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: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Steven Price <steven.price@xxxxxxx>
---
Found while debating with Steven a similar race on PG_mte_tagged. For
the latter we'll have to take a lock but hopefully in practice it will
only happen when restoring from swap. Separate thread anyway.
There's at least arch/arm with a similar race. Powerpc seems to do it
properly with separate test/set. Other architectures have a bigger
problem as they do a similar check in update_mmu_cache(), called after
the pte was already exposed to user.
I looked at fixing this in the mprotect() code but taking the page lock
will slow it down, so not sure how popular this would be for such a rare
race.
arch/arm64/mm/flush.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
index ac485163a4a7..6d44c028d1c9 100644
--- a/arch/arm64/mm/flush.c
+++ b/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);
