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AHgh+RqRFkMqeC6iblBVkiOv8yw+mnlFdeHj9MIwOko3RUOUHPBBHp1D4/usl0iLwbU+YROR5/+0QkTtuqa5TOc=@vger.kernel.org X-Gm-Message-State: AOJu0Yxi5ZQWkwuDgh+LM14oDY6gp+V1iBRxU4caos8qjSMY/RuekoNo WbiHiGXZvt3Y5wqKoj1ZXDHm6Bu/HndqYdr29oaAnssD4hCBdQUfAH0Quv71wk0YLbBNConzH9j OZBtziRn4Lthe77P6tHvyUZHIufrxDuA= X-Gm-Gg: AfdE7cmqay8ivuZqPojPW6Xbpp1/1UrKAcgCVJVWQWHAMB45K1B6VmfLh97Hm+05NsK 6lmB7C/wJRmYQryCW5ffOe2AGtvxMRX7UCqiIOdLJielJFqXodkwho4TiXzW3pyKlDiwkUbNxru BkiYMAk9CPzpAxDXPNYSRtqEo9DRpFhtknhAayDiBuIHg0US6E8NZpHednaFITGhlChZV7+z3Ve N3zzX0EFjhtTpudBBqUga7JdC4esyzmrhY65rlTID/1H7maqfuC1zgFBxIuVymzxEigidn8lSvx x8ZjmTDcEPRiB3hBgA== X-Received: by 2002:a17:907:97d3:b0:c11:f4d6:48fc with SMTP id a640c23a62f3a-c16b48261c8mr204637466b.45.1784324151069; Fri, 17 Jul 2026 14:35:51 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20260717210143.4734-1-blbllhy@gmail.com> In-Reply-To: <20260717210143.4734-1-blbllhy@gmail.com> From: Mateusz Guzik Date: Fri, 17 Jul 2026 23:35:39 +0200 X-Gm-Features: AUfX_mwP3S2KwmbkP5eepUHmugHbXAMFAfKfjfgwEebjLs_0J1AEbV3zkyG2fqA Message-ID: Subject: Re: [PATCH] pid: fix cad_pid use-after-free race in proc_do_cad_pid() To: "Cen Zhang (Microsoft)" Cc: brauner@kernel.org, oleg@redhat.com, jack@suse.cz, avagin@gmail.com, ptikhomirov@virtuozzo.com, linux-kernel@vger.kernel.org, AutonomousCodeSecurity@microsoft.com, tgopinath@linux.microsoft.com, kys@microsoft.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Jul 17, 2026 at 11:01=E2=80=AFPM Cen Zhang (Microsoft) wrote: > > proc_do_cad_pid() reads the global cad_pid pointer and passes it to > pid_vnr() without protecting the lifetime of the referenced struct pid. > A concurrent writer can replace cad_pid and drop the final reference to > the old struct pid after the reader has loaded the pointer but before > pid_vnr() has finished dereferencing it, causing a use-after-free. > > Fix this by wrapping the read side in an RCU read-side critical section > and waiting for a grace period before dropping the old cad_pid reference > on the write side. Do not use call_rcu(&old_pid->rcu, ...) here: > struct pid::rcu is already used by free_pid(), so queueing it again can > corrupt the RCU callback list. > > KASAN crash stack: > kernel/pid.c:545 pid_nr_ns() # reads freed pid->level > kernel/pid.c:556 pid_vnr() > kernel/pid.c:775 proc_do_cad_pid() > fs/proc/proc_sysctl.c proc_sys_call_handler() > fs/read_write.c vfs_read() > fs/read_write.c __x64_sys_pread64() > > Fixes: 9ec52099e4b8 ("[PATCH] replace cad_pid by a struct pid") > Reported-by: AutonomousCodeSecurity@microsoft.com > Signed-off-by: Cen Zhang (Microsoft) > --- > kernel/pid.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/kernel/pid.c b/kernel/pid.c > index f55189a3d07d..fee62b2c0c7b 100644 > --- a/kernel/pid.c > +++ b/kernel/pid.c > @@ -768,11 +768,15 @@ static int proc_do_cad_pid(const struct ctl_table *= table, int write, void *buffe > size_t *lenp, loff_t *ppos) > { > struct pid *new_pid; > + struct pid *old_pid; > pid_t tmp_pid; > int r; > struct ctl_table tmp_table =3D *table; > > + rcu_read_lock(); > tmp_pid =3D pid_vnr(cad_pid); > + rcu_read_unlock(); > + > tmp_table.data =3D &tmp_pid; > > r =3D proc_dointvec(&tmp_table, write, buffer, lenp, ppos); > @@ -783,7 +787,9 @@ static int proc_do_cad_pid(const struct ctl_table *ta= ble, int write, void *buffe > if (!new_pid) > return -ESRCH; > > - put_pid(xchg(&cad_pid, new_pid)); > + old_pid =3D xchg(&cad_pid, new_pid); > + synchronize_rcu(); > + put_pid(old_pid); > return 0; > } > I don't understand what this call to synchronize_rcu() is accomplishing. sashiko points out the actual use of cad_pid in kill_cad_pid() suffers the same liveness issue, i.e., the routine needs to enter rcu and grab a ref on the found obj before issuing kill_pid(). I would deinline it while patching the problem.