[PATCH v2 08/30] ext2: Define usercopy region in ext2_inode_cache slab cache

[Kees Cook <keescook@chromium.org>]

From: David Windsor <dave@nullcore.net>

The ext2 symlink pathnames, stored in struct ext2_inode_info.i_data and
therefore contained in the ext2_inode_cache slab cache, need to be copied
to/from userspace.

cache object allocation:
    fs/ext2/super.c:
        ext2_alloc_inode(...):
            struct ext2_inode_info *ei;
            ...
            ei = kmem_cache_alloc(ext2_inode_cachep, GFP_NOFS);
            ...
            return &ei->vfs_inode;

    fs/ext2/ext2.h:
        EXT2_I(struct inode *inode):
            return container_of(inode, struct ext2_inode_info, vfs_inode);

    fs/ext2/namei.c:
        ext2_symlink(...):
            ...
            inode->i_link = (char *)&EXT2_I(inode)->i_data;

example usage trace:
    readlink_copy+0x43/0x70
    vfs_readlink+0x62/0x110
    SyS_readlinkat+0x100/0x130

    fs/namei.c:
        readlink_copy(..., link):
            ...
            copy_to_user(..., link, len);

        (inlined into vfs_readlink)
        generic_readlink(dentry, ...):
            struct inode *inode = d_inode(dentry);
            const char *link = inode->i_link;
            ...
            readlink_copy(..., link);

In support of usercopy hardening, this patch defines a region in the
ext2_inode_cache slab cache in which userspace copy operations are
allowed.

This region is known as the slab cache's usercopy region. Slab caches can
now check that each copy operation involving cache-managed memory falls
entirely within the slab's usercopy region.

This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <dave@nullcore.net>
[kees: adjust commit log, provide usage trace]
Cc: Jan Kara <jack@suse.com>
Cc: linux-ext4@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/ext2/super.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 7b1bc9059863..670142cde59d 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -219,11 +219,13 @@ static void init_once(void *foo)
 
 static int __init init_inodecache(void)
 {
-	ext2_inode_cachep = kmem_cache_create("ext2_inode_cache",
-					     sizeof(struct ext2_inode_info),
-					     0, (SLAB_RECLAIM_ACCOUNT|
-						SLAB_MEM_SPREAD|SLAB_ACCOUNT),
-					     init_once);
+	ext2_inode_cachep = kmem_cache_create_usercopy("ext2_inode_cache",
+				sizeof(struct ext2_inode_info), 0,
+				(SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD|
+					SLAB_ACCOUNT),
+				offsetof(struct ext2_inode_info, i_data),
+				sizeof_field(struct ext2_inode_info, i_data),
+				init_once);
 	if (ext2_inode_cachep == NULL)
 		return -ENOMEM;
 	return 0;
-- 
2.7.4