Re: [PATCH v2 05/40] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma
From: sashiko-bot@kernel.org
Date: 2026-09-14 16:45:18
Also in:
bpf, dri-devel, kvm, kvmarm, linux-perf-users, linux-rdma, linux-s390, linux-scsi, selinux
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [Critical] State modification applied to `desc->vma_flags` during action preparation is not propagated back to `map->vma_flags`, resulting in incorrect VMA merging and the silent loss of critical flags like `VM_PFNMAP` and `VM_IO`. -- commit 31f214221abd5db93fe1f46874d29ea3c59a0cc7 Author: Lorenzo Stoakes (ARM) [off-list ref] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma When a user requests an mmap_action be performed in mmap_prepare, this involves populating the VMA range with data. This patch asserts that no future mmap action is performed on a mergeable VMA upon mmap_prepare validation.
quoted hunk ↗ jump to hunk
diff --git a/mm/vma.c b/mm/vma.c index 626a18c08a269..2648fb0e19959 100644 --- a/mm/vma.c +++ b/mm/vma.c@@ -2809,6 +2809,15 @@ static int mmap_validate(unsigned long prev_start, int mmap_prepare_validate(const struct vm_area_desc *prev_desc, const struct vm_area_desc *desc) { + /* + * It is not valid to execute mmap actions for VMAs which can be merged, + * as any such merge would leave portions of the mapping incorrectly + * unmapped. + */ + if (vma_flags_can_merge(&desc->vma_flags) && + WARN_ON_ONCE(desc->action.type != MMAP_NOTHING)) + return -EINVAL; +
[Severity: Critical]
Does this check actually prevent the merge? Looking at call_mmap_prepare(),
map->vma_flags is copied from desc->vma_flags before call_action_prepare()
is called:
mm/vma.c:call_mmap_prepare() {
...
map->vma_flags = desc->vma_flags;
...
err = call_action_prepare(map, desc);
if (err)
return err;
err = mmap_prepare_validate(&prev_desc, desc);
...
}
If call_action_prepare() mutates desc->vma_flags to include action-specific
unmergeable flags, vma_flags_can_merge(&desc->vma_flags) will be false here,
allowing the validation to pass. However, these changes don't appear to be
synchronized back to map->vma_flags.
Could this cause the actual VMA operations to operate on the stale and
mergeable map->vma_flags?
If a VMA lacks VM_PFNMAP or VM_IO but contains PFN-mapped pages, could this
lead to memory corruption if kernel mechanisms assume the pages are normal
memory and call put_page() on raw PFNs?
return mmap_validate(prev_desc->start, desc->start, &prev_desc->vma_flags, &desc->vma_flags); }
-- Sashiko AI review · https://sashiko.dev/#/patchset/20260914-b4-mmap-prepare-vma-flag-sanify-v2-0-7d9781ed5361@kernel.org?part=5