[RFC][PATCH] mm: Split locks in remap_file_pages()

[Roberto Sassu]

On Fri, 2024-10-18 at 16:42 +0100, Lorenzo Stoakes wrote:
> On Fri, Oct 18, 2024 at 04:47:10PM +0200, Roberto Sassu wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> > 
> > Commit ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in
> > remap_file_pages()") fixed a security issue, it added an LSM check when
> > trying to remap file pages, so that LSMs have the opportunity to evaluate
> > such action like for other memory operations such as mmap() and mprotect().
> > 
> > However, that commit called security_mmap_file() inside the mmap_lock lock,
> > while the other calls do it before taking the lock, after commit
> > 8b3ec6814c83 ("take security_mmap_file() outside of ->mmap_sem").
> > 
> > This caused lock inversion issue with IMA which was taking the mmap_lock
> > and i_mutex lock in the opposite way when the remap_file_pages() system
> > call was called.
> > 
> > Solve the issue by splitting the critical region in remap_file_pages() in
> > two regions: the first takes a read lock of mmap_lock and retrieves the VMA
> > and the file associated, and calculate the 'prot' and 'flags' variable; the
> > second takes a write lock on mmap_lock, checks that the VMA flags and the
> > VMA file descriptor are the same as the ones obtained in the first critical
> > region (otherwise the system call fails), and calls do_mmap().
> > 
> > In between, after releasing the read lock and taking the write lock, call
> > security_mmap_file(), and solve the lock inversion issue.
> 
> Great description!
> 
> > 
> > Cc: stable at vger.kernel.org
> > Fixes: ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in remap_file_pages()")
> > Reported-by: syzbot+91ae49e1c1a2634d20c0 at syzkaller.appspotmail.com
> > Closes: https://lore.kernel.org/linux-security-module/66f7b10e.050a0220.46d20.0036.GAE@google.com/
> > Reviewed-by: Roberto Sassu <roberto.sassu at huawei.com> (Calculate prot and flags earlier)
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> 
> Other than some nits below:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
> 
> I think you're definitely good to un-RFC here.

Perfect, will do. Thank you!

Roberto

> > ---
> >  mm/mmap.c | 62 ++++++++++++++++++++++++++++++++++++++++---------------
> >  1 file changed, 45 insertions(+), 17 deletions(-)
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 9c0fb43064b5..762944427e03 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1640,6 +1640,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> >  	unsigned long populate = 0;
> >  	unsigned long ret = -EINVAL;
> >  	struct file *file;
> > +	vm_flags_t vm_flags;
> > 
> >  	pr_warn_once("%s (%d) uses deprecated remap_file_pages() syscall. See Documentation/mm/remap_file_pages.rst.\n",
> >  		     current->comm, current->pid);
> > @@ -1656,12 +1657,53 @@ 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;
> 
> I'm kinda verbose generally, but I'd love a comment like:
> 
> 	/*
> 	 * Look up VMA under read lock first so we can perform the security
> 	 * without holding locks (which can be problematic). We reacquire a
> 	 * write lock later and check nothing changed underneath us.
> 	 */
> 
> > +
> > +	vma = vma_lookup(mm, start);
> > +
> > +	if (!vma || !(vma->vm_flags & VM_SHARED)) {
> > +		mmap_read_unlock(mm);
> > +		return -EINVAL;
> > +	}
> > +
> > +	prot |= vma->vm_flags & VM_READ ? PROT_READ : 0;
> > +	prot |= vma->vm_flags & VM_WRITE ? PROT_WRITE : 0;
> > +	prot |= vma->vm_flags & VM_EXEC ? PROT_EXEC : 0;
> > +
> > +	flags &= MAP_NONBLOCK;
> > +	flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
> > +	if (vma->vm_flags & VM_LOCKED)
> > +		flags |= MAP_LOCKED;
> > +
> > +	/* Save vm_flags used to calculate prot and flags, and recheck later. */
> > +	vm_flags = vma->vm_flags;
> > +	file = get_file(vma->vm_file);
> > +
> > +	mmap_read_unlock(mm);
> > +
> 
> Maybe worth adding a comment to explain why you're doing this without the
> lock so somebody looking at this later can understand the dance?
> 
> > +	ret = security_mmap_file(file, prot, flags);
> > +	if (ret) {
> > +		fput(file);
> > +		return ret;
> > +	}
> > +
> > +	ret = -EINVAL;
> > +
> 
> Again, being verbose, I'd put something here like:
> 
> 	/* OK security check passed, take write lock + let it rip */
> 
> > +	if (mmap_write_lock_killable(mm)) {
> > +		fput(file);
> >  		return -EINTR;
> > +	}
> > 
> >  	vma = vma_lookup(mm, start);
> > 
> > -	if (!vma || !(vma->vm_flags & VM_SHARED))
> > +	if (!vma)
> > +		goto out;
> > +
> 
> I'd also add something like:
> 
> 	/* Make sure things didn't change under us. */
> 
> > +	if (vma->vm_flags != vm_flags)
> > +		goto out;
> > +
> 
> And drop this newline to group them together (super nitty I know, sorry!)
> 
> > +	if (vma->vm_file != file)
> >  		goto out;
> > 
> >  	if (start + size > vma->vm_end) {
> > @@ -1689,25 +1731,11 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> >  			goto out;
> >  	}
> > 
> > -	prot |= vma->vm_flags & VM_READ ? PROT_READ : 0;
> > -	prot |= vma->vm_flags & VM_WRITE ? PROT_WRITE : 0;
> > -	prot |= vma->vm_flags & VM_EXEC ? PROT_EXEC : 0;
> > -
> > -	flags &= MAP_NONBLOCK;
> > -	flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
> > -	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))
> > --
> > 2.34.1
> > 
> 
> These are just nits, this looks good to me!