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

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Message-ID: <gl4pf7gezpjtvnbp4lzyb65wqaiw3xzjjrs3476j5odxsfzvsj@oouue73v3cgr>
Date: Fri, 18 Oct 2024 14:05:27 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>, 
	Paul Moore <paul@...l-moore.com>, ebpqwerty472123@...il.com, kirill.shutemov@...ux.intel.com, 
	zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, eric.snowberg@...cle.com, 
	jmorris@...ei.org, serge@...lyn.com, linux-integrity@...r.kernel.org, 
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	Roberto Sassu <roberto.sassu@...wei.com>, linux-mm@...ck.org, akpm@...ux-foundation.org, vbabka@...e.cz, 
	linux-fsdevel@...r.kernel.org, Liam Howlett <liam.howlett@...cle.com>, 
	Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/3] ima: Remove inode lock

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);
+	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))
-- 
  Kiryl Shutsemau / Kirill A. Shutemov