[PATCH 5.11 104/122] ARM: 9063/1: mm: reduce maximum number of CPUs if DEBUG_KMAP_LOCAL is enabled

Greg Kroah-Hartman
2021-04-19T13:17:43Z

From: Ard Biesheuvel <ardb@kernel.org>

[ Upstream commit d624833f5984d484c5e3196f34b926f9e71dafee ]

The debugging code for kmap_local() doubles the number of per-CPU fixmap
slots allocated for kmap_local(), in order to use half of them as guard
regions. This causes the fixmap region to grow downwards beyond the start
of its reserved window if the supported number of CPUs is large, and collide
with the newly added virtual DT mapping right below it, which is obviously
not good.

One manifestation of this is EFI boot on a kernel built with NR_CPUS=32
and CONFIG_DEBUG_KMAP_LOCAL=y, which may pass the FDT in highmem, resulting
in block entries below the fixmap region that the fixmap code misidentifies
as fixmap table entries, and subsequently tries to dereference using a
phys-to-virt translation that is only valid for lowmem. This results in a
cryptic splat such as the one below.

ftrace: allocating 45548 entries in 89 pages
8<--- cut here ---
Unable to handle kernel paging request at virtual address fc6006f0
pgd = (ptrval)
[fc6006f0] *pgd=80000040207003, *pmd=00000000
Internal error: Oops: a06 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 0 Comm: swapper Not tainted 5.11.0+ #382
Hardware name: Generic DT based system
PC is at cpu_ca15_set_pte_ext+0x24/0x30
LR is at __set_fixmap+0xe4/0x118
pc : [<c041ac9c>] lr : [<c04189d8>] psr: 400000d3
sp : c1601ed8 ip : 00400000 fp : 00800000
r10: 0000071f r9 : 00421000 r8 : 00c00000
r7 : 00c00000 r6 : 0000071f r5 : ffade000 r4 : 4040171f
r3 : 00c00000 r2 : 4040171f r1 : c041ac78 r0 : fc6006f0
Flags: nZcv IRQs off FIQs off Mode SVC_32 ISA ARM Segment none
Control: 30c5387d Table: 40203000 DAC: 00000001
Process swapper (pid: 0, stack limit = 0x(ptrval))

So let's limit CONFIG_NR_CPUS to 16 when CONFIG_DEBUG_KMAP_LOCAL=y. Also,
fix the BUILD_BUG_ON() check that was supposed to catch this, by checking
whether the region grows below the start address rather than above the end
address.

Fixes: 2a15ba82fa6ca3f3 ("ARM: highmem: Switch to generic kmap atomic")
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm/Kconfig | 8 +++++++-
arch/arm/mm/mmu.c | 3 +--
2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 138248999df7..3d2c684eab77 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1310,9 +1310,15 @@ config KASAN_SHADOW_OFFSET

config NR_CPUS
int "Maximum number of CPUs (2-32)"
- range 2 32
+ range 2 16 if DEBUG_KMAP_LOCAL
+ range 2 32 if !DEBUG_KMAP_LOCAL
depends on SMP
default "4"
+ help
+ The maximum number of CPUs that the kernel can support.
+ Up to 32 CPUs can be supported, or up to 16 if kmap_local()
+ debugging is enabled, which uses half of the per-CPU fixmap
+ slots as guard regions.

config HOTPLUG_CPU
bool "Support for hot-pluggable CPUs"
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index c06ebfbc48c4..56c7954cb626 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -388,8 +388,7 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
pte_t *pte = pte_offset_fixmap(pmd_off_k(vaddr), vaddr);

/* Make sure fixmap region does not exceed available allocation. */
- BUILD_BUG_ON(FIXADDR_START + (__end_of_fixed_addresses * PAGE_SIZE) >
- FIXADDR_END);
+ BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) < FIXADDR_START);
BUG_ON(idx >= __end_of_fixed_addresses);

/* we only support device mappings until pgprot_kernel has been set */
--
2.30.2