Re: [PATCH v3 05/40] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma
From: Suren Baghdasaryan <surenb@google.com>
Date: 2026-09-25 07:29:02
Also in:
bpf, fuse-devel, kvm, kvm-riscv, kvmarm, linux-arch, linux-doc, linux-fbdev, linux-fsdevel, linux-mm, linux-perf-users, linux-rdma, linux-s390, linux-scsi, linux-sound, linux-usb, linuxppc-dev, lkml, selinux, sparclinux
On Thu, Sep 17, 2026 at 9:25 AM Lorenzo Stoakes (ARM) [off-list ref] wrote:
When a user requests an mmap_action be performed in mmap_prepare, this involves populating the VMA range with data. However, if the VMA is mergeable, it might then mistakenly be merged with another VMA without having populated the range. Every mmap action currently available sets VMA flags such that the VMA cannot be merged. However, to ensure that no future mmap action falls foul of this, assert that this is the case upon mmap_prepare validation. Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
quoted hunk ↗ jump to hunk
--- mm/vma.c | 9 +++++++++ 1 file changed, 9 insertions(+)diff --git a/mm/vma.c b/mm/vma.c index d6ed10cefc8f..62f2ce1ad5a1 100644 --- a/mm/vma.c +++ b/mm/vma.c@@ -2809,6 +2809,15 @@ static int mmap_validate(unsigned long prev_start, unsigned long prev_end, 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))
Any reason you chose this "if (A && WARN_ON_ONCE(B))" pattern instead of a simpler "if (WARN_ON_ONCE(A && B))"? Unless there are races between A and B updates, I think these would be equivalent, right?
+ return -EINVAL;
+
return mmap_validate(prev_desc->start, prev_desc->end,
desc->start, desc->end,
&prev_desc->vma_flags, &desc->vma_flags);
--
2.55.0