From 22213318af7ae265bc6cd8aef2febbc2d69a2440 Mon Sep 17 00:00:00 2001 From: Al Viro Date: Sat, 19 Apr 2014 12:30:58 -0400 Subject: [PATCH] fix races between __d_instantiate() and checks of dentry flags in non-lazy walk we need to be careful about dentry switching from negative to positive - both ->d_flags and ->d_inode are updated, and in some places we might see only one store. The cases where dentry has been obtained by dcache lookup with ->i_mutex held on parent are safe - ->d_lock and ->i_mutex provide all the barriers we need. However, there are several places where we run into trouble: * do_last() fetches ->d_inode, then checks ->d_flags and assumes that inode won't be NULL unless d_is_negative() is true. Race with e.g. creat() - we might have fetched the old value of ->d_inode (still NULL) and new value of ->d_flags (already not DCACHE_MISS_TYPE). Lin Ming has observed and reported the resulting oops. * a bunch of places checks ->d_inode for being non-NULL, then checks ->d_flags for "is it a symlink". Race with symlink(2) in case if our CPU sees ->d_inode update first - we see non-NULL there, but ->d_flags still contains DCACHE_MISS_TYPE instead of DCACHE_SYMLINK_TYPE. Result: false negative on "should we follow link here?", with subsequent unpleasantness. Cc: stable@vger.kernel.org # 3.13 and 3.14 need that one Reported-and-tested-by: Lin Ming Signed-off-by: Al Viro --- fs/dcache.c | 3 +-- fs/namei.c | 6 +++--- 2 files changed, 4 insertions(+), 5 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 40707d88a945..494a9def5dce 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1647,8 +1647,7 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode) unsigned add_flags = d_flags_for_inode(inode); spin_lock(&dentry->d_lock); - dentry->d_flags &= ~DCACHE_ENTRY_TYPE; - dentry->d_flags |= add_flags; + __d_set_type(dentry, add_flags); if (inode) hlist_add_head(&dentry->d_alias, &inode->i_dentry); dentry->d_inode = inode; diff --git a/fs/namei.c b/fs/namei.c index c6157c894fce..80168273396b 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1542,7 +1542,7 @@ static inline int walk_component(struct nameidata *nd, struct path *path, inode = path->dentry->d_inode; } err = -ENOENT; - if (!inode) + if (!inode || d_is_negative(path->dentry)) goto out_path_put; if (should_follow_link(path->dentry, follow)) { @@ -2249,7 +2249,7 @@ mountpoint_last(struct nameidata *nd, struct path *path) mutex_unlock(&dir->d_inode->i_mutex); done: - if (!dentry->d_inode) { + if (!dentry->d_inode || d_is_negative(dentry)) { error = -ENOENT; dput(dentry); goto out; @@ -2994,7 +2994,7 @@ retry_lookup: finish_lookup: /* we _can_ be in RCU mode here */ error = -ENOENT; - if (d_is_negative(path->dentry)) { + if (!inode || d_is_negative(path->dentry)) { path_to_nameidata(path, nd); goto out; }