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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Milton Miller <miltonm@us.ibm.com>,
        Anton Blanchard <anton@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.7 204/393] powerpc/vdso: Fix vdso cpu truncation
Date:   Mon, 17 Aug 2020 17:14:14 +0200
Message-Id: <20200817143829.518416563@linuxfoundation.org>
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From: Milton Miller <miltonm@us.ibm.com>

[ Upstream commit a9f675f950a07d5c1dbcbb97aabac56f5ed085e3 ]

The code in vdso_cpu_init that exposes the cpu and numa node to
userspace via SPRG_VDSO incorrctly masks the cpu to 12 bits. This means
that any kernel running on a box with more than 4096 threads (NR_CPUS
advertises a limit of of 8192 cpus) would expose userspace to two cpu
contexts running at the same time with the same cpu number.

Note: I'm not aware of any distro shipping a kernel with support for more
than 4096 threads today, nor of any system image that currently exceeds
4096 threads. Found via code browsing.

Fixes: 18ad51dd342a7eb09dbcd059d0b451b616d4dafc ("powerpc: Add VDSO version of getcpu")
Signed-off-by: Milton Miller <miltonm@us.ibm.com>
Signed-off-by: Anton Blanchard <anton@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200715233704.1352257-1-anton@ozlabs.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/powerpc/kernel/vdso.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/vdso.c b/arch/powerpc/kernel/vdso.c
index f38f26e844b64..1c07df1ad560e 100644
--- a/arch/powerpc/kernel/vdso.c
+++ b/arch/powerpc/kernel/vdso.c
@@ -678,7 +678,7 @@ int vdso_getcpu_init(void)
 	node = cpu_to_node(cpu);
 	WARN_ON_ONCE(node > 0xffff);
 
-	val = (cpu & 0xfff) | ((node & 0xffff) << 16);
+	val = (cpu & 0xffff) | ((node & 0xffff) << 16);
 	mtspr(SPRN_SPRG_VDSO_WRITE, val);
 	get_paca()->sprg_vdso = val;
 
-- 
2.25.1