linux-kernel - Re: [PATCH 3/4] rseq: Make rseq work with protection keys

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Message-ID: <90a36a64-8ea5-4ea1-965f-bcec604c7d5b@intel.com>
Date: Fri, 21 Feb 2025 11:48:49 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Dmitry Vyukov <dvyukov@...gle.com>, peterz@...radead.org,
 boqun.feng@...il.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, aruna.ramakrishna@...cle.com,
 elver@...gle.com
Cc: "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] rseq: Make rseq work with protection keys

On 2/21/25 11:38, Mathieu Desnoyers wrote:
> I agree that switching to permissive key in the fast path would be
> simpler. AFAIU, the switch_to_permissive_pkey_reg() is only a pkey
> read when the key is already permissive.

Unfortunately, on x86, PKRU is almost never in its permissive state. We
chose a policy (stored in the global init_pkru_value variable) that
allows R/W access to pkey 0, but disables access to everything else.
It's 0xfffffff5, IIRC.

This ensures deny-by-default behavior and ensures that threads cloned
off long ago don't have a dangerous PKRU value for newly-allocated and
pkey-protected memory.

If I had a time machine, it'd be interesting to go back and try to make
PKRU's default value be all 0's and also represent the logically most
restrictive value.