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powerpc/vdso: Fix vdso cpu truncation

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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: 18ad51dd34 ("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
This commit is contained in:
Milton Miller 2020-07-16 09:37:04 +10:00 committed by Michael Ellerman
parent 77ca3951cc
commit a9f675f950
1 changed files with 1 additions and 1 deletions
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2
arch/powerpc/kernel/vdso.c
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@ -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;