[PATCH v3] arm: Fix memory attribute inconsistencies when using fixmap

[Ard Biesheuvel]

From: Jon Medhurst <tixy at linaro.org>

To cope with the variety in ARM architectures and configurations, the
pagetable attributes for kernel memory are generated at runtime to match
the system the kernel finds itself on. This calculated value is stored
in pgprot_kernel.

However, when early fixmap support was added for ARM (commit
a5f4c561b3b1) the attributes used for mappings were hard coded because
pgprot_kernel is not set up early enough. Unfortunately, when fixmap is
used after early boot this means the memory being mapped can have
different attributes to existing mappings, potentially leading to
unpredictable behaviour. A specific problem also exists due to the hard
coded values not include the 'shareable' attribute which means on
systems where this matters (e.g. those with multiple CPU clusters) the
cache contents for a memory location can become inconsistent between
CPUs.

To resolve these issues we change fixmap to use the same memory
attributes (from pgprot_kernel) that the rest of the kernel uses. To
enable this we need to refactor the initialisation code so
build_mem_type_table() is called early enough. Note, that relies on early
param parsing for memory type overrides passed via the kernel command
line, so we need to make sure this call is still after
parse_early_params().

Fixes: a5f4c561b3b1 ("ARM: 8415/1: early fixmap support for earlycon")
Cc: <stable at vger.kernel.org> # v4.3+
Signed-off-by: Jon Medhurst <tixy at linaro.org>
[ardb: keep early_fixmap_init() before param parsing, for earlycon]
Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
v3: - Unfortunately, I failed to spot an issue with earlycon when testing
      Tixy's v2. Earlycon depends on fixmap, but only for device mappings,
      which are not affected by this change. So instead, preserve the calls
      to early_fixmap_init() and early_ioremap_init() in their original
      locations, and only move build_mem_type_table() to the earliest
      possible moment in boot, which is right after early param parsing
      (which may set cachepolicy/nocache/nowb variables)

 arch/arm/include/asm/fixmap.h |  2 +-
 arch/arm/include/asm/mmu.h    |  6 ++++++
 arch/arm/kernel/setup.c       |  5 ++---
 arch/arm/mm/mmu.c             | 12 +++++++++++-
 4 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/arm/include/asm/fixmap.h b/arch/arm/include/asm/fixmap.h
index 5c17d2dec777..8f967d1373f6 100644
--- a/arch/arm/include/asm/fixmap.h
+++ b/arch/arm/include/asm/fixmap.h
@@ -41,7 +41,7 @@ static const enum fixed_addresses __end_of_fixed_addresses =
 
 #define FIXMAP_PAGE_COMMON	(L_PTE_YOUNG | L_PTE_PRESENT | L_PTE_XN | L_PTE_DIRTY)
 
-#define FIXMAP_PAGE_NORMAL	(FIXMAP_PAGE_COMMON | L_PTE_MT_WRITEBACK)
+#define FIXMAP_PAGE_NORMAL	(pgprot_kernel | L_PTE_XN)
 #define FIXMAP_PAGE_RO		(FIXMAP_PAGE_NORMAL | L_PTE_RDONLY)
 
 /* Used by set_fixmap_(io|nocache), both meant for mapping a device */
diff --git a/arch/arm/include/asm/mmu.h b/arch/arm/include/asm/mmu.h
index a5b47421059d..80dfeb8ce8f2 100644
--- a/arch/arm/include/asm/mmu.h
+++ b/arch/arm/include/asm/mmu.h
@@ -1,6 +1,8 @@
 #ifndef __ARM_MMU_H
 #define __ARM_MMU_H
 
+struct machine_desc;
+
 #ifdef CONFIG_MMU
 
 typedef struct {
@@ -24,6 +26,8 @@ typedef struct {
 #define ASID(mm)	(0)
 #endif
 
+void early_mm_init(const struct machine_desc *mdesc);
+
 #else
 
 /*
@@ -35,6 +39,8 @@ typedef struct {
 	unsigned long	end_brk;
 } mm_context_t;
 
+static inline void early_mm_init(const struct machine_desc *mdesc) { }
+
 #endif
 
 #endif
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index f4e54503afa9..f5db5548ad42 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -1087,9 +1087,8 @@ void __init setup_arch(char **cmdline_p)
 
 	parse_early_param();
 
-#ifdef CONFIG_MMU
-	early_paging_init(mdesc);
-#endif
+	early_mm_init(mdesc);
+
 	setup_dma_zone(mdesc);
 	xen_early_init();
 	efi_init();
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 4e016d7f37b3..ec81a30479aa 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -414,6 +414,11 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 		     FIXADDR_END);
 	BUG_ON(idx >= __end_of_fixed_addresses);
 
+	/* we only support device mappings until pgprot_kernel has been set */
+	if (WARN_ON(pgprot_val(prot) != pgprot_val(FIXMAP_PAGE_IO) &&
+		    pgprot_val(pgprot_kernel) == 0))
+		return;
+
 	if (pgprot_val(prot))
 		set_pte_at(NULL, vaddr, pte,
 			pfn_pte(phys >> PAGE_SHIFT, prot));
@@ -1616,7 +1621,6 @@ void __init paging_init(const struct machine_desc *mdesc)
 {
 	void *zero_page;
 
-	build_mem_type_table();
 	prepare_page_table();
 	map_lowmem();
 	memblock_set_current_limit(arm_lowmem_limit);
@@ -1636,3 +1640,9 @@ void __init paging_init(const struct machine_desc *mdesc)
 	empty_zero_page = virt_to_page(zero_page);
 	__flush_dcache_page(NULL, empty_zero_page);
 }
+
+void __init early_mm_init(const struct machine_desc *mdesc)
+{
+	build_mem_type_table();
+	early_paging_init(mdesc);
+}
-- 
2.7.4