[PATCH] 9p: add missing locking around taking dentry fid list

[PATCH] 9p: add missing locking around taking dentry fid list

[Dominique Martinet]

Fix a use-after-free on dentry's d_fsdata fid list when a thread
lookups a fid through dentry while another thread unlinks it:

UAF thread:
refcount_t: addition on 0; use-after-free.
 p9_fid_get linux/./include/net/9p/client.h:262
 v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129
 v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181
 v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314
 v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400
 vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248

Freed by:
 p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456
 p9_fid_put linux/./include/net/9p/client.h:278
 v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55
 v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518
 vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335

The problem is that d_fsdata was not accessed under d_lock, because
d_release() normally is only called once the dentry is otherwise no
longer accessible but since we also call it explicitly in v9fs_remove
that lock is required:
move the hlist out of the dentry under lock then unref its fids once
they are no longer accessible.

Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom")
Cc: stable@vger.kernel.org
Reported-by: Meysam Firouzi
Reported-by: Amirmohammad Eftekhar
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
---
 fs/9p/vfs_dentry.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
index f16f73581634..01338d4c2d9e 100644
--- a/fs/9p/vfs_dentry.c
+++ b/fs/9p/vfs_dentry.c
@@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry)
 static void v9fs_dentry_release(struct dentry *dentry)
 {
 	struct hlist_node *p, *n;
+	struct hlist_head head;
 
 	p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n",
 		 dentry, dentry);
-	hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
+
+	spin_lock(&dentry->d_lock);
+	hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
+	spin_unlock(&dentry->d_lock);
+
+	hlist_for_each_safe(p, n, &head)
 		p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
-	dentry->d_fsdata = NULL;
 }
 
 static int v9fs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
-- 
2.44.0

Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Christian Schoenebeck]

On Tuesday, May 21, 2024 2:29:46 PM CEST Dominique Martinet wrote:
> Fix a use-after-free on dentry's d_fsdata fid list when a thread
> lookups a fid through dentry while another thread unlinks it:

I guess that's "looks up". :)

> UAF thread:
> refcount_t: addition on 0; use-after-free.
>  p9_fid_get linux/./include/net/9p/client.h:262
>  v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129
>  v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181
>  v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314
>  v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400
>  vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
> 
> Freed by:
>  p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456

That line number looks weird.

>  p9_fid_put linux/./include/net/9p/client.h:278
>  v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55
>  v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518
>  vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
> 
> The problem is that d_fsdata was not accessed under d_lock, because
> d_release() normally is only called once the dentry is otherwise no
> longer accessible but since we also call it explicitly in v9fs_remove
> that lock is required:
> move the hlist out of the dentry under lock then unref its fids once
> they are no longer accessible.
> 
> Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom")
> Cc: stable@vger.kernel.org
> Reported-by: Meysam Firouzi
> Reported-by: Amirmohammad Eftekhar
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> ---
>  fs/9p/vfs_dentry.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
> index f16f73581634..01338d4c2d9e 100644
> --- a/fs/9p/vfs_dentry.c
> +++ b/fs/9p/vfs_dentry.c
> @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry)
>  static void v9fs_dentry_release(struct dentry *dentry)
>  {
>  	struct hlist_node *p, *n;
> +	struct hlist_head head;
>  
>  	p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n",
>  		 dentry, dentry);
> -	hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
> +
> +	spin_lock(&dentry->d_lock);
> +	hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
> +	spin_unlock(&dentry->d_lock);
> +
> +	hlist_for_each_safe(p, n, &head)
>  		p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
> -	dentry->d_fsdata = NULL;
>  }

I'm not sure if that works out. So you are moving the list from dentry to a
local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads
dentry->d_fsdata (twice) and holds it as local variable before taking a
lock. So the lock in v9fs_fid_find() should happen earlier, no?

>  
>  static int v9fs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
> 

Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Dominique Martinet]

Christian Schoenebeck wrote on Wed, May 22, 2024 at 04:35:19PM +0200:

Thanks for the review!

> On Tuesday, May 21, 2024 2:29:46 PM CEST Dominique Martinet wrote:
> > Fix a use-after-free on dentry's d_fsdata fid list when a thread
> > lookups a fid through dentry while another thread unlinks it:
> 
> I guess that's "looks up". :)

Err, I guess.

> > UAF thread:
> > refcount_t: addition on 0; use-after-free.
> >  p9_fid_get linux/./include/net/9p/client.h:262
> >  v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129
> >  v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181
> >  v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314
> >  v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400
> >  vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
> > 
> > Freed by:
> >  p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456
> 
> That line number looks weird.

I have a p9_fid_destroy there (as of a v6.9-rc5 tree); might have moved
a bit though.
Unfortunately it's inlined so the stack trace only has kfree() next
which is why I cut the trace there; I don't think it really matters?

> >  p9_fid_put linux/./include/net/9p/client.h:278
> >  v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55
> >  v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518
> >  vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
> > 
> > The problem is that d_fsdata was not accessed under d_lock, because
> > d_release() normally is only called once the dentry is otherwise no
> > longer accessible but since we also call it explicitly in v9fs_remove
> > that lock is required:
> > move the hlist out of the dentry under lock then unref its fids once
> > they are no longer accessible.
> > 
> > Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom")
> > Cc: stable@vger.kernel.org
> > Reported-by: Meysam Firouzi
> > Reported-by: Amirmohammad Eftekhar
> > Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> > ---
> >  fs/9p/vfs_dentry.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
> > index f16f73581634..01338d4c2d9e 100644
> > --- a/fs/9p/vfs_dentry.c
> > +++ b/fs/9p/vfs_dentry.c
> > @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry)
> >  static void v9fs_dentry_release(struct dentry *dentry)
> >  {
> >  	struct hlist_node *p, *n;
> > +	struct hlist_head head;
> >  
> >  	p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n",
> >  		 dentry, dentry);
> > -	hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
> > +
> > +	spin_lock(&dentry->d_lock);
> > +	hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
> > +	spin_unlock(&dentry->d_lock);
> > +
> > +	hlist_for_each_safe(p, n, &head)
> >  		p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
> > -	dentry->d_fsdata = NULL;
> >  }
> 
> I'm not sure if that works out. So you are moving the list from dentry to a
> local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads
> dentry->d_fsdata (twice) and holds it as local variable before taking a
> lock. So the lock in v9fs_fid_find() should happen earlier, no?

The comment still works -- if detry->d_fsdata is NULL then
hlist_for_each_entry will stop short and not iterate over anything (it
won't bug out), so that part is fine in my opinion.

What should be improved though is that if dentry->d_inode we can still
look by inode even if there was a d_fsdata as log as fid wasn't found,
e.g.:
-----
diff --git a/fs/9p/fid.c b/fs/9p/fid.c
index de009a33e0e2..c72825fb0ece 100644
--- a/fs/9p/fid.c
+++ b/fs/9p/fid.c
@@ -131,9 +131,9 @@ static struct p9_fid *v9fs_fid_find(struct dentry *dentry, kuid_t uid, int any)
 			}
 		}
 		spin_unlock(&dentry->d_lock);
-	} else {
-		if (dentry->d_inode)
-			ret = v9fs_fid_find_inode(dentry->d_inode, false, uid, any);
+	}
+	if (!ret && dentry->d_inode)
+		ret = v9fs_fid_find_inode(dentry->d_inode, false, uid, any);
 	}
 
 	return ret;
----

I don't think that has to be part of this commit though, the worst that
can happen here is an extra lookup to server instead of a use after
free; I'll send a separate patch for this.

-- 
Dominique Martinet | Asmadeus

Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Christian Schoenebeck]

On Wednesday, May 22, 2024 7:25:06 PM CEST Dominique Martinet wrote:
> Christian Schoenebeck wrote on Wed, May 22, 2024 at 04:35:19PM +0200:
[...]
> > > diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
> > > index f16f73581634..01338d4c2d9e 100644
> > > --- a/fs/9p/vfs_dentry.c
> > > +++ b/fs/9p/vfs_dentry.c
> > > @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry)
> > >  static void v9fs_dentry_release(struct dentry *dentry)
> > >  {
> > >  	struct hlist_node *p, *n;
> > > +	struct hlist_head head;
> > >  
> > >  	p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n",
> > >  		 dentry, dentry);
> > > -	hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
> > > +
> > > +	spin_lock(&dentry->d_lock);
> > > +	hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
> > > +	spin_unlock(&dentry->d_lock);
> > > +
> > > +	hlist_for_each_safe(p, n, &head)
> > >  		p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
> > > -	dentry->d_fsdata = NULL;
> > >  }
> > 
> > I'm not sure if that works out. So you are moving the list from dentry to a
> > local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads
> > dentry->d_fsdata (twice) and holds it as local variable before taking a
> > lock. So the lock in v9fs_fid_find() should happen earlier, no?
> 
> The comment still works -- if detry->d_fsdata is NULL then
> hlist_for_each_entry will stop short and not iterate over anything (it
> won't bug out), so that part is fine in my opinion.

I meant the opposite: dentry->d_fsdata not being NULL. In this case
v9fs_fid_find() takes a local copy of the list head pointer as `h` without
taking a lock before.

Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this
point `h` could already point at garbage.

/Christian

Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Dominique Martinet]

Christian Schoenebeck wrote on Thu, May 23, 2024 at 10:34:14AM +0200:
> > The comment still works -- if detry->d_fsdata is NULL then
> > hlist_for_each_entry will stop short and not iterate over anything (it
> > won't bug out), so that part is fine in my opinion.
> 
> I meant the opposite: dentry->d_fsdata not being NULL.

I also meant that in the d_fsdata not being NULL branch, if d_fsdata
turns out to be NULL when it is read under lock later.

> In this case v9fs_fid_find() takes a local copy of the list head
> pointer as `h` without taking a lock before.

It doesn't, it takes &dentry->d_fsdata so the address of d_fsdata before
the lock, but that address cannot change here (another thread cannot
change the address of the dentry) ...(continuing below)

> Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this
> point `h` could already point at garbage.

... so *h (in practice, head->first in hlist_for_each_entry()) will
properly contain the first node of the list under lock: either NULL if
we just cleared it (at which point the loop won't iterate anything), or
a new list if other items have been added meanwhile.


I really think it's safe, but I do agree that it's hard to read, happy
to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it
compiles to the same code for me (x86_64/gcc 13.2.0)
-- 
Dominique Martinet | Asmadeus

Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Christian Schoenebeck]

On Thursday, May 23, 2024 11:27:28 AM CEST Dominique Martinet wrote:
> Christian Schoenebeck wrote on Thu, May 23, 2024 at 10:34:14AM +0200:
> > > The comment still works -- if detry->d_fsdata is NULL then
> > > hlist_for_each_entry will stop short and not iterate over anything (it
> > > won't bug out), so that part is fine in my opinion.
> > 
> > I meant the opposite: dentry->d_fsdata not being NULL.
> 
> I also meant that in the d_fsdata not being NULL branch, if d_fsdata
> turns out to be NULL when it is read under lock later.
> 
> > In this case v9fs_fid_find() takes a local copy of the list head
> > pointer as `h` without taking a lock before.
> 
> It doesn't, it takes &dentry->d_fsdata so the address of d_fsdata before
> the lock, but that address cannot change here (another thread cannot
> change the address of the dentry) ...(continuing below)

Aaah right, I was missing the `&`, my bad!

> > Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this
> > point `h` could already point at garbage.
> 
> ... so *h (in practice, head->first in hlist_for_each_entry()) will
> properly contain the first node of the list under lock: either NULL if
> we just cleared it (at which point the loop won't iterate anything), or
> a new list if other items have been added meanwhile.

Yeah, looks fine to me.

> I really think it's safe, but I do agree that it's hard to read, happy
> to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it
> compiles to the same code for me (x86_64/gcc 13.2.0)

No need, you can add my RB. Thanks for the clarification!

Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>

Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Dominique Martinet]

Christian Schoenebeck wrote on Thu, May 23, 2024 at 12:05:44PM +0200:
> > I really think it's safe, but I do agree that it's hard to read, happy
> > to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it
> > compiles to the same code for me (x86_64/gcc 13.2.0)
> 
> No need, you can add my RB. Thanks for the clarification!
> 
> Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>

Thanks!
I've fixed the typo in the commit message and queued it up in -next,
will send this patch and the trace uninit fix to Linus early next week.

-- 
Dominique Martinet | Asmadeus