[PATCH v2] arm64: kpti: Fix the interaction between ASID switching and software PAN

[James Morse]

Hi Catalin,

On 15/01/18 17:58, Catalin Marinas wrote:
> With ARM64_SW_TTBR0_PAN enabled, the exception entry code checks the
> active ASID to decide whether user access was enabled (non-zero ASID)
> when the exception was taken. On return from exception, if user access
> was previously disabled, it re-instates TTBR0_EL1 from the per-thread
> saved value (updated in switch_mm() or efi_set_pgd()).
> 
> Commit 7655abb95386 ("arm64: mm: Move ASID from TTBR0 to TTBR1") makes a
> TTBR0_EL1 + ASID switching non-atomic. Subsequently, commit 27a921e75711
> ("arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN") changes the
> __uaccess_ttbr0_disable() function and asm macro to first write the
> reserved TTBR0_EL1 followed by the ASID=0 update in TTBR1_EL1. If an
> exception occurs between these two, the exception return code will
> re-instate a valid TTBR0_EL1. Similar scenario can happen in
> cpu_switch_mm() between setting the reserved TTBR0_EL1 and the ASID
> update in cpu_do_switch_mm().
> 
> This patch reverts the entry.S check for ASID == 0 to TTBR0_EL1 and
> disables the interrupts around the TTBR0_EL1 and ASID switching code in
> __uaccess_ttbr0_disable(). It also ensures that, when returning from the
> EFI runtime services, efi_set_pgd() doesn't leave a non-zero ASID in
> TTBR1_EL1 by using uaccess_ttbr0_{enable,disable}.
> 
> The accesses to current_thread_info()->ttbr0 are updated to use
> READ_ONCE/WRITE_ONCE.
> 
> As a safety measure, __uaccess_ttbr0_enable() always masks out any
> existing non-zero ASID TTBR1_EL1 before writing in the new ASID.
> 
> Fixes: 27a921e75711 ("arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN")
> Acked-by: Will Deacon <will.deacon at arm.com>
> Reported-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Reviewed-by: James Morse <james.morse at arm.com>
> Cc: James Morse <james.morse at arm.com>
> Co-developed-by: Marc Zyngier <marc.zyngier at arm.com>
> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
> 
> Changes since v1 (no functional change):
> 
> - Modified efi_set_pgd() to pair uaccess_ttbr0_disable/enable
> - Accesses to current_thread_info()->ttbr0 are passed through READ/WRITE_ONCE
> 
> I dropped James' tested-by, just in case I messed something up.

Tested as before:
Tested-by: James Morse <james.morse at arm.com>


Thanks,

James