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@xxxxxxxxxxxxxxxxxxxxxxxxx Fixes: 01b39dcc9568 ("ovl: use inode_insert5() to hash a newly...") Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx> --- fs/overlayfs/dir.c | 2 +- fs/overlayfs/inode.c | 3 ++- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c index 82c129bfe58d..93872bb50230 100644 --- a/fs/overlayfs/dir.c +++ b/fs/overlayfs/dir.c @@ -260,7 +260,7 @@ static int ovl_instantiate(struct dentry *dentry, struct inode *inode, * 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)); diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c index 3b7ed5d2279c..b48273e846ad 100644 --- a/fs/overlayfs/inode.c +++ b/fs/overlayfs/inode.c @@ -832,7 +832,7 @@ struct inode *ovl_get_inode(struct super_block *sb, 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 @@ struct inode *ovl_get_inode(struct super_block *sb, return inode; out_err: + pr_warn_ratelimited("overlayfs: failed to get inode (%i)\n", err); inode = ERR_PTR(err); goto out; } -- 2.17.1
