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rcu pathwalk: prevent bogus hard errors from may_lookup()

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If lazy call of ->permission() returns a hard error, check that
try_to_unlazy() succeeds before returning it.  That both makes
life easier for ->permission() instances and closes the race
in ENOTDIR handling - it is possible that positive d_can_lookup()
seen in link_path_walk() applies to the state *after* unlink() +
mkdir(), while nd->inode matches the state prior to that.

Normally seeing e.g. EACCES from permission check in rcu pathwalk
means that with some timings non-rcu pathwalk would've run into
the same; however, running into a non-executable regular file
in the middle of a pathname would not get to permission check -
it would fail with ENOTDIR instead.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2023-09-29 21:11:41 -04:00
parent 583340de1d
commit cdb67fdeed
1 changed files with 5 additions and 1 deletions
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6
fs/namei.c
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@ -1717,7 +1717,11 @@ static inline int may_lookup(struct mnt_idmap *idmap,
{
if (nd->flags & LOOKUP_RCU) {
int err = inode_permission(idmap, nd->inode, MAY_EXEC|MAY_NOT_BLOCK);
if (err != -ECHILD || !try_to_unlazy(nd))
if (!err) // success, keep going
return 0;
if (!try_to_unlazy(nd))
return -ECHILD; // redo it all non-lazy
if (err != -ECHILD) // hard error
return err;
}
return inode_permission(idmap, nd->inode, MAY_EXEC);