arm64: mte: Enable TCO in functions that can read beyond buffer limits

load_unaligned_zeropad() and __get/put_kernel_nofault() functions can
read past some buffer limits which may include some MTE granule with a
different tag.

When MTE async mode is enabled, the load operation crosses the boundaries
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: Will Deacon <will@kernel.org>
Reported-by: Branislav Rankov <Branislav.Rankov@arm.com>
Tested-by: Branislav Rankov <Branislav.Rankov@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210315132019.33202-6-vincenzo.frascino@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 4 insertions, 0 deletions

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;
 }