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Message-ID: <cff8d16e-c1a9-3f10-7c7f-06fb569741ce@arm.com>
Date: Thu, 8 Apr 2021 16:26:25 +0100
From: Vincenzo Frascino <vincenzo.frascino@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
kasan-dev@...glegroups.com,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>
Subject: Re: [PATCH] arm64: mte: Move MTE TCF0 check in entry-common
On 4/8/21 4:18 PM, Mark Rutland wrote:
> Hi Vincenzo,
>
> On Thu, Apr 08, 2021 at 03:37:23PM +0100, Vincenzo Frascino wrote:
>> The check_mte_async_tcf macro sets the TIF flag non-atomically. This can
>> race with another CPU doing a set_tsk_thread_flag() and the flag can be
>> lost in the process.
>>
>> Move the tcf0 check to enter_from_user_mode() and clear tcf0 in
>> exit_to_user_mode() to address the problem.
>
> Beware that these are called at critical points of the entry sequence,
> so we need to take care that nothing is instrumented (e.g. we can only
> safely use noinstr functions here).
>
Sure, I will add noinstr in the next version of the patch.
>> Note: Moving the check in entry-common allows to use set_thread_flag()
>> which is safe.
>>
>> Fixes: 637ec831ea4f ("arm64: mte: Handle synchronous and asynchronous
>> tag check faults")
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Reported-by: Will Deacon <will@...nel.org>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
>> ---
>> arch/arm64/include/asm/mte.h | 8 ++++++++
>> arch/arm64/kernel/entry-common.c | 6 ++++++
>> arch/arm64/kernel/entry.S | 30 ------------------------------
>> arch/arm64/kernel/mte.c | 25 +++++++++++++++++++++++--
>> 4 files changed, 37 insertions(+), 32 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
>> index 9b557a457f24..188f778c6f7b 100644
>> --- a/arch/arm64/include/asm/mte.h
>> +++ b/arch/arm64/include/asm/mte.h
>> @@ -31,6 +31,8 @@ void mte_invalidate_tags(int type, pgoff_t offset);
>> void mte_invalidate_tags_area(int type);
>> void *mte_allocate_tag_storage(void);
>> void mte_free_tag_storage(char *storage);
>> +void check_mte_async_tcf0(void);
>> +void clear_mte_async_tcf0(void);
>>
>> #ifdef CONFIG_ARM64_MTE
>>
>> @@ -83,6 +85,12 @@ static inline int mte_ptrace_copy_tags(struct task_struct *child,
>> {
>> return -EIO;
>> }
>> +void check_mte_async_tcf0(void)
>> +{
>> +}
>> +void clear_mte_async_tcf0(void)
>> +{
>> +}
>
> Were these meant to be static inline?
>
Agree, it definitely needs static inline here.
>> static inline void mte_assign_mem_tag_range(void *addr, size_t size)
>> {
>> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
>> index 9d3588450473..837d3624a1d5 100644
>> --- a/arch/arm64/kernel/entry-common.c
>> +++ b/arch/arm64/kernel/entry-common.c
>> @@ -289,10 +289,16 @@ asmlinkage void noinstr enter_from_user_mode(void)
>> CT_WARN_ON(ct_state() != CONTEXT_USER);
>> user_exit_irqoff();
>> trace_hardirqs_off_finish();
>> +
>> + /* Check for asynchronous tag check faults in user space */
>> + check_mte_async_tcf0();
>> }
>>
>> asmlinkage void noinstr exit_to_user_mode(void)
>> {
>> + /* Ignore asynchronous tag check faults in the uaccess routines */
>> + clear_mte_async_tcf0();
>> +
>> trace_hardirqs_on_prepare();
>> lockdep_hardirqs_on_prepare(CALLER_ADDR0);
>> user_enter_irqoff();
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index a31a0a713c85..fafd74ae5021 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -147,32 +147,6 @@ alternative_cb_end
>> .L__asm_ssbd_skip\@:
>> .endm
>>
>> - /* Check for MTE asynchronous tag check faults */
>> - .macro check_mte_async_tcf, flgs, tmp
>> -#ifdef CONFIG_ARM64_MTE
>> -alternative_if_not ARM64_MTE
>> - b 1f
>> -alternative_else_nop_endif
>> - mrs_s \tmp, SYS_TFSRE0_EL1
>> - tbz \tmp, #SYS_TFSR_EL1_TF0_SHIFT, 1f
>> - /* Asynchronous TCF occurred for TTBR0 access, set the TI flag */
>> - orr \flgs, \flgs, #_TIF_MTE_ASYNC_FAULT
>> - str \flgs, [tsk, #TSK_TI_FLAGS]
>> - msr_s SYS_TFSRE0_EL1, xzr
>> -1:
>> -#endif
>> - .endm
>> -
>> - /* Clear the MTE asynchronous tag check faults */
>> - .macro clear_mte_async_tcf
>> -#ifdef CONFIG_ARM64_MTE
>> -alternative_if ARM64_MTE
>> - dsb ish
>> - msr_s SYS_TFSRE0_EL1, xzr
>> -alternative_else_nop_endif
>> -#endif
>> - .endm
>> -
>> .macro mte_set_gcr, tmp, tmp2
>> #ifdef CONFIG_ARM64_MTE
>> /*
>> @@ -243,8 +217,6 @@ alternative_else_nop_endif
>> ldr x19, [tsk, #TSK_TI_FLAGS]
>> disable_step_tsk x19, x20
>>
>> - /* Check for asynchronous tag check faults in user space */
>> - check_mte_async_tcf x19, x22
>> apply_ssbd 1, x22, x23
>>
>> ptrauth_keys_install_kernel tsk, x20, x22, x23
>> @@ -775,8 +747,6 @@ SYM_CODE_START_LOCAL(ret_to_user)
>> cbnz x2, work_pending
>> finish_ret_to_user:
>> user_enter_irqoff
>> - /* Ignore asynchronous tag check faults in the uaccess routines */
>> - clear_mte_async_tcf
>> enable_step_tsk x19, x2
>> #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> bl stackleak_erase
>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index b3c70a612c7a..e759b0eca47e 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -166,14 +166,35 @@ static void set_gcr_el1_excl(u64 excl)
>> */
>> }
>>
>> +void check_mte_async_tcf0(void)
>
> As above, this'll need to be noinstr. I also reckon we should put this
> in the header so that it can be inlined.
>
Yes, I agree.
>> +{
>> + /*
>> + * dsb(ish) is not required before the register read
>> + * because the TFSRE0_EL1 is automatically synchronized
>> + * by the hardware on exception entry as SCTLR_EL1.ITFSB
>> + * is set.
>> + */
>> + u64 tcf0 = read_sysreg_s(SYS_TFSRE0_EL1);
>
> Shouldn't we have an MTE feature check first?
>
Indeed, I will add it in the next version.
>> +
>> + if (tcf0 & SYS_TFSR_EL1_TF0)
>> + set_thread_flag(TIF_MTE_ASYNC_FAULT);
>> +
>> + write_sysreg_s(0, SYS_TFSRE0_EL1);
>> +}
>> +
>> +void clear_mte_async_tcf0(void)
>> +{
>> + dsb(ish);
>> + write_sysreg_s(0, SYS_TFSRE0_EL1);
>> +}
>
> Likewise here on all counts.
>
I will add noinstr and the check in the next version.
> Thanks,
> Mark.
>
>> void flush_mte_state(void)
>> {
>> if (!system_supports_mte())
>> return;
>>
>> /* clear any pending asynchronous tag fault */
>> - dsb(ish);
>> - write_sysreg_s(0, SYS_TFSRE0_EL1);
>> + clear_mte_async_tcf0();
>> clear_thread_flag(TIF_MTE_ASYNC_FAULT);
>> /* disable tag checking */
>> set_sctlr_el1_tcf0(SCTLR_EL1_TCF0_NONE);
>> --
>> 2.30.2
>>
--
Regards,
Vincenzo
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