5.4-stable review patch. If anyone has any objections, please let me know.
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From: Al Viro viro@zeniv.linux.org.uk
[ Upstream commit b969f9614885c20f903e1d1f9445611daf161d6d ]
There's one case where ->d_compare() can be called for an in-lookup dentry; usually that's nothing special from ->d_compare() point of view, but... proc_sys_compare() is weird.
The thing is, /proc/sys subdirectories can look differently for different processes. Up to and including having the same name resolve to different dentries - all of them hashed.
The way it's done is ->d_compare() refusing to admit a match unless this dentry is supposed to be visible to this caller. The information needed to discriminate between them is stored in inode; it is set during proc_sys_lookup() and until it's done d_splice_alias() we really can't tell who should that dentry be visible for.
Normally there's no negative dentries in /proc/sys; we can run into a dying dentry in RCU dcache lookup, but those can be safely rejected.
However, ->d_compare() is also called for in-lookup dentries, before they get positive - or hashed, for that matter. In case of match we will wait until dentry leaves in-lookup state and repeat ->d_compare() afterwards. In other words, the right behaviour is to treat the name match as sufficient for in-lookup dentries; if dentry is not for us, we'll see that when we recheck once proc_sys_lookup() is done with it.
While we are at it, fix the misspelled READ_ONCE and WRITE_ONCE there.
Fixes: d9171b934526 ("parallel lookups machinery, part 4 (and last)") Reported-by: NeilBrown neilb@brown.name Reviewed-by: Christian Brauner brauner@kernel.org Reviewed-by: NeilBrown neil@brown.name Signed-off-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Sasha Levin sashal@kernel.org --- fs/proc/inode.c | 2 +- fs/proc/proc_sysctl.c | 18 +++++++++++------- 2 files changed, 12 insertions(+), 8 deletions(-)
diff --git a/fs/proc/inode.c b/fs/proc/inode.c index a6bb1b5c903e6..86175a8c04b3f 100644 --- a/fs/proc/inode.c +++ b/fs/proc/inode.c @@ -53,7 +53,7 @@ static void proc_evict_inode(struct inode *inode)
head = ei->sysctl; if (head) { - RCU_INIT_POINTER(ei->sysctl, NULL); + WRITE_ONCE(ei->sysctl, NULL); proc_sys_evict_inode(inode, head); } } diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c index f4264dd4ea31b..c8dad20069803 100644 --- a/fs/proc/proc_sysctl.c +++ b/fs/proc/proc_sysctl.c @@ -928,17 +928,21 @@ static int proc_sys_compare(const struct dentry *dentry, struct ctl_table_header *head; struct inode *inode;
- /* Although proc doesn't have negative dentries, rcu-walk means - * that inode here can be NULL */ - /* AV: can it, indeed? */ - inode = d_inode_rcu(dentry); - if (!inode) - return 1; if (name->len != len) return 1; if (memcmp(name->name, str, len)) return 1; - head = rcu_dereference(PROC_I(inode)->sysctl); + + // false positive is fine here - we'll recheck anyway + if (d_in_lookup(dentry)) + return 0; + + inode = d_inode_rcu(dentry); + // we just might have run into dentry in the middle of __dentry_kill() + if (!inode) + return 1; + + head = READ_ONCE(PROC_I(inode)->sysctl); return !head || !sysctl_is_seen(head); }