Re: [PATCH] KVM: x86: Ensure liveliness of nested VM-Enter fail tracepoint message

[Wanpeng Li]

On Tue, 8 Jun 2021 at 02:00, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Use the __string() machinery provided by the tracing subystem to make a
> copy of the string literals consumed by the "nested VM-Enter failed"
> tracepoint.  A complete copy is necessary to ensure that the tracepoint
> can't outlive the data/memory it consumes and deference stale memory.
>
> Because the tracepoint itself is defined by kvm, if kvm-intel and/or
> kvm-amd are built as modules, the memory holding the string literals
> defined by the vendor modules will be freed when the module is unloaded,
> whereas the tracepoint and its data in the ring buffer will live until
> kvm is unloaded (or "indefinitely" if kvm is built-in).
>
> This bug has existed since the tracepoint was added, but was recently
> exposed by a new check in tracing to detect exactly this type of bug.
>
>   fmt: '%s%s
>   ' current_buffer: ' vmx_dirty_log_t-140127  [003] ....  kvm_nested_vmenter_failed: '
>   WARNING: CPU: 3 PID: 140134 at kernel/trace/trace.c:3759 trace_check_vprintf+0x3be/0x3e0
>   CPU: 3 PID: 140134 Comm: less Not tainted 5.13.0-rc1-ce2e73ce600a-req #184
>   Hardware name: ASUS Q87M-E/Q87M-E, BIOS 1102 03/03/2014
>   RIP: 0010:trace_check_vprintf+0x3be/0x3e0
>   Code: <0f> 0b 44 8b 4c 24 1c e9 a9 fe ff ff c6 44 02 ff 00 49 8b 97 b0 20
>   RSP: 0018:ffffa895cc37bcb0 EFLAGS: 00010282
>   RAX: 0000000000000000 RBX: ffffa895cc37bd08 RCX: 0000000000000027
>   RDX: 0000000000000027 RSI: 00000000ffffdfff RDI: ffff9766cfad74f8
>   RBP: ffffffffc0a041d4 R08: ffff9766cfad74f0 R09: ffffa895cc37bad8
>   R10: 0000000000000001 R11: 0000000000000001 R12: ffffffffc0a041d4
>   R13: ffffffffc0f4dba8 R14: 0000000000000000 R15: ffff976409f2c000
>   FS:  00007f92fa200740(0000) GS:ffff9766cfac0000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000559bd11b0000 CR3: 000000019fbaa002 CR4: 00000000001726e0
>   Call Trace:
>    trace_event_printf+0x5e/0x80
>    trace_raw_output_kvm_nested_vmenter_failed+0x3a/0x60 [kvm]
>    print_trace_line+0x1dd/0x4e0
>    s_show+0x45/0x150
>    seq_read_iter+0x2d5/0x4c0
>    seq_read+0x106/0x150
>    vfs_read+0x98/0x180
>    ksys_read+0x5f/0xe0
>    do_syscall_64+0x40/0xb0
>    entry_SYSCALL_64_after_hwframe+0x44/0xae

I can observe tons of other kvm tracepoints warning like this after
commit 9a6944fee68e25 (tracing: Add a verifier to check string
pointers for trace events), just echo 1 >
/sys/kernel/tracing/events/kvm/enable and boot a linux guest.

    Wanpeng