Re: [PATCH 1/3] ima: Remove inode lock

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Fri, 2024-10-18 at 14:05 +0300, Kirill A. Shutemov wrote:
> On Fri, Oct 18, 2024 at 12:00:22PM +0100, Lorenzo Stoakes wrote:
> > + Liam, Jann
> > 
> > On Fri, Oct 18, 2024 at 01:49:06PM +0300, Kirill A. Shutemov wrote:
> > > On Fri, Oct 18, 2024 at 11:24:06AM +0200, Roberto Sassu wrote:
> > > > Probably it is hard, @Kirill would there be any way to safely move
> > > > security_mmap_file() out of the mmap_lock lock?
> > > 
> > > What about something like this (untested):
> > > 
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index dd4b35a25aeb..03473e77d356 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1646,6 +1646,26 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> > >  	if (pgoff + (size >> PAGE_SHIFT) < pgoff)
> > >  		return ret;
> > > 
> > > +	if (mmap_read_lock_killable(mm))
> > > +		return -EINTR;
> > > +
> > > +	vma = vma_lookup(mm, start);
> > > +
> > > +	if (!vma || !(vma->vm_flags & VM_SHARED)) {
> > > +		mmap_read_unlock(mm);
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	file = get_file(vma->vm_file);
> > > +
> > > +	mmap_read_unlock(mm);
> > > +
> > > +	ret = security_mmap_file(vma->vm_file, prot, flags);
> > 
> > Accessing VMA fields without any kind of lock is... very much not advised.
> > 
> > I'm guessing you meant to say:
> > 
> > 	ret = security_mmap_file(file, prot, flags);
> > 
> > Here? :)
> 
> Sure. My bad.
> 
> Patch with all fixups:
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index dd4b35a25aeb..541787d526b6 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1646,14 +1646,41 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
>  	if (pgoff + (size >> PAGE_SHIFT) < pgoff)
>  		return ret;
>  
> -	if (mmap_write_lock_killable(mm))
> +	if (mmap_read_lock_killable(mm))
>  		return -EINTR;
>  
>  	vma = vma_lookup(mm, start);
>  
> +	if (!vma || !(vma->vm_flags & VM_SHARED)) {
> +		mmap_read_unlock(mm);
> +		return -EINVAL;
> +	}
> +
> +	file = get_file(vma->vm_file);
> +
> +	mmap_read_unlock(mm);
> +
> +	ret = security_mmap_file(file, prot, flags);

Uhm, I have to calculate prot and flags before. I can check if what I
used here changed in the next lock, and refuse.

Roberto

> +	if (ret) {
> +		fput(file);
> +		return ret;
> +	}
> +
> +	ret = -EINVAL;
> +
> +	if (mmap_write_lock_killable(mm)) {
> +		fput(file);
> +		return -EINTR;
> +	}
> +
> +	vma = vma_lookup(mm, start);
> +
>  	if (!vma || !(vma->vm_flags & VM_SHARED))
>  		goto out;
>  
> +	if (vma->vm_file != file)
> +		goto out;
> +
>  	if (start + size > vma->vm_end) {
>  		VMA_ITERATOR(vmi, mm, vma->vm_end);
>  		struct vm_area_struct *next, *prev = vma;
> @@ -1688,16 +1715,11 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
>  	if (vma->vm_flags & VM_LOCKED)
>  		flags |= MAP_LOCKED;
>  
> -	file = get_file(vma->vm_file);
> -	ret = security_mmap_file(vma->vm_file, prot, flags);
> -	if (ret)
> -		goto out_fput;
>  	ret = do_mmap(vma->vm_file, start, size,
>  			prot, flags, 0, pgoff, &populate, NULL);
> -out_fput:
> -	fput(file);
>  out:
>  	mmap_write_unlock(mm);
> +	fput(file);
>  	if (populate)
>  		mm_populate(ret, populate);
>  	if (!IS_ERR_VALUE(ret))