6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Lorenzo Stoakes (ARM) [ Upstream commit 597e4b9ab48a3f841080f627722064853d3f650e ] The MREMAP_DONTUNMAP feature is highly unusual in that it permits mremap() operations that keep the original VMA in place. Historically this has led to a lot of bugs where non-obvious interactions occur between existing mremap() operations and the original VMA. Fix another of these - partial copies. The long-standing mremap() partial VMA logic has the baked-in assumption that the originating VMA is unmapped and thus moved. However MREMAP_DONTUNMAP defeats this by performing a partial copy instead since it keeps the source VMA around. An mremap(..., MREMAP_DONTUNMAP) operation disallows resizing of the VMA, but the operation can be performed partially: |-----------------| | | | v <------> <------> .new_sz. new_sz |--.------.--| |------| | .source. | | dest | |--.------.--| |------| <------------> old_sz The page tables in the specified range are moved, but the original VMA is kept intact. This interacts poorly with mlock()'d VMAs, as the VMA_LOCKED_BIT flag is cleared for the entire source VMA and set for the entire destination VMA. This results in an mm->locked_vm leak as the change is therefore not accounted correctly. The clear solution here is to make the portion of the source VMA which is mremap()'d distinct from the rest of it, a.k.a. split it. Therefore resolve this issue by splitting it ahead of the rest of the mremap() operation if the VMA is mlock()'d. This is valid, as the source VMA will lose its VMA_LOCKED_BIT flag, so if a partial remap it will become distinct from the rest of the VMA. In order to make this change re-expose split_vma() in vma.h for CONFIG_MMU (nommu doesn't compile mremap.c and uses a static helper instead). Finally, update the sys_map_count check to account for this case. Note that the early check does not use needs_pre_split() - this is because the VMA has not been looked up by this point, so be conservative and assume that the VMA is mlock()'d in this case for the purposes of the sys_map_count check. Link: https://lore.kernel.org/20260930-fix-dontunmap-partial-self-merge-v2-2-f388985a0f0a@kernel.org Fixes: e346b3813067 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()") Signed-off-by: Lorenzo Stoakes (ARM) Signed-off-by: Andrew Morton Reviewed-by: Pedro Falcato Acked-by: Kiryl Shutsemau (Meta) Reviewed-by: Jose A. Perez de Azpillaga Tested-by: Anirudh Srinivasan Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Jann Horn Cc: Brian Geffon Cc: Minchan Kim Cc: [ sashal: Moved split preparation from prep_move_vma() into the existing move_vma() implementation. ] Signed-off-by: Sasha Levin --- mm/mremap.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/mm/mremap.c b/mm/mremap.c index d35f603c9738e..db18cacb795b5 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -605,6 +605,8 @@ static unsigned long move_vma(struct vm_area_struct *vma, int err = 0; bool need_rmap_locks; struct vma_iterator vmi; + const bool split_before = vma->vm_start != old_addr; + const bool split_after = vma->vm_end != old_addr + old_len; /* * We'd prefer to avoid failure later on in do_munmap: @@ -617,9 +619,9 @@ static unsigned long move_vma(struct vm_area_struct *vma, to_account = new_len; if (vma->vm_ops && vma->vm_ops->may_split) { - if (vma->vm_start != old_addr) + if (split_before) err = vma->vm_ops->may_split(vma, old_addr); - if (!err && vma->vm_end != old_addr + old_len) + if (!err && split_after) err = vma->vm_ops->may_split(vma, old_addr + old_len); if (err) return err; @@ -637,6 +639,21 @@ static unsigned long move_vma(struct vm_area_struct *vma, if (err) return err; + /* + * To account mlock()'d pages correctly in the MREMAP_DONTUNMAP + * case perform any split ahead of time for an mlock()'d VMA. + */ + if ((flags & MREMAP_DONTUNMAP) && (vma->vm_flags & VM_LOCKED)) { + vma_iter_init(&vmi, mm, old_addr); + + if (split_before) + err = split_vma(&vmi, vma, old_addr, 1); + if (!err && split_after) + err = split_vma(&vmi, vma, old_addr + old_len, 0); + if (err) + return err; + } + if (vm_flags & VM_ACCOUNT) { if (security_vm_enough_memory_mm(mm, to_account >> PAGE_SHIFT)) return -ENOMEM; -- 2.53.0