[PATCH] fs/inode.c: change the order of initialization in inode_init_always()

[PATCH] fs/inode.c: change the order of initialization in inode_init_always()

[Alexey Khoroshilov]

From: Rustam Subkhankulov <subkhankulov@ispras.ru>

If function 'security_inode_alloc()' returns a nonzero value at
[fs/inode.c: 195] due to an error (e.g. fail to allocate memory),
then some of the fields, including 'i_private', will not be
initialized.

After that, if the fs-specfic free_inode function is called in
'i_callback', the nonzero value of 'i_private' field can be interpreted
as initialized. As a result, this can cause dereferencing of random
value pointer (e.g. nilfs2).

In earlier versions, a similar situation could occur with the 'u' union
in 'inode' structure.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Signed-off-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
---
 fs/inode.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index bd4da9c5207e..08d093737e8c 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -192,8 +192,6 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	inode->i_wb_frn_history = 0;
 #endif
 
-	if (security_inode_alloc(inode))
-		goto out;
 	spin_lock_init(&inode->i_lock);
 	lockdep_set_class(&inode->i_lock, &sb->s_type->i_lock_key);
 
@@ -230,9 +228,10 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	inode->i_flctx = NULL;
 	this_cpu_inc(nr_inodes);
 
+	if (security_inode_alloc(inode))
+		return -ENOMEM;
+
 	return 0;
-out:
-	return -ENOMEM;
 }
 EXPORT_SYMBOL(inode_init_always);
 
-- 
2.34.1

Re: [PATCH] fs/inode.c: change the order of initialization in inode_init_always()

[Christian Brauner]

On Sat, Aug 27, 2022 at 05:09:26PM +0300, Alexey Khoroshilov wrote:
> From: Rustam Subkhankulov <subkhankulov@ispras.ru>
> 
> If function 'security_inode_alloc()' returns a nonzero value at
> [fs/inode.c: 195] due to an error (e.g. fail to allocate memory),
> then some of the fields, including 'i_private', will not be
> initialized.
> 
> After that, if the fs-specfic free_inode function is called in
> 'i_callback', the nonzero value of 'i_private' field can be interpreted
> as initialized. As a result, this can cause dereferencing of random
> value pointer (e.g. nilfs2).
> 
> In earlier versions, a similar situation could occur with the 'u' union
> in 'inode' structure.
> 
> Found by Linux Verification Center (linuxtesting.org) with syzkaller.
> 
> Signed-off-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> ---
>  fs/inode.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index bd4da9c5207e..08d093737e8c 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -192,8 +192,6 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
>  	inode->i_wb_frn_history = 0;
>  #endif
>  
> -	if (security_inode_alloc(inode))
> -		goto out;
>  	spin_lock_init(&inode->i_lock);
>  	lockdep_set_class(&inode->i_lock, &sb->s_type->i_lock_key);
>  
> @@ -230,9 +228,10 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
>  	inode->i_flctx = NULL;
>  	this_cpu_inc(nr_inodes);
>  
> +	if (security_inode_alloc(inode))
> +		return -ENOMEM;

This should probably be before this_cpu_inc(nr_inodes).

[PATCH v2] fs/inode.c: change the order of initialization in inode_init_always()

[Rustam Subkhankulov]

If function security_inode_alloc() returns a nonzero value due to an
error (e.g. fail to allocate memory), then some of the fields, including
'i_private', will not be initialized.

After that, if the fs-specfic free_inode function is called in
i_callback(), the nonzero value of 'i_private' field can be interpreted
as initialized. As a result, this can cause dereferencing of random
value pointer (e.g. nilfs2).

In earlier versions, a similar situation could occur with the 'u' union
in 'inode' structure.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Signed-off-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
---
 fs/inode.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index ba1de23c13c1..a2892d85993d 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -192,8 +192,6 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	inode->i_wb_frn_history = 0;
 #endif
 
-	if (security_inode_alloc(inode))
-		goto out;
 	spin_lock_init(&inode->i_lock);
 	lockdep_set_class(&inode->i_lock, &sb->s_type->i_lock_key);
 
@@ -228,6 +226,10 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
 	inode->i_fsnotify_mask = 0;
 #endif
 	inode->i_flctx = NULL;
+
+	if (security_inode_alloc(inode))
+		goto out;
+
 	this_cpu_inc(nr_inodes);
 
 	return 0;
-- 
2.34.1

Re: [PATCH v2] fs/inode.c: change the order of initialization in inode_init_always()

[Al Viro]

On Mon, Aug 29, 2022 at 10:25:21PM +0300, Rustam Subkhankulov wrote:
> If function security_inode_alloc() returns a nonzero value due to an
> error (e.g. fail to allocate memory), then some of the fields, including
> 'i_private', will not be initialized.
> 
> After that, if the fs-specfic free_inode function is called in
> i_callback(), the nonzero value of 'i_private' field can be interpreted
> as initialized. As a result, this can cause dereferencing of random
> value pointer (e.g. nilfs2).
> 
> In earlier versions, a similar situation could occur with the 'u' union
> in 'inode' structure.

See vfs.git#work.inode (included into #for-next); I agree that your
commit message looks better, but...