[PATCH 03/24] arm64: Fix volatility of IRG inline assembly

[Pierre-Clément Tosi]

Add the 'volatile' qualifier, without which both GCC and Clang treat the
function as pure, potentially collapsing multiple inlined calls into a
single one (returning the same not-so-random tag) through CSE or moving
an inlined call within a loop out of it. This is not compatible with
IRG's architecturally-defined RNG side effects.

As existing KASAN callers are either not inlined or guarded by runtime
conditions, the compiler isn't allowed to hoist the non-volatile asm.
Note that commit 85f49cae4dfc ("arm64: mte: add in-kernel MTE helpers")
originally introduced the helper as an out-of-line function (in mte.c),
so the risk then was even smaller than when defined in a header.

I'll mark this 'Fixes' since it addresses UB but won't request stable
backports, as none of the issues mentioned above were actually hit.

Fixes: 2cb34276427a ("arm64: kasan: simplify and inline MTE functions")
Signed-off-by: Pierre-Clément Tosi <ptosi@xxxxxxxxxx>
---
 arch/arm64/include/asm/mte-kasan.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
index de910f1c19e1..43510341b965 100644
--- a/arch/arm64/include/asm/mte-kasan.h
+++ b/arch/arm64/include/asm/mte-kasan.h
@@ -111,7 +111,7 @@ static inline u8 mte_get_random_tag(void)
 {
 	void *addr;
 
-	asm(__MTE_PREAMBLE "irg %0, %0"
+	asm volatile(__MTE_PREAMBLE "irg %0, %0"
 		: "=r" (addr));
 
 	return mte_get_ptr_tag(addr);

-- 
2.56.0.455.g6c5a503b38-goog