[PATCH 0/2] ovl: Fix null ptr dereference at realinode in rcu-walk

[PATCH 0/2] ovl: Fix null ptr dereference at realinode in rcu-walk

[Zhihao Cheng]

Zhihao Cheng (2):
  ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk
    mode
  ovl: ovl_permission: Fix null pointer dereference at realinode in
    rcu-walk mode

 fs/overlayfs/inode.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

-- 
2.31.1

[PATCH 1/2] ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk mode

[Zhihao Cheng]

Following process:
         P1                     P2
 path_openat
  link_path_walk
   may_lookup
    inode_permission(rcu)
     ovl_permission
      acl_permission_check
       check_acl
        get_cached_acl_rcu
	 ovl_get_inode_acl
	  realinode = ovl_inode_real(ovl_inode)
	                      drop_cache
		               __dentry_kill(ovl_dentry)
				iput(ovl_inode)
		                 ovl_destroy_inode(ovl_inode)
		                  dput(oi->__upperdentry)
		                   dentry_kill(upperdentry)
		                    dentry_unlink_inode
				     upperdentry->d_inode = NULL
	    ovl_inode_upper
	     upperdentry = ovl_i_dentry_upper(ovl_inode)
	     d_inode(upperdentry) // returns NULL
	  IS_POSIXACL(realinode) // NULL pointer dereference
, will trigger an null pointer dereference at realinode:
  [  205.472797] BUG: kernel NULL pointer dereference, address:
                 0000000000000028
  [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
                 6.3.0-12064-g2edfa098e750-dirty #1216
  [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
  [  205.489584] Call Trace:
  [  205.489812]  <TASK>
  [  205.490014]  ovl_get_inode_acl+0x26/0x30
  [  205.490466]  get_cached_acl_rcu+0x61/0xa0
  [  205.490908]  generic_permission+0x1bf/0x4e0
  [  205.491447]  ovl_permission+0x79/0x1b0
  [  205.491917]  inode_permission+0x15e/0x2c0
  [  205.492425]  link_path_walk+0x115/0x550
  [  205.493311]  path_lookupat.isra.0+0xb2/0x200
  [  205.493803]  filename_lookup+0xda/0x240
  [  205.495747]  vfs_fstatat+0x7b/0xb0

Fetch a reproducer in [Link].

Fix it by checking realinode whether to be NULL before accessing it.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
---
 fs/overlayfs/inode.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 541cf3717fc2..38cfdf9e2b44 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -563,16 +563,16 @@ struct posix_acl *do_ovl_get_acl(struct mnt_idmap *idmap,
 	struct posix_acl *acl;
 	struct path realpath;
 
-	if (!IS_POSIXACL(realinode))
-		return NULL;
-
 	/* Careful in RCU walk mode */
 	ovl_i_path_real(inode, &realpath);
-	if (!realpath.dentry) {
+	if (!realpath.dentry || !realinode) {
 		WARN_ON(!rcu);
 		return ERR_PTR(-ECHILD);
 	}
 
+	if (!IS_POSIXACL(realinode))
+		return NULL;
+
 	if (rcu) {
 		/*
 		 * If the layer is idmapped drop out of RCU path walk
-- 
2.31.1

[PATCH 2/2] ovl: ovl_permission: Fix null pointer dereference at realinode in rcu-walk mode

[Zhihao Cheng]

Following process:
          P1                     P2
 path_lookupat
  link_path_walk
   inode_permission
    ovl_permission
      ovl_i_path_real(inode, &realpath)
        path->dentry = ovl_i_dentry_upper(inode)
                          drop_cache
			   __dentry_kill(ovl_dentry)
		            iput(ovl_inode)
		             ovl_destroy_inode(ovl_inode)
		              dput(oi->__upperdentry)
		               dentry_kill(upperdentry)
		                dentry_unlink_inode
				 upperdentry->d_inode = NULL
      realinode = d_inode(realpath.dentry) // return NULL
      inode_permission(realinode)
       inode->i_sb  // NULL pointer dereference
, will trigger an null pointer dereference at realinode:
  [  335.664979] BUG: kernel NULL pointer dereference,
                 address: 0000000000000002
  [  335.668032] CPU: 0 PID: 2592 Comm: ls Not tainted 6.3.0
  [  335.669956] RIP: 0010:inode_permission+0x33/0x2c0
  [  335.678939] Call Trace:
  [  335.679165]  <TASK>
  [  335.679371]  ovl_permission+0xde/0x320
  [  335.679723]  inode_permission+0x15e/0x2c0
  [  335.680090]  link_path_walk+0x115/0x550
  [  335.680771]  path_lookupat.isra.0+0xb2/0x200
  [  335.681170]  filename_lookup+0xda/0x240
  [  335.681922]  vfs_statx+0xa6/0x1f0
  [  335.682233]  vfs_fstatat+0x7b/0xb0

Fetch a reproducer in [Link].

Fix it by checking realinode whether to be NULL before accessing it.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217405
Fixes: 4b7791b2e958 ("ovl: handle idmappings in ovl_permission()")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
---
 fs/overlayfs/inode.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 38cfdf9e2b44..a4c56584b8c2 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -293,6 +293,11 @@ int ovl_permission(struct mnt_idmap *idmap,
 		WARN_ON(!(mask & MAY_NOT_BLOCK));
 		return -ECHILD;
 	}
+	realinode = d_inode(realpath.dentry);
+	if (!realinode) {
+		WARN_ON(!(mask & MAY_NOT_BLOCK));
+		return -ECHILD;
+	}
 
 	/*
 	 * Check overlay inode with the creds of task and underlying inode
@@ -302,7 +307,6 @@ int ovl_permission(struct mnt_idmap *idmap,
 	if (err)
 		return err;
 
-	realinode = d_inode(realpath.dentry);
 	old_cred = ovl_override_creds(inode->i_sb);
 	if (!upperinode &&
 	    !special_file(realinode->i_mode) && mask & MAY_WRITE) {
-- 
2.31.1

Re: [PATCH 0/2] ovl: Fix null ptr dereference at realinode in rcu-walk

[Christian Brauner]

On Fri, May 05, 2023 at 08:24:50PM +0800, Zhihao Cheng wrote:
> Zhihao Cheng (2):
>   ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk
>     mode
>   ovl: ovl_permission: Fix null pointer dereference at realinode in
>     rcu-walk mode
> 
>  fs/overlayfs/inode.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)

Seems fine to me. Thanks for fixing this,
Reviewed-by: Christian Brauner <brauner@kernel.org>

(One thing that might be possible is to move this into a tiny static
inline helper but might not be worth it.)

Re: [PATCH 1/2] ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk mode

[Christian Brauner]

On Fri, May 05, 2023 at 08:24:51PM +0800, Zhihao Cheng wrote:
> Following process:
>          P1                     P2
>  path_openat
>   link_path_walk
>    may_lookup
>     inode_permission(rcu)
>      ovl_permission
>       acl_permission_check
>        check_acl
>         get_cached_acl_rcu
> 	 ovl_get_inode_acl
> 	  realinode = ovl_inode_real(ovl_inode)
> 	                      drop_cache
> 		               __dentry_kill(ovl_dentry)
> 				iput(ovl_inode)
> 		                 ovl_destroy_inode(ovl_inode)
> 		                  dput(oi->__upperdentry)
> 		                   dentry_kill(upperdentry)
> 		                    dentry_unlink_inode
> 				     upperdentry->d_inode = NULL
> 	    ovl_inode_upper
> 	     upperdentry = ovl_i_dentry_upper(ovl_inode)
> 	     d_inode(upperdentry) // returns NULL
> 	  IS_POSIXACL(realinode) // NULL pointer dereference
> , will trigger an null pointer dereference at realinode:
>   [  205.472797] BUG: kernel NULL pointer dereference, address:
>                  0000000000000028
>   [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
>                  6.3.0-12064-g2edfa098e750-dirty #1216
>   [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
>   [  205.489584] Call Trace:
>   [  205.489812]  <TASK>
>   [  205.490014]  ovl_get_inode_acl+0x26/0x30
>   [  205.490466]  get_cached_acl_rcu+0x61/0xa0
>   [  205.490908]  generic_permission+0x1bf/0x4e0
>   [  205.491447]  ovl_permission+0x79/0x1b0
>   [  205.491917]  inode_permission+0x15e/0x2c0
>   [  205.492425]  link_path_walk+0x115/0x550
>   [  205.493311]  path_lookupat.isra.0+0xb2/0x200
>   [  205.493803]  filename_lookup+0xda/0x240
>   [  205.495747]  vfs_fstatat+0x7b/0xb0
> 
> Fetch a reproducer in [Link].
> 
> Fix it by checking realinode whether to be NULL before accessing it.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
> Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
> Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
> ---
>  fs/overlayfs/inode.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 541cf3717fc2..38cfdf9e2b44 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -563,16 +563,16 @@ struct posix_acl *do_ovl_get_acl(struct mnt_idmap *idmap,
>  	struct posix_acl *acl;
>  	struct path realpath;
>  
> -	if (!IS_POSIXACL(realinode))
> -		return NULL;
> -
>  	/* Careful in RCU walk mode */
>  	ovl_i_path_real(inode, &realpath);
> -	if (!realpath.dentry) {
> +	if (!realpath.dentry || !realinode) {
>  		WARN_ON(!rcu);
>  		return ERR_PTR(-ECHILD);
>  	}

I think the logic here is now a bit strange. I would just not bother
calling ovl_inode_real() anymore and simply use the same logic as in
ovl_permission() (Thus my comment about using a tiny helper.).