arm64: Fix race condition on PG_dcache_clean in __sync_icache_dcache()

To ensure that instructions are observable in a new mapping, the arm64
set_pte_at() implementation cleans the D-cache and invalidates the
I-cache to the PoU. As an optimisation, this is only done on executable
mappings and the PG_dcache_clean page flag is set to avoid future cache
maintenance on the same page.

When two different processes map the same page (e.g. private executable
file or shared mapping) there's a potential race on checking and setting
PG_dcache_clean via set_pte_at() -> __sync_icache_dcache(). While on the
fault paths the page is locked (PG_locked), mprotect() does not take the
page lock. The result is that one process may see the PG_dcache_clean
flag set but the I/D cache maintenance not yet performed.

Avoid test_and_set_bit(PG_dcache_clean) in favour of separate test_bit()
and set_bit(). In the rare event of a race, the cache maintenance is
done twice.
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>
Cc: <stable@vger.kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Reviewed-by: NSteven Price <steven.price@arm.com>
Acked-by: NWill Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210514095001.13236-1-catalin.marinas@arm.comSigned-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/flush.c

@@ -55,8 +55,10 @@ void __sync_icache_dcache(pte_t pte)
 {
 	struct page *page = pte_page(pte);
 
-	if (!test_and_set_bit(PG_dcache_clean, &page->flags))
+	if (!test_bit(PG_dcache_clean, &page->flags)) {
 		sync_icache_aliases(page_address(page), page_size(page));
+		set_bit(PG_dcache_clean, &page->flags);
+	}
 }
 EXPORT_SYMBOL_GPL(__sync_icache_dcache);