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Subject[PATCH 1/2] vfs: check dentry is still valid in get_link()
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When following a trailing symlink in rcu-walk mode it's possible for
the dentry to become invalid between the last dentry seq lock check
and getting the link (eg. an unlink) leading to a backtrace similar
to this:

crash> bt
PID: 10964 TASK: ffff951c8aa92f80 CPU: 3 COMMAND: "TaniumCX"

#7 [ffffae44d0a6fbe0] page_fault at ffffffff8d6010fe
[exception RIP: unknown or invalid address]
RIP: 0000000000000000 RSP: ffffae44d0a6fc90 RFLAGS: 00010246
RAX: ffffffff8da3cc80 RBX: ffffae44d0a6fd30 RCX: 0000000000000000
RDX: ffffae44d0a6fd98 RSI: ffff951aa9af3008 RDI: 0000000000000000
RBP: 0000000000000000 R8: ffffae44d0a6fb94 R9: 0000000000000000
R10: ffff951c95d8c318 R11: 0000000000080000 R12: ffffae44d0a6fd98
R13: ffff951aa9af3008 R14: ffff951c8c9eb840 R15: 0000000000000000
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
#8 [ffffae44d0a6fc90] trailing_symlink at ffffffff8cf24e61
#9 [ffffae44d0a6fcc8] path_lookupat at ffffffff8cf261d1
#10 [ffffae44d0a6fd28] filename_lookup at ffffffff8cf2a700
#11 [ffffae44d0a6fe40] vfs_statx at ffffffff8cf1dbc4
#12 [ffffae44d0a6fe98] __do_sys_newstat at ffffffff8cf1e1f9
#13 [ffffae44d0a6ff38] do_syscall_64 at ffffffff8cc0420b

Most of the time this is not a problem because the inode is unchanged
while the rcu read lock is held.

But xfs can re-use inodes which can result in the inode ->get_link()
method becoming invalid (or NULL).

This case needs to be checked for in fs/namei.c:get_link() and if
detected the walk re-started.

Signed-off-by: Ian Kent <raven@themaw.net>
---
fs/namei.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1946d9667790..9a48a6106516 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1760,8 +1760,11 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
if (!res) {
const char * (*get)(struct dentry *, struct inode *,
struct delayed_call *);
- get = inode->i_op->get_link;
+ get = READ_ONCE(inode->i_op->get_link);
if (nd->flags & LOOKUP_RCU) {
+ /* Does the inode still match the associated dentry? */
+ if (unlikely(read_seqcount_retry(&link->dentry->d_seq, last->seq)))
+ return ERR_PTR(-ECHILD);
res = get(NULL, inode, &last->done);
if (res == ERR_PTR(-ECHILD) && try_to_unlazy(nd))
res = get(link->dentry, inode, &last->done);

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