[PATCH v1] mm: Close theoretical race where stale TLB entries could linger

[Ryan Roberts <ryan.roberts@arm.com>]

Commit 3ea277194daa ("mm, mprotect: flush TLB if potentially racing with
a parallel reclaim leaving stale TLB entries") described a theoretical
race as such:

"""
Nadav Amit identified a theoritical race between page reclaim and
mprotect due to TLB flushes being batched outside of the PTL being held.

He described the race as follows:

	CPU0                            CPU1
	----                            ----
					user accesses memory using RW PTE
					[PTE now cached in TLB]
	try_to_unmap_one()
	==> ptep_get_and_clear()
	==> set_tlb_ubc_flush_pending()
					mprotect(addr, PROT_READ)
					==> change_pte_range()
					==> [ PTE non-present - no flush ]

					user writes using cached RW PTE
	...

	try_to_unmap_flush()

The same type of race exists for reads when protecting for PROT_NONE and
also exists for operations that can leave an old TLB entry behind such
as munmap, mremap and madvise.
"""

The solution was to introduce flush_tlb_batched_pending() and call it
under the PTL from mprotect/madvise/munmap/mremap to complete any
pending tlb flushes.

However, while madvise_free_pte_range() and
madvise_cold_or_pageout_pte_range() were both retro-fitted to call
flush_tlb_batched_pending() immediately after initially acquiring the
PTL, they both temporarily release the PTL to split a large folio if
they stumble upon one. In this case, where re-acquiring the PTL
flush_tlb_batched_pending() must be called again, but it previously was
not. Let's fix that.

There are 2 Fixes: tags here: the first is the commit that fixed
madvise_free_pte_range(). The second is the commit that added
madvise_cold_or_pageout_pte_range(), which looks like it copy/pasted the
faulty pattern from madvise_free_pte_range().

This is a theoretical bug discovered during code review.

Cc: stable@vger.kernel.org
Fixes: 3ea277194daa ("mm, mprotect: flush TLB if potentially racing with a parallel reclaim leaving stale TLB entries")
Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

Applies on today's mm-unstable (3f676fe5c7a0). All mm selftests continue to
pass.

Thanks,
Ryan

 mm/madvise.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index 5f7a66a1617e..1d44a35ae85c 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -508,6 +508,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 					pte_offset_map_lock(mm, pmd, addr, &ptl);
 				if (!start_pte)
 					break;
+				flush_tlb_batched_pending(mm);
 				arch_enter_lazy_mmu_mode();
 				if (!err)
 					nr = 0;
@@ -741,6 +742,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 				start_pte = pte;
 				if (!start_pte)
 					break;
+				flush_tlb_batched_pending(mm);
 				arch_enter_lazy_mmu_mode();
 				if (!err)
 					nr = 0;
--
2.43.0