From b7012d513f81959596a01083415597edb04cf509 Mon Sep 17 00:00:00 2001 From: "Liam R. Howlett" Date: Fri, 30 Aug 2024 00:00:41 -0400 Subject: [PATCH] mm/vma: correctly position vma_iterator in __split_vma() Patch series "Avoid MAP_FIXED gap exposure", v8. It is now possible to walk the vma tree using the rcu read locks and is beneficial to do so to reduce lock contention. Doing so while a MAP_FIXED mapping is executing means that a reader may see a gap in the vma tree that should never logically exist - and does not when using the mmap lock in read mode. The temporal gap exists because mmap_region() calls munmap() prior to installing the new mapping. This patch set stops rcu readers from seeing the temporal gap by splitting up the munmap() function into two parts. The first part prepares the vma tree for modifications by doing the necessary splits and tracks the vmas marked for removal in a side tree. The second part completes the munmapping of the vmas after the vma tree has been overwritten (either by a MAP_FIXED replacement vma or by a NULL in the munmap() case). Please note that rcu walkers will still be able to see a temporary state of split vmas that may be in the process of being removed, but the temporal gap will not be exposed. vma_start_write() are called on both parts of the split vma, so this state is detectable. If existing vmas have a vm_ops->close(), then they will be called prior to mapping the new vmas (and ptes are cleared out). Without calling ->close(), hugetlbfs tests fail (hugemmap06 specifically) due to resources still being marked as 'busy'. Unfortunately, calling the corresponding ->open() may not restore the state of the vmas, so it is safer to keep the existing failure scenario where a gap is inserted and never replaced. The failure scenario is in its own patch (0015) for traceability. This patch (of 21): The vma iterator may be left pointing to the newly created vma. This happens when inserting the new vma at the end of the old vma (!new_below). The incorrect position in the vma iterator is not exposed currently since the vma iterator is repositioned in the munmap path and is not reused in any of the other paths. This has limited impact in the current code, but is required for future changes. Link: https://lkml.kernel.org/r/20240830040101.822209-2-Liam.Howlett@oracle.com Fixes: b2b3b886738f ("mm: don't use __vma_adjust() in __split_vma()") Signed-off-by: Liam R. Howlett Reviewed-by: Suren Baghdasaryan Reviewed-by: Lorenzo Stoakes Cc: Bert Karwatzki Cc: Jeff Xu Cc: Jiri Olsa Cc: Kees Cook Cc: Matthew Wilcox Cc: "Paul E. McKenney" Cc: Sidhartha Kumar Cc: Vlastimil Babka Cc: Lorenzo Stoakes Cc: Mark Brown Cc: Paul Moore Signed-off-by: Andrew Morton --- mm/vma.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/vma.c b/mm/vma.c index 5850f7c0949b..066de79b7b73 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -177,7 +177,7 @@ void unmap_region(struct mm_struct *mm, struct ma_state *mas, /* * __split_vma() bypasses sysctl_max_map_count checking. We use this where it * has already been checked or doesn't make sense to fail. - * VMA Iterator will point to the end VMA. + * VMA Iterator will point to the original VMA. */ static int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma, unsigned long addr, int new_below) @@ -246,6 +246,9 @@ static int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma, /* Success. */ if (new_below) vma_next(vmi); + else + vma_prev(vmi); + return 0; out_free_mpol: