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Date: Wed, 17 Aug 2022 09:45:45 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Jeff Layton <jlayton@kernel.org>, Ilya Dryomov <idryomov@gmail.com>,
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	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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	clang-built-linux <llvm@lists.linux.dev>
Subject: Re: Simplify load_unaligned_zeropad() (was Re: [GIT PULL] Ceph
 updates for 5.20-rc1)
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On Tue, Aug 16, 2022 at 10:57:45AM -0700, Linus Torvalds wrote:

> > > +     if (insn_decode(&insn, (void *) regs->ip, len, INSN_MODE_KERN))
> > > +             return false;
> >
> > We have insn_decode_kernel() for exactly this (very) common case.
> 
> I did that originally, and then I undid it in disgust, because that
> interface is too simple.
> 
> In particular, it just uses MAX_INSN_SIZE blindly. Which I didn't want
> to do when I actually had the instruction size.
> 
> Yes, yes, I also check the decode size after-the-fact, but I didn't
> want the decoder to even look at the invalid bytes.
> 
> This exception case is about the data being at the end of the page, I
> wanted the fixup to be aware of code being at the end of a page too.

I don't want to argue this point too much; but I will anyway :-)

IMO if the decoder ends up out of bounds its a decoder bug either way
around. That is, we *know* there is a full instruction at the given IP
because we got into this exception path.

( it would be possible to add further constraints on trapnr )

Irrespective of the length constraint given to the decoder, it should
not decode/access things past this instruction (without being careful
about it).

Anyway, I'm fine with the patch as you have it.