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From:   Heiko Stuebner <heiko@sntech.de>
To:     palmer@dabbelt.com, paul.walmsley@sifive.com, aou@eecs.berkeley.edu
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        wefu@redhat.com, liush@allwinnertech.com, guoren@kernel.org,
        atishp@atishpatra.org, anup@brainfault.org, drew@beagleboard.org,
        hch@lst.de, arnd@arndb.de, wens@csie.org, maxime@cerno.tech,
        gfavor@ventanamicro.com, andrea.mondelli@huawei.com,
        behrensj@mit.edu, xinhaoqu@huawei.com, mick@ics.forth.gr,
        allen.baum@esperantotech.com, jscheid@ventanamicro.com,
        rtrauben@gmail.com, samuel@sholland.org, cmuellner@linux.com,
        philipp.tomsich@vrull.eu, Heiko Stuebner <heiko@sntech.de>,
        Atish Patra <atishp@rivosinc.com>
Subject: [PATCH v7 01/13] riscv: prevent null-pointer dereference with sbi_remote_fence_i
Date:   Mon,  7 Mar 2022 21:52:58 +0100
Message-Id: <20220307205310.1905628-2-heiko@sntech.de>
In-Reply-To: <20220307205310.1905628-1-heiko@sntech.de>
References: <20220307205310.1905628-1-heiko@sntech.de>
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The callback used inside sbi_remote_fence_i is set at sbi probe time
to the needed variant. Before that it is a NULL pointer.

Some users like the flush_icache_*() functions suggest a generic
functionality, that doesn't depend on a specific boot-stage but
uses sbi_remote_fence_i as one option to flush other cpu cores.

So they definitely shouldn't run into null-pointer dereference
issues when called "too early" during boot.

So introduce an empty function to be the standard for the __sbi_rfence
function pointer until sbi_init has run.

Users of sbi_remote_fence_i will have separate code for the local
cpu and sbi_init() is called before other cpus are brought up.
So there are no other cpus present at the time when the issue
might happen.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
---
 arch/riscv/kernel/sbi.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
index 775d3322b422..5a60a458c0b7 100644
--- a/arch/riscv/kernel/sbi.c
+++ b/arch/riscv/kernel/sbi.c
@@ -16,11 +16,19 @@
 unsigned long sbi_spec_version __ro_after_init = SBI_SPEC_VERSION_DEFAULT;
 EXPORT_SYMBOL(sbi_spec_version);
 
+static int __sbi_rfence_none(int fid, const struct cpumask *cpu_mask,
+			     unsigned long start, unsigned long size,
+			     unsigned long arg4, unsigned long arg5)
+{
+	return -EOPNOTSUPP;
+}
+
 static void (*__sbi_set_timer)(uint64_t stime) __ro_after_init;
 static int (*__sbi_send_ipi)(const struct cpumask *cpu_mask) __ro_after_init;
 static int (*__sbi_rfence)(int fid, const struct cpumask *cpu_mask,
 			   unsigned long start, unsigned long size,
-			   unsigned long arg4, unsigned long arg5) __ro_after_init;
+			   unsigned long arg4, unsigned long arg5)
+			   __ro_after_init = __sbi_rfence_none;
 
 struct sbiret sbi_ecall(int ext, int fid, unsigned long arg0,
 			unsigned long arg1, unsigned long arg2,
-- 
2.30.2