powerpc/vdso: Fix vdso cpu truncation

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

diff --git a/arch/powerpc/kernel/vdso.c b/arch/powerpc/kernel/vdso.c
index e0f4ba4..8dad442 100644 (file)
--- a/arch/powerpc/kernel/vdso.c
+++ b/arch/powerpc/kernel/vdso.c
@@ -677,7 +677,7 @@ 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;