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Message-ID: <20210308180910.GB17002@C02TD0UTHF1T.local>
Date: Mon, 8 Mar 2021 18:09:10 +0000
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,
Andrew Morton <akpm@...ux-foundation.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Dmitry Vyukov <dvyukov@...gle.com>,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Alexander Potapenko <glider@...gle.com>,
Marco Elver <elver@...gle.com>,
Evgenii Stepanov <eugenis@...gle.com>,
Branislav Rankov <Branislav.Rankov@....com>,
Andrey Konovalov <andreyknvl@...gle.com>,
Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v14 5/8] arm64: mte: Enable TCO in functions that can
read beyond buffer limits
On Mon, Mar 08, 2021 at 04:14:31PM +0000, Vincenzo Frascino wrote:
> load_unaligned_zeropad() and __get/put_kernel_nofault() functions can
> read passed some buffer limits which may include some MTE granule with a
> different tag.
s/passed/past/
> When MTE async mode is enable, the load operation crosses the boundaries
s/enabel/enabled/
> and the next granule has a different tag the PE sets the TFSR_EL1.TF1 bit
> as if an asynchronous tag fault is happened.
>
> Enable Tag Check Override (TCO) in these functions before the load and
> disable it afterwards to prevent this to happen.
>
> Note: The same condition can be hit in MTE sync mode but we deal with it
> through the exception handling.
> In the current implementation, mte_async_mode flag is set only at boot
> time but in future kasan might acquire some runtime features that
> that change the mode dynamically, hence we disable it when sync mode is
> selected for future proof.
>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Reported-by: Branislav Rankov <Branislav.Rankov@....com>
> Tested-by: Branislav Rankov <Branislav.Rankov@....com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> ---
> arch/arm64/include/asm/uaccess.h | 24 ++++++++++++++++++++++++
> arch/arm64/include/asm/word-at-a-time.h | 4 ++++
> arch/arm64/kernel/mte.c | 22 ++++++++++++++++++++++
> 3 files changed, 50 insertions(+)
>
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 0deb88467111..a857f8f82aeb 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -188,6 +188,26 @@ static inline void __uaccess_enable_tco(void)
> ARM64_MTE, CONFIG_KASAN_HW_TAGS));
> }
>
> +/* Whether the MTE asynchronous mode is enabled. */
> +DECLARE_STATIC_KEY_FALSE(mte_async_mode);
Can we please hide this behind something like:
static inline bool system_uses_mte_async_mode(void)
{
return IS_ENABLED(CONFIG_KASAN_HW_TAGS) &&
static_branch_unlikely(&mte_async_mode);
}
... like we do for system_uses_ttbr0_pan()?
That way the callers are easier to read, and kernels built without
CONFIG_KASAN_HW_TAGS don't have the static branch at all. I reckon you
can put that in one of hte mte headers and include it where needed.
Thanks,
Mark.
> +
> +/*
> + * These functions disable tag checking only if in MTE async mode
> + * since the sync mode generates exceptions synchronously and the
> + * nofault or load_unaligned_zeropad can handle them.
> + */
> +static inline void __uaccess_disable_tco_async(void)
> +{
> + if (static_branch_unlikely(&mte_async_mode))
> + __uaccess_disable_tco();
> +}
> +
> +static inline void __uaccess_enable_tco_async(void)
> +{
> + if (static_branch_unlikely(&mte_async_mode))
> + __uaccess_enable_tco();
> +}
> +
> static inline void uaccess_disable_privileged(void)
> {
> __uaccess_disable_tco();
> @@ -307,8 +327,10 @@ do { \
> do { \
> int __gkn_err = 0; \
> \
> + __uaccess_enable_tco_async(); \
> __raw_get_mem("ldr", *((type *)(dst)), \
> (__force type *)(src), __gkn_err); \
> + __uaccess_disable_tco_async(); \
> if (unlikely(__gkn_err)) \
> goto err_label; \
> } while (0)
> @@ -380,8 +402,10 @@ do { \
> do { \
> int __pkn_err = 0; \
> \
> + __uaccess_enable_tco_async(); \
> __raw_put_mem("str", *((type *)(src)), \
> (__force type *)(dst), __pkn_err); \
> + __uaccess_disable_tco_async(); \
> if (unlikely(__pkn_err)) \
> goto err_label; \
> } while(0)
> diff --git a/arch/arm64/include/asm/word-at-a-time.h b/arch/arm64/include/asm/word-at-a-time.h
> index 3333950b5909..c62d9fa791aa 100644
> --- a/arch/arm64/include/asm/word-at-a-time.h
> +++ b/arch/arm64/include/asm/word-at-a-time.h
> @@ -55,6 +55,8 @@ static inline unsigned long load_unaligned_zeropad(const void *addr)
> {
> unsigned long ret, offset;
>
> + __uaccess_enable_tco_async();
> +
> /* Load word from unaligned pointer addr */
> asm(
> "1: ldr %0, %3\n"
> @@ -76,6 +78,8 @@ static inline unsigned long load_unaligned_zeropad(const void *addr)
> : "=&r" (ret), "=&r" (offset)
> : "r" (addr), "Q" (*(unsigned long *)addr));
>
> + __uaccess_disable_tco_async();
> +
> return ret;
> }
>
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index fa755cf94e01..1ad9be4c8376 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -26,6 +26,10 @@ u64 gcr_kernel_excl __ro_after_init;
>
> static bool report_fault_once = true;
>
> +/* Whether the MTE asynchronous mode is enabled. */
> +DEFINE_STATIC_KEY_FALSE(mte_async_mode);
> +EXPORT_SYMBOL_GPL(mte_async_mode);
> +
> static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
> {
> pte_t old_pte = READ_ONCE(*ptep);
> @@ -118,12 +122,30 @@ static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
>
> void mte_enable_kernel_sync(void)
> {
> + /*
> + * Make sure we enter this function when no PE has set
> + * async mode previously.
> + */
> + WARN_ONCE(static_key_enabled(&mte_async_mode),
> + "MTE async mode enabled system wide!");
> +
> __mte_enable_kernel("synchronous", SCTLR_ELx_TCF_SYNC);
> }
>
> void mte_enable_kernel_async(void)
> {
> __mte_enable_kernel("asynchronous", SCTLR_ELx_TCF_ASYNC);
> +
> + /*
> + * MTE async mode is set system wide by the first PE that
> + * executes this function.
> + *
> + * Note: If in future KASAN acquires a runtime switching
> + * mode in between sync and async, this strategy needs
> + * to be reviewed.
> + */
> + if (!static_branch_unlikely(&mte_async_mode))
> + static_branch_enable(&mte_async_mode);
> }
>
> void mte_set_report_once(bool state)
> --
> 2.30.0
>
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