Re: [PATCH v3] arm64: mte: Move MTE TCF0 check in entry-common

[Mark Rutland]

On Fri, Apr 09, 2021 at 03:32:47PM +0100, Mark Rutland wrote:
> Hi Vincenzo,
> 
> On Fri, Apr 09, 2021 at 02:24:19PM +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 all the other flags
> > 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.
> > 
> > Note: Moving the check in entry-common allows to use set_thread_flag()
> > which is safe.

I've dug into this a bit more, and as set_thread_flag() calls some
potentially-instrumented helpers I don't think this is safe after all
(as e.g. those might cause an EL1 exception and clobber the ESR/FAR/etc
before the EL0 exception handler reads it).

Making that watertight is pretty hairy, as we either need to open-code
set_thread_flag() or go rework a load of core code. If we can use STSET
in the entry asm that'd be simpler, otherwise we'll need something more
involved.

Thanks,
Mark.

> > 
> > Fixes: 637ec831ea4f ("arm64: mte: Handle synchronous and asynchronous tag check faults")
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Will Deacon <will@xxxxxxxxxx>
> > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> > ---
> >  arch/arm64/include/asm/mte.h     |  9 +++++++++
> >  arch/arm64/kernel/entry-common.c |  6 ++++++
> >  arch/arm64/kernel/entry.S        | 34 --------------------------------
> >  arch/arm64/kernel/mte.c          | 33 +++++++++++++++++++++++++++++--
> >  4 files changed, 46 insertions(+), 36 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> > index 9b557a457f24..c7ab681a95c3 100644
> > --- a/arch/arm64/include/asm/mte.h
> > +++ b/arch/arm64/include/asm/mte.h
> > @@ -49,6 +49,9 @@ int mte_ptrace_copy_tags(struct task_struct *child, long request,
> >  
> >  void mte_assign_mem_tag_range(void *addr, size_t size);
> >  
> > +void noinstr check_mte_async_tcf0(void);
> > +void noinstr clear_mte_async_tcf0(void);
> 
> Can we please put the implementations in the header so that they can be
> inlined? Otherwise when the HW doesn't support MTE we'll always do a pointless
> branch to the out-of-line implementation.
> 
> With that, we can mark them __always_inline to avoid weirdness with an inline
> noinstr function.
> 
> Otherwise, this looks good to me.
> 
> Thanks,
> Mark.
> 
> > +
> >  #else /* CONFIG_ARM64_MTE */
> >  
> >  /* unused if !CONFIG_ARM64_MTE, silence the compiler */
> > @@ -83,6 +86,12 @@ static inline int mte_ptrace_copy_tags(struct task_struct *child,
> >  {
> >  	return -EIO;
> >  }
> > +static inline void check_mte_async_tcf0(void)
> > +{
> > +}
> > +static inline void clear_mte_async_tcf0(void)
> > +{
> > +}
> >  
> >  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..fb57df0d453f 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -34,15 +34,11 @@
> >   * user and kernel mode.
> >   */
> >  	.macro user_exit_irqoff
> > -#if defined(CONFIG_CONTEXT_TRACKING) || defined(CONFIG_TRACE_IRQFLAGS)
> >  	bl	enter_from_user_mode
> > -#endif
> >  	.endm
> >  
> >  	.macro user_enter_irqoff
> > -#if defined(CONFIG_CONTEXT_TRACKING) || defined(CONFIG_TRACE_IRQFLAGS)
> >  	bl	exit_to_user_mode
> > -#endif
> >  	.endm
> >  
> >  	.macro	clear_gp_regs
> > @@ -147,32 +143,6 @@ 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 +213,6 @@ 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 +743,6 @@ 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..84a942c25870 100644
> > --- a/arch/arm64/kernel/mte.c
> > +++ b/arch/arm64/kernel/mte.c
> > @@ -166,14 +166,43 @@ static void set_gcr_el1_excl(u64 excl)
> >  	 */
> >  }
> >  
> > -void flush_mte_state(void)
> > +void noinstr check_mte_async_tcf0(void)
> > +{
> > +	u64 tcf0;
> > +
> > +	if (!system_supports_mte())
> > +		return;
> > +
> > +	/*
> > +	 * 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.
> > +	 */
> > +	tcf0 = read_sysreg_s(SYS_TFSRE0_EL1);
> > +
> > +	if (tcf0 & SYS_TFSR_EL1_TF0)
> > +		set_thread_flag(TIF_MTE_ASYNC_FAULT);
> > +
> > +	write_sysreg_s(0, SYS_TFSRE0_EL1);
> > +}
> > +
> > +void noinstr clear_mte_async_tcf0(void)
> >  {
> >  	if (!system_supports_mte())
> >  		return;
> >  
> > -	/* clear any pending asynchronous tag fault */
> >  	dsb(ish);
> >  	write_sysreg_s(0, SYS_TFSRE0_EL1);
> > +}
> > +
> > +void flush_mte_state(void)
> > +{
> > +	if (!system_supports_mte())
> > +		return;
> > +
> > +	/* clear any pending asynchronous tag fault */
> > +	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
> >