Re: [RFC PATCH 1/7] mseal: expose interface to seal / unseal user memory ranges

Liam R. Howlett
2024-09-12T16:41:39Z

* Fares Mehanna <faresx@amazon.de> [240911 10:36]:
> To make sure the kernel mm-local mapping is untouched by the user, we will seal
> the VMA before changing the protection to be used by the kernel.
>
> This will guarantee that userspace can't unmap or alter this VMA while it is
> being used by the kernel.
>
> After the kernel is done with the secret memory, it will unseal the VMA to be
> able to unmap and free it.
>
> Unseal operation is not exposed to userspace.

We can't use the mseal feature for this; it is supposed to be a one way
transition.

Willy describes the feature best here [1].

It is not clear from the change log above or the cover letter as to why
you need to go this route instead of using the mmap lock.

[1] https://lore.kernel.org/lkml/ZS%2F3GCKvNn5qzhC4@casper.infradead.org/

>
> Signed-off-by: Fares Mehanna <faresx@amazon.de>
> Signed-off-by: Roman Kagan <rkagan@amazon.de>
> ---
> mm/internal.h | 7 +++++
> mm/mseal.c | 81 ++++++++++++++++++++++++++++++++-------------------
> 2 files changed, 58 insertions(+), 30 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index b4d86436565b..cf7280d101e9 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1501,6 +1501,8 @@ bool can_modify_mm(struct mm_struct *mm, unsigned long start,
> unsigned long end);
> bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start,
> unsigned long end, int behavior);
> +/* mm's mmap write lock must be taken before seal/unseal operation */
> +int do_mseal(unsigned long start, unsigned long end, bool seal);
> #else
> static inline int can_do_mseal(unsigned long flags)
> {
> @@ -1518,6 +1520,11 @@ static inline bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start,
> {
> return true;
> }
> +
> +static inline int do_mseal(unsigned long start, unsigned long end, bool seal)
> +{
> + return -EINVAL;
> +}
> #endif
>
> #ifdef CONFIG_SHRINKER_DEBUG
> diff --git a/mm/mseal.c b/mm/mseal.c
> index 15bba28acc00..aac9399ffd5d 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -26,6 +26,11 @@ static inline void set_vma_sealed(struct vm_area_struct *vma)
> vm_flags_set(vma, VM_SEALED);
> }
>
> +static inline void clear_vma_sealed(struct vm_area_struct *vma)
> +{
> + vm_flags_clear(vma, VM_SEALED);
> +}
> +
> /*
> * check if a vma is sealed for modification.
> * return true, if modification is allowed.
> @@ -117,7 +122,7 @@ bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start, unsigned long
>
> static int mseal_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
> struct vm_area_struct **prev, unsigned long start,
> - unsigned long end, vm_flags_t newflags)
> + unsigned long end, vm_flags_t newflags, bool seal)
> {
> int ret = 0;
> vm_flags_t oldflags = vma->vm_flags;
> @@ -131,7 +136,10 @@ static int mseal_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
> goto out;
> }
>
> - set_vma_sealed(vma);
> + if (seal)
> + set_vma_sealed(vma);
> + else
> + clear_vma_sealed(vma);
> out:
> *prev = vma;
> return ret;
> @@ -167,9 +175,9 @@ static int check_mm_seal(unsigned long start, unsigned long end)
> }
>
> /*
> - * Apply sealing.
> + * Apply sealing / unsealing.
> */
> -static int apply_mm_seal(unsigned long start, unsigned long end)
> +static int apply_mm_seal(unsigned long start, unsigned long end, bool seal)
> {
> unsigned long nstart;
> struct vm_area_struct *vma, *prev;
> @@ -191,11 +199,14 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
> unsigned long tmp;
> vm_flags_t newflags;
>
> - newflags = vma->vm_flags | VM_SEALED;
> + if (seal)
> + newflags = vma->vm_flags | VM_SEALED;
> + else
> + newflags = vma->vm_flags & ~(VM_SEALED);
> tmp = vma->vm_end;
> if (tmp > end)
> tmp = end;
> - error = mseal_fixup(&vmi, vma, &prev, nstart, tmp, newflags);
> + error = mseal_fixup(&vmi, vma, &prev, nstart, tmp, newflags, seal);
> if (error)
> return error;
> nstart = vma_iter_end(&vmi);
> @@ -204,6 +215,37 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
> return 0;
> }
>
> +int do_mseal(unsigned long start, unsigned long end, bool seal)
> +{
> + int ret;
> +
> + if (end < start)
> + return -EINVAL;
> +
> + if (end == start)
> + return 0;
> +
> + /*
> + * First pass, this helps to avoid
> + * partial sealing in case of error in input address range,
> + * e.g. ENOMEM error.
> + */
> + ret = check_mm_seal(start, end);
> + if (ret)
> + goto out;
> +
> + /*
> + * Second pass, this should success, unless there are errors
> + * from vma_modify_flags, e.g. merge/split error, or process
> + * reaching the max supported VMAs, however, those cases shall
> + * be rare.
> + */
> + ret = apply_mm_seal(start, end, seal);
> +
> +out:
> + return ret;
> +}
> +
> /*
> * mseal(2) seals the VM's meta data from
> * selected syscalls.
> @@ -256,7 +298,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
> *
> * unseal() is not supported.
> */
> -static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> +static int __do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> {
> size_t len;
> int ret = 0;
> @@ -277,33 +319,12 @@ static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> return -EINVAL;
>
> end = start + len;
> - if (end < start)
> - return -EINVAL;
> -
> - if (end == start)
> - return 0;
>
> if (mmap_write_lock_killable(mm))
> return -EINTR;
>
> - /*
> - * First pass, this helps to avoid
> - * partial sealing in case of error in input address range,
> - * e.g. ENOMEM error.
> - */
> - ret = check_mm_seal(start, end);
> - if (ret)
> - goto out;
> -
> - /*
> - * Second pass, this should success, unless there are errors
> - * from vma_modify_flags, e.g. merge/split error, or process
> - * reaching the max supported VMAs, however, those cases shall
> - * be rare.
> - */
> - ret = apply_mm_seal(start, end);
> + ret = do_mseal(start, end, true);
>
> -out:
> mmap_write_unlock(current->mm);
> return ret;
> }
> @@ -311,5 +332,5 @@ static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> SYSCALL_DEFINE3(mseal, unsigned long, start, size_t, len, unsigned long,
> flags)
> {
> - return do_mseal(start, len, flags);
> + return __do_mseal(start, len, flags);
> }
> --
> 2.40.1
>
>
>
>
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