820_entry *e820_table;
-
-int e820_add_entry(uint64_t address, uint64_t length, uint32_t type);
-int e820_get_num_entries(void);
+void e820_add_entry(uint64_t address, uint64_t length, uint32_t type);
 bool e820_get_entry(int index, uint32_t type,
                     uint64_t *address, uint64_t *length);
-
-
+int e820_get_table(struct e820_entry **table);
 
 #endif

hw/i386/fw_cfg.c

@@ -48,6 +48,15 @@ const char *fw_cfg_arch_key_name(uint16_t key)
     return NULL;
 }
 
+/* Add etc/e820 late, once all regions should be present */
+void fw_cfg_add_e820(FWCfgState *fw_cfg)
+{
+    struct e820_entry *table;
+    int nr_e820 = e820_get_table(&table);
+
+    fw_cfg_add_file(fw_cfg, "etc/e820", table, nr_e820 * sizeof(*table));
+}
+
 void fw_cfg_build_smbios(PCMachineState *pcms, FWCfgState *fw_cfg,
                          SmbiosEntryPointType ep_type)
 {
@@ -60,6 +69,7 @@ void fw_cfg_build_smbios(PCMachineState *pcms, FWCfgState *fw_cfg,
     PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
     MachineClass *mc = MACHINE_GET_CLASS(pcms);
     X86CPU *cpu = X86_CPU(ms->possible_cpus->cpus[0].cpu);
+    int nr_e820;
 
     if (pcmc->smbios_defaults) {
         /* These values are guest ABI, do not change */
@@ -78,8 +88,9 @@ void fw_cfg_build_smbios(PCMachineState *pcms, FWCfgState *fw_cfg,
     }
 
     /* build the array of physical mem area from e820 table */
-    mem_array = g_malloc0(sizeof(*mem_array) * e820_get_num_entries());
-    for (i = 0, array_count = 0; i < e820_get_num_entries(); i++) {
+    nr_e820 = e820_get_table(NULL);
+    mem_array = g_malloc0(sizeof(*mem_array) * nr_e820);
+    for (i = 0, array_count = 0; i < nr_e820; i++) {
         uint64_t addr, len;
 
         if (e820_get_entry(i, E820_RAM, &addr, &len)) {
@@ -138,9 +149,6 @@ FWCfgState *fw_cfg_arch_create(MachineState *ms,
 #endif
     fw_cfg_add_i32(fw_cfg, FW_CFG_IRQ0_OVERRIDE, 1);
 
-    fw_cfg_add_file(fw_cfg, "etc/e820", e820_table,
-                    sizeof(struct e820_entry) * e820_get_num_entries());
-
     fw_cfg_add_bytes(fw_cfg, FW_CFG_HPET, &hpet_cfg, sizeof(hpet_cfg));
     /* allocate memory for the NUMA channel: one (64bit) word for the number
      * of nodes, one word for each VCPU->node and one word for each node to

hw/i386/fw_cfg.h

@@ -27,5 +27,6 @@ void fw_cfg_build_smbios(PCMachineState *pcms, FWCfgState *fw_cfg,
                          SmbiosEntryPointType ep_type);
 void fw_cfg_build_feature_control(MachineState *ms, FWCfgState *fw_cfg);
 void fw_cfg_add_acpi_dsdt(Aml *scope, FWCfgState *fw_cfg);
+void fw_cfg_add_e820(FWCfgState *fw_cfg);
 
 #endif

hw/i386/microvm.c

@@ -324,8 +324,6 @@ static void microvm_memory_init(MicrovmMachineState *mms)
     fw_cfg_add_i16(fw_cfg, FW_CFG_MAX_CPUS, machine->smp.max_cpus);
     fw_cfg_add_i64(fw_cfg, FW_CFG_RAM_SIZE, (uint64_t)machine->ram_size);
     fw_cfg_add_i32(fw_cfg, FW_CFG_IRQ0_OVERRIDE, 1);
-    fw_cfg_add_file(fw_cfg, "etc/e820", e820_table,
-                    sizeof(struct e820_entry) * e820_get_num_entries());
 
     rom_set_fw(fw_cfg);
 
@@ -586,9 +584,11 @@ static void microvm_machine_done(Notifier *notifier, void *data)
 {
     MicrovmMachineState *mms = container_of(notifier, MicrovmMachineState,
                                             machine_done);
+    X86MachineState *x86ms = X86_MACHINE(mms);
 
     acpi_setup_microvm(mms);
     dt_setup_microvm(mms);
+    fw_cfg_add_e820(x86ms->fw_cfg);
 }
 
 static void microvm_powerdown_req(Notifier *notifier, void *data)

hw/i386/pc.c

@@ -625,6 +625,7 @@ void pc_machine_done(Notifier *notifier, void *data)
     acpi_setup();
     if (x86ms->fw_cfg) {
         fw_cfg_build_smbios(pcms, x86ms->fw_cfg, pcms->smbios_entry_point_type);
+        fw_cfg_add_e820(x86ms->fw_cfg);
         fw_cfg_build_feature_control(MACHINE(pcms), x86ms->fw_cfg);
         /* update FW_CFG_NB_CPUS to account for -device added CPUs */
         fw_cfg_modify_i16(x86ms->fw_cfg, FW_CFG_NB_CPUS, x86ms->boot_cpus);

target/i386/kvm/kvm.c

@@ -2706,11 +2706,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
     }
 
     /* Tell fw_cfg to notify the BIOS to reserve the range. */
-    ret = e820_add_entry(identity_base, 0x4000, E820_RESERVED);
-    if (ret < 0) {
-        fprintf(stderr, "e820_add_entry() table is full\n");
-        return ret;
-    }
+    e820_add_entry(identity_base, 0x4000, E820_RESERVED);
 
     shadow_mem = object_property_get_int(OBJECT(s), "kvm-shadow-mem", &error_abort);
     if (shadow_mem != -1) {

target/i386/kvm/xen-emu.c

@@ -176,12 +176,7 @@ int kvm_xen_init(KVMState *s, uint32_t hypercall_msr)
     s->xen_caps = xen_caps;
 
     /* Tell fw_cfg to notify the BIOS to reserve the range. */
-    ret = e820_add_entry(XEN_SPECIAL_AREA_ADDR, XEN_SPECIAL_AREA_SIZE,
-                         E820_RESERVED);
-    if (ret < 0) {
-        fprintf(stderr, "e820_add_entry() table is full\n");
-        return ret;
-    }
+    e820_add_entry(XEN_SPECIAL_AREA_ADDR, XEN_SPECIAL_AREA_SIZE, E820_RESERVED);
 
     /* The pages couldn't be overlaid until KVM was initialized */
     xen_primary_console_reset();