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Wed, 29 Jul 2026 02:51:51 -0700 (PDT) From: Jeffin Philip To: joshua.crofts1@gmail.com Cc: a.hindborg@kernel.org, leitao@debian.org, linux-kernel@vger.kernel.org, syzbot+35790eb7861f8fc57382@syzkaller.appspotmail.com, Jeffin Philip Subject: Re: [PATCH] configfs: fix refcount warning in configfs_get_config_item() Date: Wed, 29 Jul 2026 15:21:25 +0530 Message-ID: <20260729095125.96899-1-jeffinphilip14@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260729110018.00005e66@gmail.com> References: <20260729110018.00005e66@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Wed, 29 Jul 2026 09:00 +0000 Joshua Crofts wrote: >On Wed, 29 Jul 2026 08:55:05 +0000 >Joshua Crofts wrote: > >> syzbot reported a "refcount_t: addition on 0; use-after-free" warning >> in configfs_get_config_item(). >> >> This occurs when configfs_get_config_item() races with a concurrent >> teardown (e.g. rmdir). When the target config_item's refcount drops to >> 0, configfs_get_config_item() calls config_item_get(), which >> unconditionally increments the refcount via kref_get(), triggering the >> refcount warning. >> >> Fix this by using config_item_get_unless_zero(), which safely returns >> NULL if the refcount is already 0. >> Reported-by: syzbot+35790eb7861f8fc57382@syzkaller.appspotmail.com >> Closes: https://syzkaller.appspot.com/bug?extid=35790eb7861f8fc57382 >> Signed-off-by: Joshua Crofts >> --- >> fs/configfs/configfs_internal.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/fs/configfs/configfs_internal.h b/fs/configfs/configfs_internal.h >> index acdeea8e2..5448af3a8 100644 >> --- a/fs/configfs/configfs_internal.h >> +++ b/fs/configfs/configfs_internal.h >> @@ -124,7 +124,7 @@ static inline struct config_item *configfs_get_config_item(struct dentry *dentry >> spin_lock(&dentry->d_lock); >> if (!d_unhashed(dentry)) { >> struct configfs_dirent * sd = dentry->d_fsdata; >> - item = config_item_get(sd->s_element); >> + item = config_item_get_unless_zero(sd->s_element); >> } >> spin_unlock(&dentry->d_lock); I tested this solution first on syzbot and well, it errored out. But on testing it on a local build, it seems to trigger a different warning in configfs_get() regarding atomic count. Could you please look into it? Thanks, Jeffin.