Re: [RFC PATCH 3/5] mm: introduce VM_EXEC_KEEP

Kirill A. Shutemov
2020-07-29T13:52:10Z

On Mon, Jul 27, 2020 at 10:11:25AM -0700, Anthony Yznaga wrote:
> A vma with the VM_EXEC_KEEP flag is preserved across exec. For anonymous
> vmas only. For safety, overlap with fixed address VMAs created in the new
> mm during exec (e.g. the stack and elf load segments) is not permitted and
> will cause the exec to fail.
> (We are studying how to guarantee there are no conflicts. Comments welcome.)
>
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
> ---
> arch/x86/Kconfig | 1 +
> fs/exec.c | 20 ++++++++++++++++++++
> include/linux/mm.h | 5 +++++
> kernel/fork.c | 2 +-
> mm/mmap.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 74 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 883da0abf779..fc36eb2f45c0 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -30,6 +30,7 @@ config X86_64
> select MODULES_USE_ELF_RELA
> select NEED_DMA_MAP_STATE
> select SWIOTLB
> + select ARCH_USES_HIGH_VMA_FLAGS
>
> config FORCE_DYNAMIC_FTRACE
> def_bool y
> diff --git a/fs/exec.c b/fs/exec.c
> index 262112e5f9f8..1de09c4eef00 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1069,6 +1069,20 @@ ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
> EXPORT_SYMBOL(read_code);
> #endif
>
> +static int vma_dup_some(struct mm_struct *old_mm, struct mm_struct *new_mm)
> +{
> + struct vm_area_struct *vma;
> + int ret;
> +
> + for (vma = old_mm->mmap; vma; vma = vma->vm_next)
> + if (vma->vm_flags & VM_EXEC_KEEP) {
> + ret = vma_dup(vma, new_mm);
> + if (ret)
> + return ret;
> + }
> + return 0;
> +}
> +
> /*
> * Maps the mm_struct mm into the current task struct.
> * On success, this function returns with the mutex
> @@ -1104,6 +1118,12 @@ static int exec_mmap(struct mm_struct *mm)
> mutex_unlock(&tsk->signal->exec_update_mutex);
> return -EINTR;
> }
> + ret = vma_dup_some(old_mm, mm);
> + if (ret) {
> + mmap_read_unlock(old_mm);
> + mutex_unlock(&tsk->signal->exec_update_mutex);
> + return ret;
> + }
> }
>
> task_lock(tsk);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dc7b87310c10..1c538ba77f33 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -295,11 +295,15 @@ int overcommit_kbytes_handler(struct ctl_table *, int, void *, size_t *,
> #define VM_HIGH_ARCH_BIT_2 34 /* bit only usable on 64-bit architectures */
> #define VM_HIGH_ARCH_BIT_3 35 /* bit only usable on 64-bit architectures */
> #define VM_HIGH_ARCH_BIT_4 36 /* bit only usable on 64-bit architectures */
> +#define VM_HIGH_ARCH_BIT_5 37 /* bit only usable on 64-bit architectures */
> #define VM_HIGH_ARCH_0 BIT(VM_HIGH_ARCH_BIT_0)
> #define VM_HIGH_ARCH_1 BIT(VM_HIGH_ARCH_BIT_1)
> #define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2)
> #define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3)
> #define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4)
> +#define VM_EXEC_KEEP BIT(VM_HIGH_ARCH_BIT_5) /* preserve VMA across exec */
> +#else
> +#define VM_EXEC_KEEP VM_NONE
> #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
>
> #ifdef CONFIG_ARCH_HAS_PKEYS
> @@ -2534,6 +2538,7 @@ extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
> unsigned long addr, unsigned long len, pgoff_t pgoff,
> bool *need_rmap_locks);
> extern void exit_mmap(struct mm_struct *);
> +extern int vma_dup(struct vm_area_struct *vma, struct mm_struct *mm);
>
> static inline int check_data_rlimit(unsigned long rlim,
> unsigned long new,
> diff --git a/kernel/fork.c b/kernel/fork.c
> index efc5493203ae..15ead613714f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -564,7 +564,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> tmp->anon_vma = NULL;
> } else if (anon_vma_fork(tmp, mpnt))
> goto fail_nomem_anon_vma_fork;
> - tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
> + tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT | VM_EXEC_KEEP);
> file = tmp->vm_file;
> if (file) {
> struct inode *inode = file_inode(file);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 59a4682ebf3f..be2ff53743c3 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3279,6 +3279,53 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> return NULL;
> }
>
> +int vma_dup(struct vm_area_struct *old_vma, struct mm_struct *mm)
> +{
> + unsigned long npages;
> + struct mm_struct *old_mm = old_vma->vm_mm;
> + struct vm_area_struct *vma;
> + int ret = -ENOMEM;
> +
> + if (WARN_ON(old_vma->vm_file || old_vma->vm_ops))
> + return -EINVAL;
> +
> + vma = find_vma(mm, old_vma->vm_start);
> + if (vma && vma->vm_start < old_vma->vm_end)
> + return -EEXIST;
> +
> + npages = vma_pages(old_vma);
> + mm->total_vm += npages;

Why only total_vm? Where's exec_vm/stack_vm/data_vm?

> +
> + vma = vm_area_dup(old_vma);
> + if (!vma)
> + goto fail_nomem;
> +
> + ret = vma_dup_policy(old_vma, vma);
> + if (ret)
> + goto fail_nomem_policy;
> +
> + vma->vm_mm = mm;
> + ret = anon_vma_fork(vma, old_vma);
> + if (ret)
> + goto fail_nomem_anon_vma_fork;

Looks like a duplication of code form dup_mmap().
Any chance to get in one place?

> + vma->vm_flags &= ~(VM_LOCKED|VM_UFFD_MISSING|VM_UFFD_WP|VM_EXEC_KEEP);
> + vma->vm_next = vma->vm_prev = NULL;

No need. vm_area_dup() takes care of it.

> + vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

Semantics of VM_EXEC_KEEP vs userfaultfd() deserves a detailed explanation.
I feel these flags has to be mutually exclusive.

> + if (is_vm_hugetlb_page(vma))
> + reset_vma_resv_huge_pages(vma);
> + __insert_vm_struct(mm, vma);
> + ret = copy_page_range(mm, old_mm, old_vma);
> + return ret;
> +
> +fail_nomem_anon_vma_fork:
> + mpol_put(vma_policy(vma));
> +fail_nomem_policy:
> + vm_area_free(vma);
> +fail_nomem:
> + return -ENOMEM;
> +}
> +
> /*
> * Return true if the calling process may expand its vm space by the passed
> * number of pages
> --
> 1.8.3.1
>
>

--
Kirill A. Shutemov