From 30bf5728bb217a6d1ba73f44094c9b9c6bc9a567 Mon Sep 17 00:00:00 2001 From: Heiko Carstens Date: Thu, 7 Aug 2025 17:04:27 +0200 Subject: [PATCH] s390/mm: Do not map lowcore with identity mapping [ Upstream commit 93f616ff870a1fb7e84d472cad0af651b18f9f87 ] Since the identity mapping is pinned to address zero the lowcore is always also mapped to address zero, this happens regardless of the relocate_lowcore command line option. If the option is specified the lowcore is mapped twice, instead of only once. This means that NULL pointer accesses will succeed instead of causing an exception (low address protection still applies, but covers only parts). To fix this never map the first two pages of physical memory with the identity mapping. Fixes: 32db401965f1 ("s390/mm: Pin identity mapping base to zero") Reviewed-by: Alexander Gordeev Signed-off-by: Heiko Carstens Signed-off-by: Alexander Gordeev Signed-off-by: Sasha Levin --- arch/s390/boot/vmem.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/s390/boot/vmem.c b/arch/s390/boot/vmem.c index 3fa28db2fe59f..14aee85240217 100644 --- a/arch/s390/boot/vmem.c +++ b/arch/s390/boot/vmem.c @@ -471,6 +471,9 @@ void setup_vmem(unsigned long kernel_start, unsigned long kernel_end, unsigned l lowcore_address + sizeof(struct lowcore), POPULATE_LOWCORE); for_each_physmem_usable_range(i, &start, &end) { + /* Do not map lowcore with identity mapping */ + if (!start) + start = sizeof(struct lowcore); pgtable_populate((unsigned long)__identity_va(start), (unsigned long)__identity_va(end), POPULATE_IDENTITY); -- 2.47.3