Re: [RFC] is ovl_fh->fid really intended to be misaligned?

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, Nov 14, 2019 at 03:47:23PM +0000, Al Viro wrote:
> AFAICS, this
>         bytes = (fh->len - offsetof(struct ovl_fh, fid));
>         real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
>                                   bytes >> 2, (int)fh->type,
>                                   connected ? ovl_acceptable : NULL, mnt);
> in ovl_decode_real_fh() combined with
>                 origin = ovl_decode_real_fh(fh, ofs->lower_layers[i].mnt,
>                                             connected);
> in ovl_check_origin_fh(),
>         /* First lookup overlay inode in inode cache by origin fh */
>         err = ovl_check_origin_fh(ofs, fh, false, NULL, &stack);
> in ovl_lower_fh_to_d() and
>         struct ovl_fh *fh = (struct ovl_fh *) fid;
> ...
>                  ovl_lower_fh_to_d(sb, fh);
> in ovl_fh_to_dentry() leads to the pointer to struct fid passed to
> exportfs_decode_fh() being 21 bytes ahead of that passed to
> ovl_fh_to_dentry().  
> 
> However, alignment of struct fid pointers is 32 bits and quite a few
> places dealing with those (including ->fh_to_dentry() instances)
> do access them directly.  Argument of ->fh_to_dentry() is supposed
> to be 32bit-aligned, and callers generally guarantee that.  Your
> code, OTOH, violates the alignment systematically there - what
> it passes to layers' ->fh_to_dentry() (by way of exportfs_decode_fh())
> always has two lower bits different from what it got itself.
> 
> What do we do with that?  One solution would be to insert sane padding
> in ovl_fh, but the damn thing appears to be stored as-is in xattrs on
> disk, so that would require rather unpleasant operations reinserting
> the padding on the fly ;-/

Something like this?  Totally untested...

Thanks,
Miklos

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index b801c6353100..60a4ca72cb4e 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -253,7 +253,7 @@ struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper)
 
 	BUILD_BUG_ON(MAX_HANDLE_SZ + offsetof(struct ovl_fh, fid) > 255);
 	fh_len = offsetof(struct ovl_fh, fid) + buflen;
-	fh = kmalloc(fh_len, GFP_KERNEL);
+	fh = kzalloc(fh_len, GFP_KERNEL);
 	if (!fh) {
 		fh = ERR_PTR(-ENOMEM);
 		goto out;
@@ -271,7 +271,7 @@ struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper)
 	 */
 	if (is_upper)
 		fh->flags |= OVL_FH_FLAG_PATH_UPPER;
-	fh->len = fh_len;
+	fh->len = fh_len - OVL_FH_WIRE_OFFSET;
 	fh->uuid = *uuid;
 	memcpy(fh->fid, buf, buflen);
 
@@ -300,7 +300,8 @@ int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
 	/*
 	 * Do not fail when upper doesn't support xattrs.
 	 */
-	err = ovl_check_setxattr(dentry, upper, OVL_XATTR_ORIGIN, fh,
+	err = ovl_check_setxattr(dentry, upper, OVL_XATTR_ORIGIN,
+				 fh ? OVL_FH_START(fh) : NULL,
 				 fh ? fh->len : 0, 0);
 	kfree(fh);
 
@@ -317,7 +318,8 @@ static int ovl_set_upper_fh(struct dentry *upper, struct dentry *index)
 	if (IS_ERR(fh))
 		return PTR_ERR(fh);
 
-	err = ovl_do_setxattr(index, OVL_XATTR_UPPER, fh, fh->len, 0);
+	err = ovl_do_setxattr(index, OVL_XATTR_UPPER,
+			      OVL_FH_START(fh), fh->len, 0);
 
 	kfree(fh);
 	return err;
diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
index 73c9775215b3..dedda95c7746 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -234,7 +234,7 @@ static int ovl_d_to_fh(struct dentry *dentry, char *buf, int buflen)
 	if (fh->len > buflen)
 		goto fail;
 
-	memcpy(buf, (char *)fh, fh->len);
+	memcpy(buf, OVL_FH_START(fh), fh->len);
 	err = fh->len;
 
 out:
@@ -260,6 +260,7 @@ static int ovl_dentry_to_fh(struct dentry *dentry, u32 *fid, int *max_len)
 
 	/* Round up to dwords */
 	*max_len = (len + 3) >> 2;
+	memset(fid + len, 0, (*max_len << 2) - len);
 	return OVL_FILEID;
 }
 
@@ -781,7 +782,7 @@ static struct dentry *ovl_fh_to_dentry(struct super_block *sb, struct fid *fid,
 				       int fh_len, int fh_type)
 {
 	struct dentry *dentry = NULL;
-	struct ovl_fh *fh = (struct ovl_fh *) fid;
+	struct ovl_fh *fh = (void *) fid - OVL_FH_WIRE_OFFSET;
 	int len = fh_len << 2;
 	unsigned int flags = 0;
 	int err;
diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index e9717c2f7d45..f22a65359df1 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -86,7 +86,8 @@ static int ovl_acceptable(void *ctx, struct dentry *dentry)
  */
 int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
 {
-	if (fh_len < sizeof(struct ovl_fh) || fh_len < fh->len)
+	if (fh_len < sizeof(struct ovl_fh) - OVL_FH_WIRE_OFFSET ||
+	    fh_len < fh->len)
 		return -EINVAL;
 
 	if (fh->magic != OVL_FH_MAGIC)
@@ -119,11 +120,11 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 	if (res == 0)
 		return NULL;
 
-	fh = kzalloc(res, GFP_KERNEL);
+	fh = kzalloc(res + OVL_FH_WIRE_OFFSET, GFP_KERNEL);
 	if (!fh)
 		return ERR_PTR(-ENOMEM);
 
-	res = vfs_getxattr(dentry, name, fh, res);
+	res = vfs_getxattr(dentry, name, fh + OVL_FH_WIRE_OFFSET, res);
 	if (res < 0)
 		goto fail;
 
@@ -161,7 +162,7 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
 	if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
 		return NULL;
 
-	bytes = (fh->len - offsetof(struct ovl_fh, fid));
+	bytes = (fh->len + OVL_FH_WIRE_OFFSET - offsetof(struct ovl_fh, fid));
 	real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
 				  bytes >> 2, (int)fh->type,
 				  connected ? ovl_acceptable : NULL, mnt);
@@ -433,7 +434,8 @@ int ovl_verify_set_fh(struct dentry *dentry, const char *name,
 
 	err = ovl_verify_fh(dentry, name, fh);
 	if (set && err == -ENODATA)
-		err = ovl_do_setxattr(dentry, name, fh, fh->len, 0);
+		err = ovl_do_setxattr(dentry, name,
+				      OVL_FH_START(fh), fh->len, 0);
 	if (err)
 		goto fail;
 
@@ -512,12 +514,12 @@ int ovl_verify_index(struct ovl_fs *ofs, struct dentry *index)
 
 	err = -ENOMEM;
 	len = index->d_name.len / 2;
-	fh = kzalloc(len, GFP_KERNEL);
+	fh = kzalloc(len + OVL_FH_WIRE_OFFSET, GFP_KERNEL);
 	if (!fh)
 		goto fail;
 
 	err = -EINVAL;
-	if (hex2bin((u8 *)fh, index->d_name.name, len))
+	if (hex2bin(OVL_FH_START(fh), index->d_name.name, len))
 		goto fail;
 
 	err = ovl_check_fh_len(fh, len);
@@ -603,7 +605,7 @@ static int ovl_get_index_name_fh(struct ovl_fh *fh, struct qstr *name)
 	if (!n)
 		return -ENOMEM;
 
-	s  = bin2hex(n, fh, fh->len);
+	s  = bin2hex(n, OVL_FH_START(fh), fh->len);
 	*name = (struct qstr) QSTR_INIT(n, s - n);
 
 	return 0;
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 6934bcf030f0..c62083671a12 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -74,8 +74,13 @@ enum ovl_entry_flag {
 /* The type returned by overlay exportfs ops when encoding an ovl_fh handle */
 #define OVL_FILEID	0xfb
 
-/* On-disk and in-memeory format for redirect by file handle */
+#define OVL_FH_WIRE_OFFSET 3
+#define OVL_FH_START(fh) ((void *)(fh) + OVL_FH_WIRE_OFFSET)
 struct ovl_fh {
+	/* make sure fid is 32bit aligned */
+	u8 padding[OVL_FH_WIRE_OFFSET];
+
+	/* Wire/xattr encoding begins here*/
 	u8 version;	/* 0 */
 	u8 magic;	/* 0xfb */
 	u8 len;		/* size of this header + size of fid */