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

[Anthony Yznaga <anthony.yznaga@oracle.com>]

On 7/29/20 6:52 AM, Kirill A. Shutemov wrote:
> 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?
That was oversight.  Will be fixed in the next version.

>
>> +
>> +	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?
I looked at that, but dup_mmap() is dissimilar enough with
the additional fork-specific code in dup_mmap() that I think
readability would suffer.

>
>> +	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.
Will fix.

>
>> +	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.
Yes, will document this better.
I think it's okay to mark userfaultfd-enabled memory for preservation
as long as it is understood that memory would need to re-registered
with userfaultfd() after exec and restore, if desired.  Unless there's
a particular issue you see with this?

Thanks,
Anthony

>
>> +	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
>>
>>