Re: [PATCH v2] mm: userfaultfd: avoid passing an invalid range to vma_merge()

Liam R. Howlett
2023-05-15T20:23:51Z

* Lorenzo Stoakes <lstoakes@gmail.com> [230515 15:32]:
> The userfaultfd_[un]register() functions will knowingly pass an invalid
> address range to vma_merge(), then rely on it failing to merge to indicate
> that the VMA should be split into a valid one.
>
> This is not something that should be relied upon, as vma_merge() implicitly
> assumes in cases 5-8 that curr->vm_start == addr. This is now enforced
> since commit b0729ae0ae67 ("mm/mmap/vma_merge: explicitly assign res, vma,
> extend invariants") with an explicit VM_WARN_ON() check.
>
> Since commit 29417d292bd0 ("mm/mmap/vma_merge: always check invariants")
> this check is performed unconditionally, which caused this assert to arise
> in tests performed by Mark [1].
>
> This patch fixes the issue by performing the split operations before
> attempting to merge VMAs in both instances. The problematic operation is
> splitting the start of the VMA since we were clamping to the end of the VMA
> in any case, however it is useful to group both of the split operations
> together to avoid egregious goto's and to abstract the code between the
> functions.
>
> As well as fixing the repro described in [1] this also continues to pass
> uffd unit tests.
>
> [1]:https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com
>
> Reported-by: Mark Rutland <mark.rutland@arm.com>
> Closes: https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
> fs/userfaultfd.c | 108 ++++++++++++++++++++++++++---------------------
> 1 file changed, 60 insertions(+), 48 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 0fd96d6e39ce..ef5d667ea804 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1319,6 +1319,32 @@ static __always_inline int validate_range(struct mm_struct *mm,
> return 0;
> }
>
> +static int clamp_range(struct vma_iterator *vmi, struct vm_area_struct *vma,
> + unsigned long start, unsigned long end, bool *can_merge)
> +{
> + int ret;
> + bool merge = true;
> +
> + /* The range must always be clamped to the start of a VMA. */
> + if (vma->vm_start < start) {
> + ret = split_vma(vmi, vma, start, 1);
> + if (ret)
> + return ret;
> +
> + merge = false;
> + }
> +
> + /* It must also be clamped to the end of a VMA. */
> + if (vma->vm_end > end) {
> + ret = split_vma(vmi, vma, end, 0);
> + if (ret)
> + return ret;
> + }
> +
> + *can_merge = merge;
> + return 0;
> +}
> +
> static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> unsigned long arg)
> {
> @@ -1330,7 +1356,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> unsigned long vm_flags, new_flags;
> bool found;
> bool basic_ioctls;
> - unsigned long start, end, vma_end;
> + unsigned long start, end;
> struct vma_iterator vmi;
>
> user_uffdio_register = (struct uffdio_register __user *) arg;
> @@ -1462,6 +1488,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>
> ret = 0;
> for_each_vma_range(vmi, vma, end) {
> + bool can_merge;
> +
> cond_resched();
>
> BUG_ON(!vma_can_userfault(vma, vm_flags));
> @@ -1477,32 +1505,22 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> (vma->vm_flags & vm_flags) == vm_flags)
> goto skip;
>
> - if (vma->vm_start > start)
> - start = vma->vm_start;
> - vma_end = min(end, vma->vm_end);
> + ret = clamp_range(&vmi, vma, start, end, &can_merge);
> + if (ret)
> + break;
>
> new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
> - prev = vma_merge(&vmi, mm, prev, start, vma_end, new_flags,
> - vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> - vma_policy(vma),
> - ((struct vm_userfaultfd_ctx){ ctx }),
> - anon_vma_name(vma));
> - if (prev) {
> + if (can_merge) {
> + prev = vma_merge(&vmi, mm, prev, vma->vm_start, vma->vm_end, new_flags,
> + vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> + vma_policy(vma),
> + ((struct vm_userfaultfd_ctx){ ctx }),
> + anon_vma_name(vma));
> +
> /* vma_merge() invalidated the mas */
> - vma = prev;
> - goto next;
> - }
> - if (vma->vm_start < start) {
> - ret = split_vma(&vmi, vma, start, 1);
> - if (ret)
> - break;
> - }
> - if (vma->vm_end > end) {
> - ret = split_vma(&vmi, vma, end, 0);
> - if (ret)
> - break;
> + if (prev)
> + vma = prev;
> }
> - next:
> /*
> * In the vma_merge() successful mprotect-like case 8:
> * the next vma was merged into the current one and
> @@ -1560,7 +1578,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> struct uffdio_range uffdio_unregister;
> unsigned long new_flags;
> bool found;
> - unsigned long start, end, vma_end;
> + unsigned long start, end;
> const void __user *buf = (void __user *)arg;
> struct vma_iterator vmi;
>
> @@ -1627,6 +1645,8 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> prev = vma_prev(&vmi);
> ret = 0;
> for_each_vma_range(vmi, vma, end) {
> + bool can_merge;
> +
> cond_resched();
>
> BUG_ON(!vma_can_userfault(vma, vma->vm_flags));
> @@ -1640,9 +1660,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
>
> WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
>
> - if (vma->vm_start > start)
> - start = vma->vm_start;
> - vma_end = min(end, vma->vm_end);
> + ret = clamp_range(&vmi, vma, start, end, &can_merge);
> + if (ret)
> + break;
>
> if (userfaultfd_missing(vma)) {
> /*
> @@ -1652,35 +1672,27 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> * UFFDIO_WAKE explicitly.
> */
> struct userfaultfd_wake_range range;
> - range.start = start;
> - range.len = vma_end - start;
> + range.start = vma->vm_start;
> + range.len = vma->vm_end - vma->vm_start;
> wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
> }
>
> /* Reset ptes for the whole vma range if wr-protected */
> if (userfaultfd_wp(vma))
> - uffd_wp_range(vma, start, vma_end - start, false);
> + uffd_wp_range(vma, vma->vm_start,
> + vma->vm_end - vma->vm_start, false);
>
> new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
> - prev = vma_merge(&vmi, mm, prev, start, vma_end, new_flags,
> - vma->anon_vma, vma->vm_file, vma->vm_pgoff,
> - vma_policy(vma),
> - NULL_VM_UFFD_CTX, anon_vma_name(vma));
> - if (prev) {
> - vma = prev;
> - goto next;
> - }
> - if (vma->vm_start < start) {
> - ret = split_vma(&vmi, vma, start, 1);
> - if (ret)
> - break;
> - }
> - if (vma->vm_end > end) {
> - ret = split_vma(&vmi, vma, end, 0);
> - if (ret)
> - break;
> + if (can_merge) {
> + prev = vma_merge(&vmi, mm, prev, vma->vm_start,
> + vma->vm_end, new_flags, vma->anon_vma,
> + vma->vm_file, vma->vm_pgoff,
> + vma_policy(vma),
> + NULL_VM_UFFD_CTX, anon_vma_name(vma));
> + /* vma_merge() invalidated the mas */
> + if (prev)
> + vma = prev;
> }
> - next:
> /*
> * In the vma_merge() successful mprotect-like case 8:
> * the next vma was merged into the current one and
> --
> 2.40.1
>