[BUG] kernel NULL pointer dereference in kprobe_int3_handler

["Daniel Müller" <deso@posteo.net>]

Hi,

I've seen a NULL pointer dereference in kprobe_int3_handler, in code that seems
to have gotten added with 6256e668b7af9 ("x86/kprobes: Use int3 instead of debug
trap for single-step").
Specifically, our CI has reported the following (running test_progs-no_alu32):

  [ 1033.068258] test_progs-no_a[1177] is installing a program with bpf_probe_write_user helper that may corrupt user memory!
  [ 1040.264691] BUG: kernel NULL pointer dereference, address: 0000000000000058
  [ 1040.264856] #PF: supervisor read access in kernel mode
  [ 1040.264890] #PF: error_code(0x0000) - not-present page
  [ 1040.264961] PGD 0 P4D 0 
  [ 1040.265183] Oops: 0000 [#1] PREEMPT SMP NOPTI
  [ 1040.265183] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G        W  OE     5.19.0-rc7-g4129b786299d #1
  [ 1040.265183] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
  [ 1040.265183] RIP: 0010:kprobe_int3_handler+0xd4/0x1a0
  [ 1040.265183] Code: 49 8b 06 48 83 e8 02 48 a9 fd ff ff ff 75 d0 48 c7 c7 32 cc 2b 82 e8 eb d5 9a 00 48 8b 95 80 00 00 00 65 48 8b 3d 74 62 fc 7e <48> 8b 47 58 48 39 d0 73 ac 48 8d 48 0f 48 39 ca 73 a3 48 8b 4f 28
  [ 1040.265183] RSP: 0018:ffffb4140009bd40 EFLAGS: 00000092
  [ 1040.265183] RAX: 0000000000000001 RBX: ffffffff81a04cb9 RCX: 0000000000000000
  [ 1040.265183] RDX: ffffffff81a04cb9 RSI: ffffffff822bcc32 RDI: 0000000000000000
  [ 1040.265183] RBP: ffffb4140009bd98 R08: 000000000003929b R09: 0000000000000000
  [ 1040.265183] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
  [ 1040.265183] R13: ffffffff81a04cb8 R14: ffff9490b9c5b1e0 R15: 0000000000000000
  [ 1040.265183] FS:  0000000000000000(0000) GS:ffff9490b9c40000(0000) knlGS:0000000000000000
  [ 1040.265183] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [ 1040.265183] CR2: 0000000000000058 CR3: 0000000028836000 CR4: 00000000000006e0
  [ 1040.265183] Call Trace:
  [ 1040.265183]  <TASK>
  [ 1040.265183]  do_int3+0xf/0x50
  [ 1040.265183]  exc_int3+0x87/0xd0
  [ 1040.265183]  asm_exc_int3+0x35/0x40
  [ 1040.265183] RIP: 0010:__schedule+0x3f9/0xbf0
  [ 1040.265183] Code: 83 5a fe ff ff 65 ff 05 e5 61 61 7e 48 8b 05 3e cb 68 01 48 85 c0 74 16 48 8b 78 08 4c 89 f1 4c 89 ea 44 8b 45 ac 8b 75 b8 e8 <53> 6c 79 ff 65 ff 0d bc 61 61 7e 0f 85 0d fe ff ff e8 a0 cf 5f ff
  [ 1040.265183] RSP: 0018:ffffb4140009be70 EFLAGS: 00000086
  [ 1040.265183] RAX: ffff9490056e0b90 RBX: ffff9490002f39e8 RCX: ffff949008758000
  [ 1040.265183] RDX: ffff9490002f3300 RSI: 0000000000000000 RDI: 0000000000000000
  [ 1040.265183] RBP: ffffb4140009bec8 R08: 0000000000000000 R09: 1dc944f200000000
  [ 1040.265183] R10: 0000000000000001 R11: 0000000000080000 R12: ffff9490b9c6c8c0
  [ 1040.265183] R13: ffff9490002f3300 R14: ffff949008758000 R15: ffff9490b9c6c8d8
  [ 1040.265183]  ? __schedule+0x3f9/0xbf0
  [ 1040.265183]  schedule_idle+0x26/0x40
  [ 1040.265183]  do_idle+0x177/0x250
  [ 1040.265183]  cpu_startup_entry+0x19/0x20
  [ 1040.265183]  start_secondary+0xed/0xf0
  [ 1040.265183]  secondary_startup_64_no_verify+0xe0/0xeb
  [ 1040.265183]  </TASK>
  [ 1040.265183] Modules linked in: bpf_testmod(OE) [last unloaded: bpf_testmod]
  [ 1040.265183] CR2: 0000000000000058
  [ 1040.265183] ---[ end trace 0000000000000000 ]---
  [ 1040.265183] RIP: 0010:kprobe_int3_handler+0xd4/0x1a0
  [ 1040.265183] Code: 49 8b 06 48 83 e8 02 48 a9 fd ff ff ff 75 d0 48 c7 c7 32 cc 2b 82 e8 eb d5 9a 00 48 8b 95 80 00 00 00 65 48 8b 3d 74 62 fc 7e <48> 8b 47 58 48 39 d0 73 ac 48 8d 48 0f 48 39 ca 73 a3 48 8b 4f 28
  [ 1040.265183] RSP: 0018:ffffb4140009bd40 EFLAGS: 00000092
  [ 1040.265183] RAX: 0000000000000001 RBX: ffffffff81a04cb9 RCX: 0000000000000000
  [ 1040.265183] RDX: ffffffff81a04cb9 RSI: ffffffff822bcc32 RDI: 0000000000000000
  [ 1040.265183] RBP: ffffb4140009bd98 R08: 000000000003929b R09: 0000000000000000
  [ 1040.265183] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
  [ 1040.265183] R13: ffffffff81a04cb8 R14: ffff9490b9c5b1e0 R15: 0000000000000000
  [ 1040.265183] FS:  0000000000000000(0000) GS:ffff9490b9c40000(0000) knlGS:0000000000000000
  [ 1040.265183] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [ 1040.265183] CR2: 0000000000000058 CR3: 0000000028836000 CR4: 00000000000006e0
  [ 1040.265183] Kernel panic - not syncing: Fatal exception in interrupt
  [ 1040.265183] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)

(it was sync'ed to somewhere around 40b09653b1977 ("selftests/bpf: Adjust
vmtest.sh to use local kernel configuration"); I can probably piece together the
exact kernel configuration if needed, but the inquiry is of a more general
nature)

I am wondering what is the reason for us not checking whether kprobe_running
returned a non-NULL pointer here (as we do elsewhere):
https://elixir.bootlin.com/linux/v5.18.13/source/arch/x86/kernel/kprobes/core.c#L986
? Is that an oversight or should some kind of invariant be upheld at this point?

kprobe_int3_handler+0xd4/0x1a0 maps to line 987 in the above file. Address
0000000000000058 is exactly the offset that p->ainsn.insn is at, so it seems as
if p is NULL.

Thanks,
Daniel