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Message-ID: <c33c4087-eb55-de45-fdb4-b9e0a567cc37@zytor.com>
Date:   Mon, 5 Jun 2023 09:42:52 -0700
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Subject: Re: [PATCH v8 31/33] x86/fred: BUG() when ERETU with %rsp not equal
 to that when the ring 3 event was just delivered
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To:     Thomas Gleixner <tglx@linutronix.de>, Xin Li <xin3.li@intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        peterz@infradead.org, andrew.cooper3@citrix.com, seanjc@google.com,
        pbonzini@redhat.com, ravi.v.shankar@intel.com,
        jiangshanlai@gmail.com, shan.kang@intel.com
References: <20230410081438.1750-1-xin3.li@intel.com>
 <20230410081438.1750-32-xin3.li@intel.com> <87ttvm572h.ffs@tglx>
From:   "H. Peter Anvin" <hpa@zytor.com>
In-Reply-To: <87ttvm572h.ffs@tglx>
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On 6/5/23 07:15, Thomas Gleixner wrote:
> On Mon, Apr 10 2023 at 01:14, Xin Li wrote:
>> A FRED stack frame generated by a ring 3 event should never be messed up, and
>> the first thing we must make sure is that at the time an ERETU instruction is
>> executed, %rsp must have the same address as that when the user level event
>> was just delivered.
>>
>> However we don't want to bother the normal code path of ERETU because it's on
>> the hotest code path, a good choice is to do this check when ERETU
>> faults.
> 
> Which might be not catching bugs where the wrong frame makes ERETU not
> fault.
> 
> We have CONFIG_DEBUG_ENTRY for catching this at the proper place.
> 

This is true, but this BUG() is a cheap test on a slow path, and thus 
can be included in production code.

	-hpa