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Date: Tue, 11 Aug 2020 15:08:37 +0200
From: Pavel Machek <>
To: "Madhavan T. Venkataraman" <>
Cc: Mark Rutland <>,,,,,,,,,
Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor


> >> Thanks for the lively discussion. I have tried to answer some of the
> >> comments below.
> > 
> >>> There are options today, e.g.
> >>>
> >>> a) If the restriction is only per-alias, you can have distinct aliases
> >>>    where one is writable and another is executable, and you can make it
> >>>    hard to find the relationship between the two.
> >>>
> >>> b) If the restriction is only temporal, you can write instructions into
> >>>    an RW- buffer, transition the buffer to R--, verify the buffer
> >>>    contents, then transition it to --X.
> >>>
> >>> c) You can have two processes A and B where A generates instrucitons into
> >>>    a buffer that (only) B can execute (where B may be restricted from
> >>>    making syscalls like write, mprotect, etc).
> >>
> >> The general principle of the mitigation is W^X. I would argue that
> >> the above options are violations of the W^X principle. If they are
> >> allowed today, they must be fixed. And they will be. So, we cannot
> >> rely on them.
> > 
> > Would you mind describing your threat model?
> > 
> > Because I believe you are using model different from everyone else.
> > 
> > In particular, I don't believe b) is a problem or should be fixed.
> It is a problem because a kernel that implements W^X properly
> will not allow it. It has no idea what has been done in userland.
> It has no idea that the user has checked and verified the buffer
> contents after transitioning the page to R--.

No, it is not a problem. W^X is designed to protect from attackers
doing buffer overflows, not attackers doing arbitrary syscalls.

Best regards,
(cesky, pictures)

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