[BUG REPORT] USE_AFTER_FREE in complete_emulated_mmio found by KASAN/Syzkaller fuzz test (v5.10.0)

[Zhangjiaji]

Syzkaller hit 'KASAN: use-after-free Read in complete_emulated_mmio' bug.

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BUG: KASAN: use-after-free in complete_emulated_mmio+0x305/0x420
Read of size 1 at addr ffff888009c378d1 by task syz-executor417/984

CPU: 1 PID: 984 Comm: syz-executor417 Not tainted 5.10.0-182.0.0.95.h2627.eulerosv2r13.x86_64 #3 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014 Call Trace:
dump_stack+0xbe/0xfd
print_address_description.constprop.0+0x19/0x170
__kasan_report.cold+0x6c/0x84
kasan_report+0x3a/0x50
check_memory_region+0xfd/0x1f0
memcpy+0x20/0x60
complete_emulated_mmio+0x305/0x420
kvm_arch_vcpu_ioctl_run+0x63f/0x6d0
kvm_vcpu_ioctl+0x413/0xb20
__se_sys_ioctl+0x111/0x160
do_syscall_64+0x30/0x40
entry_SYSCALL_64_after_hwframe+0x67/0xd1
RIP: 0033:0x42477d
Code: 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007faa8e6890e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004d7338 RCX: 000000000042477d
RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
RBP: 00000000004d7330 R08: 00007fff28d546df R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004d733c
R13: 0000000000000000 R14: 000000000040a200 R15: 00007fff28d54720

The buggy address belongs to the page:
page:0000000029f6a428 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x9c37
flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff)
raw: 000fffffc0000000 0000000000000000 ffffea0000270dc8 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000 page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888009c37780: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888009c37800: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888009c37880: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                                 ^
ffff888009c37900: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888009c37980: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ==================================================================


Syzkaller reproducer:
# {Threaded:true Repeat:true RepeatTimes:0 Procs:1 Slowdown:1 Sandbox: SandboxArg:0 Leak:false NetInjection:false NetDevices:false NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false IEEE802154:false Sysctl:false Swap:false UseTmpDir:false HandleSegv:true Repro:false Trace:false LegacyOptions:{Collide:false Fault:false FaultCall:0 FaultNth:0}}
r0 = openat$kvm(0xffffffffffffff9c, &(0x7f00000001c0), 0x0, 0x0)
r1 = ioctl$KVM_CREATE_VM(r0, 0xae01, 0x0)
r2 = ioctl$KVM_CREATE_VCPU(r1, 0xae41, 0x0) syz_kvm_setup_cpu$x86(r1, r2, &(0x7f0000fe2000/0x18000)=nil, &(0x7f0000000080)=[@text32={0x20, &(0x7f0000000000)="44c8df2020c020c003000000000f22c0671e26653e360f2224660f65b600000000b9e0450200f5e8f5e8f30f1ed6c744240000100000c744240200000000c7442406000000000f011424eacf5700000301b8010000000f01c1", 0x59}], 0x1, 0x27, 0x0, 0x1) ioctl$KVM_RUN(r2, 0xae80, 0x0) ioctl$KVM_SMI(0xffffffffffffffff, 0xaeb7) (async) ioctl$KVM_RUN(r2, 0xae80, 0x0)


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Hi,

I've analyzed the Syzkaller output and the complete_emulated_mmio() code path.
The buggy address is created in em_enter(), where it passes its local variable `ulong rbp` to emulate_push(), finally ends in emulator_read_write_onepage() putting the address into vcpu->mmio_fragments[].data .
The bug happens when kvm guest executes an "enter" instruction, and top of the stack crosses the mem page. 
In that case, the em_enter() function cannot complete the instruction within itself, but leave the rest data (which is in the other page) to complete_emulated_mmio().
When complete_emulated_mmio() starts, em_enter() has exited, so local variable `ulong rbp` is also released.
Now complete_emulated_mmio() trys to access vcpu->mmio_fragments[].data , and the bug happened.

any idea?

--
Best regards,
Yashu Zhang