Previous commits have established the invariant that kernel page table freeing is performed while an mmap read lock on init_mm is held, which fixes races between ptdump and kernel page table freeing over init_mm. However, x86 and arm64 can perform a ptdump over an mm other than init_mm via ptdump_walk_pgd() and since kernel memory ranges are shared across non-kernel mm's, this means that the race still exists for these cases. Fix this by acquiring a nested mmap write lock for init_mm in ptdump_walk_pgd(). This is safe as we take this after mmap write locking the mm, and nothing acquires the init_mm lock first before locking an arbitrary mm, so no deadlock is possible. Also update walk_page_range_debug() to assert that init_mm is write locked, add a comment explaining why and remove some redundant code, and eliminate the unnecessary and confusing invocation of walk_kernel_page_table_range(). We can safely remove the non-NULL check for walk.mm, as the mmap lock asserts would NULL pointer deref if it was (and of course no callers do this). The first point at which ptdump can race kernel page table freeing is commit b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table"), so we target this in the Fixes tag. Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table") Cc: stable@vger.kernel.org Reviewed-by: Mike Rapoport (Microsoft) Signed-off-by: Lorenzo Stoakes --- mm/pagewalk.c | 14 +++++++++----- mm/ptdump.c | 7 +++++++ 2 files changed, 16 insertions(+), 5 deletions(-) diff --git a/mm/pagewalk.c b/mm/pagewalk.c index bbcfd68d0907..5d87c632a255 100644 --- a/mm/pagewalk.c +++ b/mm/pagewalk.c @@ -702,12 +702,16 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start, * to account for page table freeing on vmap huge page mapping. */ mmap_assert_write_locked(mm); + /* + * x86, arm64 ptdump allow walks of efi mm's and x86 ptdump allows walks + * of arbitrary mm's. + * + * However, they both must also hold the init_mm lock to account for + * concurrent kernel page table freeing. + */ + mmap_assert_write_locked(&init_mm); - /* For convenience, we allow traversal of kernel mappings. */ - if (mm == &init_mm) - return walk_kernel_page_table_range(start, end, ops, - pgd, private); - if (start >= end || !walk.mm) + if (start >= end) return -EINVAL; if (!check_ops_safe(ops)) return -EINVAL; diff --git a/mm/ptdump.c b/mm/ptdump.c index 973020000096..5851096e6f65 100644 --- a/mm/ptdump.c +++ b/mm/ptdump.c @@ -178,11 +178,18 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd) get_online_mems(); mmap_write_lock(mm); + /* To stabilise kernel page tables we must hold the init_mm lock too. */ + if (mm != &init_mm) + mmap_write_lock_nested(&init_mm, SINGLE_DEPTH_NESTING); + while (range->start != range->end) { walk_page_range_debug(mm, range->start, range->end, &ptdump_ops, pgd, st); range++; } + + if (mm != &init_mm) + mmap_write_unlock(&init_mm); mmap_write_unlock(mm); put_online_mems(); -- 2.55.0