6.6-stable review patch. If anyone has any objections, please let me know.
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From: Martin KaFai Lau martin.lau@kernel.org
[ Upstream commit 0281b919e175bb9c3128bd3872ac2903e9436e3f ]
The following race is possible between bpf_timer_cancel_and_free and bpf_timer_cancel. It will lead a UAF on the timer->timer.
bpf_timer_cancel(); spin_lock(); t = timer->time; spin_unlock();
bpf_timer_cancel_and_free(); spin_lock(); t = timer->timer; timer->timer = NULL; spin_unlock(); hrtimer_cancel(&t->timer); kfree(t);
/* UAF on t */ hrtimer_cancel(&t->timer);
In bpf_timer_cancel_and_free, this patch frees the timer->timer after a rcu grace period. This requires a rcu_head addition to the "struct bpf_hrtimer". Another kfree(t) happens in bpf_timer_init, this does not need a kfree_rcu because it is still under the spin_lock and timer->timer has not been visible by others yet.
In bpf_timer_cancel, rcu_read_lock() is added because this helper can be used in a non rcu critical section context (e.g. from a sleepable bpf prog). Other timer->timer usages in helpers.c have been audited, bpf_timer_cancel() is the only place where timer->timer is used outside of the spin_lock.
Another solution considered is to mark a t->flag in bpf_timer_cancel and clear it after hrtimer_cancel() is done. In bpf_timer_cancel_and_free, it busy waits for the flag to be cleared before kfree(t). This patch goes with a straight forward solution and frees timer->timer after a rcu grace period.
Fixes: b00628b1c7d5 ("bpf: Introduce bpf timers.") Suggested-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Martin KaFai Lau martin.lau@kernel.org Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Hou Tao houtao1@huawei.com Link: https://lore.kernel.org/bpf/20240215211218.990808-1-martin.lau@linux.dev Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/helpers.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index e68ef39cda675..a5ce840f4fbef 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1100,6 +1100,7 @@ struct bpf_hrtimer { struct bpf_prog *prog; void __rcu *callback_fn; void *value; + struct rcu_head rcu; };
/* the actual struct hidden inside uapi struct bpf_timer */ @@ -1328,6 +1329,7 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_timer_kern *, timer)
if (in_nmi()) return -EOPNOTSUPP; + rcu_read_lock(); __bpf_spin_lock_irqsave(&timer->lock); t = timer->timer; if (!t) { @@ -1349,6 +1351,7 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_timer_kern *, timer) * if it was running. */ ret = ret ?: hrtimer_cancel(&t->timer); + rcu_read_unlock(); return ret; }
@@ -1403,7 +1406,7 @@ void bpf_timer_cancel_and_free(void *val) */ if (this_cpu_read(hrtimer_running) != t) hrtimer_cancel(&t->timer); - kfree(t); + kfree_rcu(t, rcu); }
BPF_CALL_2(bpf_kptr_xchg, void *, map_value, void *, ptr)