Messages in this thread Patch in this message |  | | Date | Sun, 25 Sep 2022 21:29:32 +0900 | Subject | [PATCH] kernfs: fix UAF race condition in __kernfs_remove() | From | Tetsuo Handa <> |
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syzbot is reporting use-after-free read at __kernfs_remove() [1], for commit 35beab0635f3cdd4 ("kernfs: restructure removal path to fix possible premature return") missed that we need to keep a ref on "kn" as well as "pos".
This race condition happens when two concurrent removers "T1" and "T2" interfere due to kernfs_drain() temporarily dropping kernfs_rwsem.
T1: T2: down_write(&root->kernfs_rwsem); do { pos = kernfs_leftmost_descendant(kn); kernfs_get(pos); kernfs_drain(pos) { up_write(&root->kernfs_rwsem); down_write(&root->kernfs_rwsem); do { // Removes all children and "kn", but won't // free T1's "pos" and "kn", for T1 has a ref // on T1's "pos", and T1's "pos" in turn keeps // a ref on "kn". pos = kernfs_leftmost_descendant(kn); kernfs_put(pos); } while (pos != kn) // Will break. up_write(&root->kernfs_rwsem); down_write(&root->kernfs_rwsem); } // Frees "pos" because this was the last ref, and also frees "kn" // because a ref by "pos" was gone (i.e. "kn" no longer has ref) // via "goto repeat;" inside kernfs_put(). kernfs_put(pos); } while (pos != kn) // Will continue, despite "kn" already freed.
Link: https://syzkaller.appspot.com/bug?extid=8bee3285b9e190f1509e [1] Reported-by: syzbot+8bee3285b9e190f1509e@syzkaller.appspotmail.com Fixes: 35beab0635f3cdd4 ("kernfs: restructure removal path to fix possible premature return") Tested-by: syzbot+8bee3285b9e190f1509e@syzkaller.appspotmail.com Co-developed-by: Hillf Danton <hdanton@sina.com> Signed-off-by: Hillf Danton <hdanton@sina.com> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> --- fs/kernfs/dir.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c index 1cc88ba6de90..effb461d34fa 100644 --- a/fs/kernfs/dir.c +++ b/fs/kernfs/dir.c @@ -1365,6 +1365,11 @@ static void __kernfs_remove(struct kernfs_node *kn) atomic_add(KN_DEACTIVATED_BIAS, &pos->active); /* deactivate and unlink the subtree node-by-node */ + /* + * kernfs_put(pos) will invoke kernfs_put(kn) if @pos was the last + * reference to @kn. Make sure @kn doesn't go away underneath us. + */ + kernfs_get(kn); do { pos = kernfs_leftmost_descendant(kn); @@ -1406,6 +1411,7 @@ static void __kernfs_remove(struct kernfs_node *kn) kernfs_put(pos); } while (pos != kn); + kernfs_put(kn); } /** -- 2.34.1
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