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From:   starmiku1207184332@gmail.com
To:     ast@kernel.org, daniel@iogearbox.net, john.fastabend@gmail.com,
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Cc:     bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, Teng Qi <starmiku1207184332@gmail.com>
Subject: [PATCH] kernel: bpf: syscall: fix a possible sleep-in-atomic bug in __bpf_prog_put()
Date:   Mon, 29 May 2023 15:53:27 +0000
Message-Id: <20230529155327.585056-1-starmiku1207184332@gmail.com>
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From: Teng Qi <starmiku1207184332@gmail.com>

__bpf_prog_put() indirectly calls kvfree() through bpf_prog_put_deferred()
which is unsafe under atomic context. The current
condition ‘in_irq() || irqs_disabled()’ in __bpf_prog_put() to ensure safety
does not cover cases involving the spin lock region and rcu read lock region.
Since __bpf_prog_put() is called by various callers in kernel/, net/ and
drivers/, and potentially more in future, it is necessary to handle those
cases as well.

Although we haven`t found a proper way to identify the rcu read lock region,
we have noticed that vfree() calls vfree_atomic() with the
condition 'in_interrupt()' to ensure safety.

To make __bpf_prog_put() safe in practice, we propose calling
bpf_prog_put_deferred() with the condition 'in_interrupt()' and
using the work queue for any other context.

We also added a comment to indicate that the safety of  __bpf_prog_put()
relies implicitly on the implementation of vfree().

Signed-off-by: Teng Qi <starmiku1207184332@gmail.com>
---
 kernel/bpf/syscall.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 14f39c1e573e..48ff5d2e163a 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2099,10 +2099,12 @@ static void __bpf_prog_put(struct bpf_prog *prog)
 	struct bpf_prog_aux *aux = prog->aux;
 
 	if (atomic64_dec_and_test(&aux->refcnt)) {
-		if (in_irq() || irqs_disabled()) {
+		if (!in_interrupt()) {
+			// safely calling vfree() under any context
 			INIT_WORK(&aux->work, bpf_prog_put_deferred);
 			schedule_work(&aux->work);
 		} else {
+			// depending on the vfree_atomic() branch in vfree()
 			bpf_prog_put_deferred(&aux->work);
 		}
 	}
-- 
2.25.1