mm: close theoretical race where stale TLB entries could linger
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 theoretical 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.
Link: https://lkml.kernel.org/r/20250606092809.4194056-1-ryan.roberts@arm.com
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>
Reviewed-by: Jann Horn <jannh@google.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mel Gorman <mgorman <mgorman@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
1 file changed
