[PATCH] riscv: fix overlap of allocated page and PTR_ERR

[Nam Cao]

On riscv32, it is possible for the last page in virtual address space
(0xfffff000) to be allocated. This page overlaps with PTR_ERR, so that
shouldn't happen.

There is already some code to ensure memblock won't allocate the last page.
However, buddy allocator is left unchecked.

Fix this by reserving physical memory that would be mapped at virtual
addresses greater than 0xfffff000.

Reported-by: Björn Töpel <bjorn at kernel.org>
Closes: https://lore.kernel.org/linux-riscv/878r1ibpdn.fsf@all.your.base.are.belong.to.us
Fixes: 76d2a0493a17 ("RISC-V: Init and Halt Code")
Signed-off-by: Nam Cao <namcao at linutronix.de>
Cc: <stable at vger.kernel.org>
---
 arch/riscv/mm/init.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 968761843203..7c985435b3fc 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -235,18 +235,19 @@ static void __init setup_bootmem(void)
 		kernel_map.va_pa_offset = PAGE_OFFSET - phys_ram_base;
 
 	/*
-	 * memblock allocator is not aware of the fact that last 4K bytes of
-	 * the addressable memory can not be mapped because of IS_ERR_VALUE
-	 * macro. Make sure that last 4k bytes are not usable by memblock
-	 * if end of dram is equal to maximum addressable memory.  For 64-bit
-	 * kernel, this problem can't happen here as the end of the virtual
-	 * address space is occupied by the kernel mapping then this check must
-	 * be done as soon as the kernel mapping base address is determined.
+	 * Reserve physical address space that would be mapped to virtual
+	 * addresses greater than (void *)(-PAGE_SIZE) because:
+	 *  - This memory would overlap with ERR_PTR
+	 *  - This memory belongs to high memory, which is not supported
+	 *
+	 * This is not applicable to 64-bit kernel, because virtual addresses
+	 * after (void *)(-PAGE_SIZE) are not linearly mapped: they are
+	 * occupied by kernel mapping. Also it is unrealistic for high memory
+	 * to exist on 64-bit platforms.
 	 */
 	if (!IS_ENABLED(CONFIG_64BIT)) {
-		max_mapped_addr = __pa(~(ulong)0);
-		if (max_mapped_addr == (phys_ram_end - 1))
-			memblock_set_current_limit(max_mapped_addr - 4096);
+		max_mapped_addr = __va_to_pa_nodebug(-PAGE_SIZE);
+		memblock_reserve(max_mapped_addr, (phys_addr_t)-max_mapped_addr);
 	}
 
 	min_low_pfn = PFN_UP(phys_ram_base);
-- 
2.39.2