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Subject: Re: [PATCH] KVM: x86: Ensure liveliness of nested VM-Enter fail
 tracepoint message
To:     Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
References: <20210607175748.674002-1-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <1e8fda75-4c34-b13f-b193-4def6869b0f2@redhat.com>
Date:   Thu, 10 Jun 2021 14:12:09 +0200
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On 07/06/21 19:57, Sean Christopherson 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
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Fixes: 380e0055bc7e ("KVM: nVMX: trace nested VM-Enter failures detected by H/W")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/trace.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
> index a61c015870e3..4f839148948b 100644
> --- a/arch/x86/kvm/trace.h
> +++ b/arch/x86/kvm/trace.h
> @@ -1550,16 +1550,16 @@ TRACE_EVENT(kvm_nested_vmenter_failed,
>   	TP_ARGS(msg, err),
>   
>   	TP_STRUCT__entry(
> -		__field(const char *, msg)
> +		__string(msg, msg)
>   		__field(u32, err)
>   	),
>   
>   	TP_fast_assign(
> -		__entry->msg = msg;
> +		__assign_str(msg, msg);
>   		__entry->err = err;
>   	),
>   
> -	TP_printk("%s%s", __entry->msg, !__entry->err ? "" :
> +	TP_printk("%s%s", __get_str(msg), !__entry->err ? "" :
>   		__print_symbolic(__entry->err, VMX_VMENTER_INSTRUCTION_ERRORS))
>   );
>   
> 

Queued, thanks.

Paolo