Re: [PATCH v4 00/13] KVM: arm64: Add support for hypervisor kCFI

From: Will Deacon <will@kernel.org>
To: "Pierre-Clément Tosi" <ptosi@google.com>
Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Vincent Donnefort <vdonnefort@google.com>
Subject: Re: [PATCH v4 00/13] KVM: arm64: Add support for hypervisor kCFI
Date: Mon, 3 Jun 2024 14:59:38 +0100	[thread overview]
Message-ID: <20240603135937.GA19151@willie-the-truck> (raw)
In-Reply-To: <20240529121251.1993135-1-ptosi@google.com>

On Wed, May 29, 2024 at 01:12:06PM +0100, Pierre-Clément Tosi wrote:
> CONFIG_CFI_CLANG ("kernel Control Flow Integrity") makes the compiler inject
> runtime type checks before any indirect function call. On AArch64, it generates
> a BRK instruction to be executed on type mismatch and encodes the indices of the
> registers holding the branch target and expected type in the immediate of the
> instruction. As a result, a synchronous exception gets triggered on kCFI failure
> and the fault handler can retrieve the immediate (and indices) from ESR_ELx.
> 
> This feature has been supported at EL1 ("host") since it was introduced by
> b26e484b8bb3 ("arm64: Add CFI error handling"), where cfi_handler() decodes
> ESR_EL1, giving informative panic messages such as
> 
>   [   21.885179] CFI failure at lkdtm_indirect_call+0x2c/0x44 [lkdtm]
>   (target: lkdtm_increment_int+0x0/0x1c [lkdtm]; expected type: 0x7e0c52a)
>   [   21.886593] Internal error: Oops - CFI: 0 [#1] PREEMPT SMP
> 
> However, it is not or only partially supported at EL2: in nVHE (or pKVM),
> CONFIG_CFI_CLANG gets filtered out at build time, preventing the compiler from
> injecting the checks. In VHE, EL2 code gets compiled with the checks but the
> handlers in VBAR_EL2 are not aware of kCFI and will produce a generic and
> not-so-helpful panic message such as
> 
>   [   36.456088][  T200] Kernel panic - not syncing: HYP panic:
>   [   36.456088][  T200] PS:204003c9 PC:ffffffc080092310 ESR:f2008228
>   [   36.456088][  T200] FAR:0000000081a50000 HPFAR:000000000081a500 PAR:1de7ec7edbadc0de
>   [   36.456088][  T200] VCPU:00000000e189c7cf
> 
> To address this,
> 
> - [01/13] fixes an existing bug where the ELR_EL2 was getting clobbered on
>   synchronous exceptions, causing the wrong "PC" to be reported by
>   nvhe_hyp_panic_handler() or __hyp_call_panic(). This is particularly limiting
>   for kCFI, as it would mask the location of the failed type check.
> - [02/13] fixes a minor C/asm ABI mismatch which would trigger a kCFI failure
> - [03/13] to [09/13] prepare nVHE for CONFIG_CFI_CLANG and [10/13] enables it
> - [11/13] improves kCFI error messages by saving then parsing the CPU context
> - [12/13] adds a kCFI test module for VHE and [13/13] extends it to nVHE & pKVM
> 
> As a result, an informative kCFI panic message is printed by or on behalf of EL2
> giving the expected type and target address (possibly resolved to a symbol) for
> VHE, nVHE, and pKVM (iff CONFIG_NVHE_EL2_DEBUG=y).
> 
> Note that kCFI errors remain fatal at EL2, even when CONFIG_CFI_PERMISSIVE=y.
> 
> Changes in v4:
>   - Addressed Will's comments on v3:

nit: but please keep reviewers on CC when you post a new version. I
missed this initially.

Will

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