linux-kernel - Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED

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Date: Wed, 17 Apr 2024 22:28:42 +0200
From: Borislav Petkov <bp@...en8.de>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: "Xin Li (Intel)" <xin@...or.com>, linux-kernel@...r.kernel.org,
	luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	dave.hansen@...ux.intel.com, x86@...nel.org
Subject: Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED

On Wed, Apr 17, 2024 at 07:59:25AM -0700, H. Peter Anvin wrote:
> FRED has separate entry flows depending on if the event came from user space
> or kernel space:
> 
> asm_fred_entrypoint_user -> fred_entry_from_user
> 
> asm_fred_entrypoint_kernel -> fred_entry_from_kernel
> 
> fred_entry_from_kernel does not invoke fred_intx() if the event type is
> EVENT_TYPE_SWINT, instead it falls through to fred_bad_type(). Perhaps
> fred_intx() should be renamed fred_intx_user() for additional clarity.

Sure.

> (It might also we worth noting in that function that the reason int $0x03
> and int $0x04 are dispatched as INT3 and INTO is to be fully user space
> compatible with legacy IDT, which behaves similarly.)

Also yes.

> Thus, the int $0x80 code is simply not reachable from kernel space; if
> kernel code were to invoke int $0x80 or any other INT instruction it will
> error out before getting to this code.

Yah, that explanation is a lot better, thanks. I'll use it. :)

> I suggested putting it into a comment for future reference. Obviously
> no need to duplicate it in the commit message :)

Yap, that makes more sense.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette