From: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
linux-arm-kernel@lists.infradead.org, james.morse@arm.com,
robin.murphy@arm.com
Subject: Re: [PATCH] arm64: uaccess: simplify uaccess_mask_ptr()
Date: Fri, 23 Sep 2022 11:34:46 +0100 [thread overview]
Message-ID: <Yy2LxmNAGybHHCn6@arm.com> (raw)
In-Reply-To: <20220922205545.GA12945@willie-the-truck>
On Thu, Sep 22, 2022 at 09:55:46PM +0100, Will Deacon wrote:
> On Thu, Sep 22, 2022 at 04:10:53PM +0100, Mark Rutland wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index 2fc9f0861769a..e69559826cb8c 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -203,9 +203,11 @@ static inline void uaccess_enable_privileged(void)
> > }
> >
> > /*
> > - * Sanitise a uaccess pointer such that it becomes NULL if above the maximum
> > - * user address. In case the pointer is tagged (has the top byte set), untag
> > - * the pointer before checking.
> > + * Sanitize a uaccess pointer such that it cannot reach any kernel address.
> > + *
> > + * Clearing bit 55 ensures the pointer cannot address any portion of the TTBR1
> > + * address range (i.e. any kernel address), and either the pointer falls within
> > + * the TTBR0 address range or must cause a fault.
> > */
> > #define uaccess_mask_ptr(ptr) (__typeof__(ptr))__uaccess_mask_ptr(ptr)
> > static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
> > @@ -213,12 +215,11 @@ static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
> > void __user *safe_ptr;
> >
> > asm volatile(
> > - " bics xzr, %3, %2\n"
> > - " csel %0, %1, xzr, eq\n"
> > - : "=&r" (safe_ptr)
> > - : "r" (ptr), "r" (TASK_SIZE_MAX - 1),
> > - "r" (untagged_addr(ptr))
> > - : "cc");
> > + " bic %0, %1, %2\n"
> > + : "=r" (safe_ptr)
> > + : "r" (ptr),
> > + "i" (BIT(55))
> > + );
> >
> > csdb();
>
> Why do we still need the CSDB after your change?
Good point, we no longer do a check against TASK_SIZE so no need for
flags prediction (IIRC that was the reason for CSDB).
--
Catalin
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next prev parent reply other threads:[~2022-09-23 10:35 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-22 15:10 [PATCH] arm64: uaccess: simplify uaccess_mask_ptr() Mark Rutland
2022-09-22 16:46 ` Robin Murphy
2022-09-22 20:55 ` Will Deacon
2022-09-23 10:34 ` Catalin Marinas [this message]
2022-09-23 12:29 ` Mark Rutland
2022-09-23 17:26 ` Catalin Marinas
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