Hi,
On 2022/7/28 5:01, Daniel Müller wrote:
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?
I think it's kprobe_is_ss(kcb) promises this.
KPROBE_HIT_SS and KPROBE_REENTER are both only set in setup_singlestep,
which set_current_kprobe must be run before that.
And p for set_current_kprobe have been checked not NULL in
kprobe_int3_handle.
I didn't find any path that current_kprobe can be set NULL here by
viewing code. Is this bug reproducible? It is not a multi threads
problem I guess it can be reproduced easily.
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
Best,
Chen
