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

[Kirill A. Shutemov]

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@xxxxxxxxxx>
> Signed-off-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
> ---
>  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