linux-kernel - Re: [PATCH v9 20/39] arm64/gcs: Ensure that new threads have a GCS

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Message-ID: <87bk2wu627.fsf@linaro.org>
Date: Tue, 16 Jul 2024 23:05:04 -0300
From: Thiago Jung Bauermann <thiago.bauermann@...aro.org>
To: Mark Brown <broonie@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,  Will Deacon
 <will@...nel.org>,  Jonathan Corbet <corbet@....net>,  Andrew Morton
 <akpm@...ux-foundation.org>,  Marc Zyngier <maz@...nel.org>,  Oliver Upton
 <oliver.upton@...ux.dev>,  James Morse <james.morse@....com>,  Suzuki K
 Poulose <suzuki.poulose@....com>,  Arnd Bergmann <arnd@...db.de>,  Oleg
 Nesterov <oleg@...hat.com>,  Eric Biederman <ebiederm@...ssion.com>,
  Shuah Khan <shuah@...nel.org>,  "Rick P. Edgecombe"
 <rick.p.edgecombe@...el.com>,  Deepak Gupta <debug@...osinc.com>,  Ard
 Biesheuvel <ardb@...nel.org>,  Szabolcs Nagy <Szabolcs.Nagy@....com>,
  Kees Cook <kees@...nel.org>,  "H.J. Lu" <hjl.tools@...il.com>,  Paul
 Walmsley <paul.walmsley@...ive.com>,  Palmer Dabbelt <palmer@...belt.com>,
  Albert Ou <aou@...s.berkeley.edu>,  Florian Weimer <fweimer@...hat.com>,
  Christian Brauner <brauner@...nel.org>,  Ross Burton
 <ross.burton@....com>,  linux-arm-kernel@...ts.infradead.org,
  linux-doc@...r.kernel.org,  kvmarm@...ts.linux.dev,
  linux-fsdevel@...r.kernel.org,  linux-arch@...r.kernel.org,
  linux-mm@...ck.org,  linux-kselftest@...r.kernel.org,
  linux-kernel@...r.kernel.org,  linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v9 20/39] arm64/gcs: Ensure that new threads have a GCS

Mark Brown <broonie@...nel.org> writes:

> diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
> index b0a67efc522b..4a3ce8e3bdfb 100644
> --- a/arch/arm64/mm/gcs.c
> +++ b/arch/arm64/mm/gcs.c
> @@ -8,6 +8,139 @@
>  #include <asm/cpufeature.h>
>  #include <asm/page.h>
>  
> +static unsigned long alloc_gcs(unsigned long addr, unsigned long size,
> +			       unsigned long token_offset, bool set_res_tok)

The token_offset and set_res_tok arguments aren't used in this function,
so they can be removed.

> +{
> +	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
> +	struct mm_struct *mm = current->mm;
> +	unsigned long mapped_addr, unused;
> +
> +	if (addr)
> +		flags |= MAP_FIXED_NOREPLACE;
> +
> +	mmap_write_lock(mm);
> +	mapped_addr = do_mmap(NULL, addr, size, PROT_READ | PROT_WRITE, flags,
> +			      VM_SHADOW_STACK, 0, &unused, NULL);
> +	mmap_write_unlock(mm);
> +
> +	return mapped_addr;
> +}
> +
> +static unsigned long gcs_size(unsigned long size)
> +{
> +	if (size)
> +		return PAGE_ALIGN(size);
> +
> +	/* Allocate RLIMIT_STACK/2 with limits of PAGE_SIZE..2G */
> +	size = PAGE_ALIGN(min_t(unsigned long long,
> +				rlimit(RLIMIT_STACK) / 2, SZ_2G));
> +	return max(PAGE_SIZE, size);
> +}
> +
> +static bool gcs_consume_token(struct mm_struct *mm, unsigned long user_addr)
> +{
> +	u64 expected = GCS_CAP(user_addr);
> +	u64 val;
> +	int ret;
> +
> +	/* This should really be an atomic cpmxchg.  It is not. */

s/cpmxchg/cmpxchg/

The same typo is also in arch/x86/kernel/shstk.c, from the "fork:
Support shadow stacks in clone3()" patch series.

> +	ret = access_remote_vm(mm, user_addr, &val, sizeof(val),
> +			       FOLL_FORCE);
> +	if (ret != sizeof(val))
> +		return false;
> +
> +	if (val != expected)
> +		return false;
> +
> +	val = 0;
> +	ret = access_remote_vm(mm, user_addr, &val, sizeof(val),
> +			       FOLL_FORCE | FOLL_WRITE);
> +	if (ret != sizeof(val))
> +		return false;
> +
> +	return true;
> +}

-- 
Thiago