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Date: Fri, 9 Apr 2021 17:18:45 +0100
From: Mark Rutland <mark.rutland@....com>
To: Vincenzo Frascino <vincenzo.frascino@....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>, stable@...r.kernel.org
Subject: Re: [PATCH v3] arm64: mte: Move MTE TCF0 check in entry-common
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@....com>
> > Cc: Will Deacon <will@...nel.org>
> > Cc: stable@...r.kernel.org
> > Reported-by: Will Deacon <will@...nel.org>
> > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> > ---
> > 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
> >
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