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Subject[PATCH] dcache: ensure d_flags & d_inode are consistent in lookup_fast()
Date
After extending the size of dentry from 192-bytes to 208-bytes
under aarch64, we got oops during the running of xfstests generic/429:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000002
CPU: 3 PID: 2725 Comm: t_encrypted_d_r Tainted: G D 5.1.0-rc4
pc : inode_permission+0x28/0x160
lr : link_path_walk.part.11+0x27c/0x528
......
Call trace:
inode_permission+0x28/0x160
link_path_walk.part.11+0x27c/0x528
path_lookupat+0x64/0x208
filename_lookup+0xa0/0x178
user_path_at_empty+0x58/0x70
vfs_statx+0x94/0x118
__se_sys_newfstatat+0x58/0x98
__arm64_sys_newfstatat+0x24/0x30
el0_svc_common+0x7c/0x148
el0_svc_handler+0x38/0x88
el0_svc+0x8/0xc

If we revert the size extension of dentry, the oops will be gone.
However if we just move the d_inode field from the begin of dentry
struct to the end of dentry struct (poorly simulate the way how
__randomize_layout works), the oops will reoccur.

The following scenario illustrates the problem:

precondition:
* dentry A has just been unlinked and becomes a negative dentry
* dentry A is encrypted, so it has d_revalidate hook: fscrypt_d_revalidate()
* lookup process is looking A/file, and creation process is creating A

lookup process: creation process:

lookup_fast
__d_lookup_rcu returns dentry A

d_revalidate returns -ECHILD

d_revalidate again succeed
__d_set_inode_and_type
dentry->d_inode = inode
WRITE_ONCE(dentry->d_flags, flags)

d_is_negative(dentry) return false
follow_managed doesn't nothing
// inconsistent with d_flags
d_backing_inode() return NULL
nd->inode = NULL

may_lookup()
// oops occurs
inode_permission(nd->inode

The root cause is the inconsistency between d_flags & d_inode
during the REF-walk in lookup_fast(): d_is_negative(dentry)
returns false, but d_backing_inode() still returns a NULL pointer.

The RCU-walk path in lookup_fast() uses d_seq to ensure d_flags & d_inode
are consistent, and lookup_slow() use inode lock to ensure that, so only
the REF-walk path in lookup_fast() is problematic.

Fixing it by adding a paired smp_rmb/smp_wmb between the reading/writing
of d_inode & d_flags to ensure the consistency.

Signed-off-by: Hou Tao <houtao1@huawei.com>
---
fs/dcache.c | 2 ++
fs/namei.c | 7 +++++++
2 files changed, 9 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index aac41adf4743..1eb85f9fcb0f 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -316,6 +316,8 @@ static inline void __d_set_inode_and_type(struct dentry *dentry,
unsigned flags;

dentry->d_inode = inode;
+ /* paired with smp_rmb() in lookup_fast() */
+ smp_wmb();
flags = READ_ONCE(dentry->d_flags);
flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
flags |= type_flags;
diff --git a/fs/namei.c b/fs/namei.c
index dede0147b3f6..833f760c70b2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1628,6 +1628,13 @@ static int lookup_fast(struct nameidata *nd,
return -ENOENT;
}

+ /*
+ * Paired with smp_wmb() in __d_set_inode_and_type() to ensure
+ * d_backing_inode is not NULL after the checking of d_flags
+ * in d_is_negative() completes.
+ */
+ smp_rmb();
+
path->mnt = mnt;
path->dentry = dentry;
err = follow_managed(path, nd);
--
2.16.2.dirty
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