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Message-ID: <c13c626d-344f-4d4b-a495-5006613f9bad@efficios.com>
Date: Mon, 24 Feb 2025 14:04:29 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: 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 v4 1/4] pkeys: add API to switch to permissive/zero pkey
register
On 2025-02-24 08:20, Dmitry Vyukov wrote:
> The API allows to switch to permissive pkey register that allows accesses
> to all PKEYs, and to a value that allows acccess to the 0 (default) PKEY.
> This functionality is already used in x86 signal delivery, and will be
> needed for rseq.
AFAIU the signal delivery uses the "allow access to all PKEYs"
approach only, not the "allow access to 0-pkey". It would be
good to clarify this in the commit message.
>
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
> Cc: x86@...nel.org
> Cc: linux-kernel@...r.kernel.org
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Fixes: d7822b1e24f2 ("rseq: Introduce restartable sequences system call")
>
> ---
> Changes in v4:
> - Added Fixes tag
>
> Changes in v3:
> - Renamed API functions to write_permissive_pkey_val/write_pkey_val
> - Added enable_zero_pkey_val for rseq
> - Added Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>
> Changes in v2:
> - Fixed typo in commit description
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/include/asm/pkeys.h | 33 +++++++++++++++++++++++++++++++++
> arch/x86/include/asm/pkru.h | 10 +++++++---
> include/linux/pkeys.h | 31 +++++++++++++++++++++++++++++++
> mm/Kconfig | 2 ++
> 5 files changed, 74 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index be2c311f5118d..43af2840d098f 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1881,6 +1881,7 @@ config X86_INTEL_MEMORY_PROTECTION_KEYS
> depends on X86_64 && (CPU_SUP_INTEL || CPU_SUP_AMD)
> select ARCH_USES_HIGH_VMA_FLAGS
> select ARCH_HAS_PKEYS
> + select ARCH_HAS_PERMISSIVE_PKEY
> help
> Memory Protection Keys provides a mechanism for enforcing
> page-based protections, but without requiring modification of the
> diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
> index 2e6c04d8a45b4..d6e35ab5c3d22 100644
> --- a/arch/x86/include/asm/pkeys.h
> +++ b/arch/x86/include/asm/pkeys.h
> @@ -2,6 +2,8 @@
> #ifndef _ASM_X86_PKEYS_H
> #define _ASM_X86_PKEYS_H
>
> +#include "pkru.h"
> +
> /*
> * If more than 16 keys are ever supported, a thorough audit
> * will be necessary to ensure that the types that store key
> @@ -123,4 +125,35 @@ static inline int vma_pkey(struct vm_area_struct *vma)
> return (vma->vm_flags & vma_pkey_mask) >> VM_PKEY_SHIFT;
> }
>
> +typedef u32 pkey_reg_t;
> +
> +static inline pkey_reg_t write_permissive_pkey_val(void)
> +{
> + return write_pkru(0);
> +}
> +
> +static inline pkey_reg_t enable_zero_pkey_val(void)
> +{
> + u32 pkru;
> +
> + if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
> + return 0;
> + /*
> + * WRPKRU is relatively expensive compared to RDPKRU,
> + * avoid it if possible.
> + */
> + pkru = rdpkru();
> + if ((pkru & (PKRU_AD_BIT|PKRU_WD_BIT)) != 0)
> + wrpkru(pkru & ~(PKRU_AD_BIT|PKRU_WD_BIT));
> + return pkru;
> +
> +
> + return write_pkru(0);
This is dead code. What I am missing ?
With those fixed, please keep my reviewed-by :)
Thanks,
Mathieu
> +}
> +
> +static inline void write_pkey_val(pkey_reg_t val)
> +{
> + write_pkru(val);
> +}
> +
> #endif /*_ASM_X86_PKEYS_H */
> diff --git a/arch/x86/include/asm/pkru.h b/arch/x86/include/asm/pkru.h
> index 74f0a2d34ffdd..b9bf9b7f2753b 100644
> --- a/arch/x86/include/asm/pkru.h
> +++ b/arch/x86/include/asm/pkru.h
> @@ -39,16 +39,20 @@ static inline u32 read_pkru(void)
> return 0;
> }
>
> -static inline void write_pkru(u32 pkru)
> +static inline u32 write_pkru(u32 pkru)
> {
> + u32 old_pkru;
> +
> if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
> - return;
> + return 0;
> /*
> * WRPKRU is relatively expensive compared to RDPKRU.
> * Avoid WRPKRU when it would not change the value.
> */
> - if (pkru != rdpkru())
> + old_pkru = rdpkru();
> + if (pkru != old_pkru)
> wrpkru(pkru);
> + return old_pkru;
> }
>
> static inline void pkru_write_default(void)
> diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
> index 86be8bf27b41b..262d60f6a15f8 100644
> --- a/include/linux/pkeys.h
> +++ b/include/linux/pkeys.h
> @@ -48,4 +48,35 @@ static inline bool arch_pkeys_enabled(void)
>
> #endif /* ! CONFIG_ARCH_HAS_PKEYS */
>
> +#ifndef CONFIG_ARCH_HAS_PERMISSIVE_PKEY
> +
> +/*
> + * Common name for value of the register that controls access to PKEYs
> + * (called differently on different arches: PKRU, POR, AMR).
> + */
> +typedef char pkey_reg_t;
> +
> +/*
> + * Sets PKEY access register to the most permissive value that allows
> + * accesses to all PKEYs. Returns the current value of PKEY register.
> + * Code should generally arrange switching back to the old value
> + * using write_pkey_val(old_value).
> + */
> +static inline pkey_reg_t write_permissive_pkey_val(void)
> +{
> + return 0;
> +}
> +
> +/*
> + * Sets PKEY access register to a value that allows access to the 0 (default)
> + * PKEY. Returns the current value of PKEY register.
> + */
> +static inline pkey_reg_t enable_zero_pkey_val(void)
> +{
> + return 0;
> +}
> +
> +static inline void write_pkey_val(pkey_reg_t val) {}
> +#endif /* ! CONFIG_ARCH_HAS_PERMISSIVE_PKEY */
> +
> #endif /* _LINUX_PKEYS_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 1b501db064172..9e874f7713a2b 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1147,6 +1147,8 @@ config ARCH_USES_HIGH_VMA_FLAGS
> bool
> config ARCH_HAS_PKEYS
> bool
> +config ARCH_HAS_PERMISSIVE_PKEY
> + bool
>
> config ARCH_USES_PG_ARCH_2
> bool
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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