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[163.53.146.70]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ad60f36a5asm3487902a91.9.2026.10.20.03.50.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 03:50:36 -0700 (PDT) From: Matt Bobrowski To: bpf@vger.kernel.org Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai , Christian Brauner , Alexander Viro , Jan Kara , linux-fsdevel@vger.kernel.org, Matt Bobrowski , stable@vger.kernel.org Subject: [PATCH bpf-next 1/2] bpf: fix crash in bpf_path_d_path() for exiting tasks Date: Tue, 20 Oct 2026 21:50:13 +1100 Message-ID: <20261020105014.97985-2-matt@bobrowski.net> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261020105014.97985-1-matt@bobrowski.net> References: <20261020105014.97985-1-matt@bobrowski.net> X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit bpf_path_d_path() passes its path to d_path(), which resolves it against current->fs through get_fs_root_rcu() without checking that the calling task still has one. An exiting task does not. do_exit() calls exit_fs(), which clears tsk->fs, before exit_task_work(), and a file whose last reference the task drops in exit_files() has its final __fput() deferred to that task work. A BPF LSM program attached to security_file_release, which may pass file->f_path to bpf_path_d_path() as a trusted argument, dereferences a NULL fs whenever a task exits with a file still open: Call trace: lock_acquire+0x8c/0x218 d_path+0x270/0x378 bpf_path_d_path+0x28/0x80 bpf_prog_9801cf2b982f05b2_file_release+0x1d0/0xc60 bpf_trampoline_6442515074+0x7c/0x100 bpf_lsm_file_release+0x8/0x20 security_file_release+0x58/0x90 __fput+0xcc/0x320 ____fput+0x1c/0x30 task_work_run+0x88/0x100 do_exit+0x210/0x3e8 do_group_exit+0x3c/0xa0 __arm64_sys_exit_group+0x20/0x30 With lockdep enabled the fault is taken in lock_acquire(), from the seqcount read in get_fs_root_rcu(); without it, on the read of fs->seq itself. Fix this by returning -EINVAL from bpf_path_d_path() when current->fs is NULL. Only the task itself clears its fs, in exit_fs(), so the check cannot race with that. Fixes: d08e2045ebf0 ("bpf: introduce new VFS based BPF kfuncs") Cc: stable@vger.kernel.org Signed-off-by: Matt Bobrowski --- fs/bpf_fs_kfuncs.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c index 08f0847c4970..6665a78218e1 100644 --- a/fs/bpf_fs_kfuncs.c +++ b/fs/bpf_fs_kfuncs.c @@ -87,6 +87,14 @@ __bpf_kfunc int bpf_path_d_path(const struct path *path, char *buf, size_t buf__ if (!buf__sz) return -EINVAL; + /* + * An exiting task releases its last files from exit_task_work(), after + * exit_fs() has already dropped its ->fs, which d_path() uses to + * resolve against. + */ + if (!current->fs) + return -EINVAL; + ret = d_path(path, buf, buf__sz); if (IS_ERR(ret)) return PTR_ERR(ret); -- 2.47.3