| From acf3062a7e1ccf67c6f7e7c28671a6708fde63b0 Mon Sep 17 00:00:00 2001 |
| From: Amir Goldstein <amir73il@gmail.com> |
| Date: Thu, 28 Mar 2019 17:38:29 +0200 |
| Subject: ovl: relax WARN_ON() for overlapping layers use case |
| |
| From: Amir Goldstein <amir73il@gmail.com> |
| |
| commit acf3062a7e1ccf67c6f7e7c28671a6708fde63b0 upstream. |
| |
| This nasty little syzbot repro: |
| https://syzkaller.appspot.com/x/repro.syz?x=12c7a94f400000 |
| |
| Creates overlay mounts where the same directory is both in upper and lower |
| layers. Simplified example: |
| |
| mkdir foo work |
| mount -t overlay none foo -o"lowerdir=.,upperdir=foo,workdir=work" |
| |
| The repro runs several threads in parallel that attempt to chdir into foo |
| and attempt to symlink/rename/exec/mkdir the file bar. |
| |
| The repro hits a WARN_ON() I placed in ovl_instantiate(), which suggests |
| that an overlay inode already exists in cache and is hashed by the pointer |
| of the real upper dentry that ovl_create_real() has just created. At the |
| point of the WARN_ON(), for overlay dir inode lock is held and upper dir |
| inode lock, so at first, I did not see how this was possible. |
| |
| On a closer look, I see that after ovl_create_real(), because of the |
| overlapping upper and lower layers, a lookup by another thread can find the |
| file foo/bar that was just created in upper layer, at overlay path |
| foo/foo/bar and hash the an overlay inode with the new real dentry as lower |
| dentry. This is possible because the overlay directory foo/foo is not |
| locked and the upper dentry foo/bar is in dcache, so ovl_lookup() can find |
| it without taking upper dir inode shared lock. |
| |
| Overlapping layers is considered a wrong setup which would result in |
| unexpected behavior, but it shouldn't crash the kernel and it shouldn't |
| trigger WARN_ON() either, so relax this WARN_ON() and leave a pr_warn() |
| instead to cover all cases of failure to get an overlay inode. |
| |
| The error returned from failure to insert new inode to cache with |
| inode_insert5() was changed to -EEXIST, to distinguish from the error |
| -ENOMEM returned on failure to get/allocate inode with iget5_locked(). |
| |
| Reported-by: syzbot+9c69c282adc4edd2b540@syzkaller.appspotmail.com |
| Fixes: 01b39dcc9568 ("ovl: use inode_insert5() to hash a newly...") |
| Signed-off-by: Amir Goldstein <amir73il@gmail.com> |
| Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| fs/overlayfs/dir.c | 2 +- |
| fs/overlayfs/inode.c | 3 ++- |
| 2 files changed, 3 insertions(+), 2 deletions(-) |
| |
| --- a/fs/overlayfs/dir.c |
| +++ b/fs/overlayfs/dir.c |
| @@ -260,7 +260,7 @@ static int ovl_instantiate(struct dentry |
| * hashed directory inode aliases. |
| */ |
| inode = ovl_get_inode(dentry->d_sb, &oip); |
| - if (WARN_ON(IS_ERR(inode))) |
| + if (IS_ERR(inode)) |
| return PTR_ERR(inode); |
| } else { |
| WARN_ON(ovl_inode_real(inode) != d_inode(newdentry)); |
| --- a/fs/overlayfs/inode.c |
| +++ b/fs/overlayfs/inode.c |
| @@ -832,7 +832,7 @@ struct inode *ovl_get_inode(struct super |
| int fsid = bylower ? oip->lowerpath->layer->fsid : 0; |
| bool is_dir, metacopy = false; |
| unsigned long ino = 0; |
| - int err = -ENOMEM; |
| + int err = oip->newinode ? -EEXIST : -ENOMEM; |
| |
| if (!realinode) |
| realinode = d_inode(lowerdentry); |
| @@ -917,6 +917,7 @@ out: |
| return inode; |
| |
| out_err: |
| + pr_warn_ratelimited("overlayfs: failed to get inode (%i)\n", err); |
| inode = ERR_PTR(err); |
| goto out; |
| } |