[PATCH 1/2] arm: Fix cache inconsistency when using fixmap

[Jon Medhurst (Tixy)]

On Fri, 2017-03-17 at 11:53 +0000, Russell King - ARM Linux wrote:
> On Thu, Mar 16, 2017 at 01:29:57PM +0000, Jon Medhurst wrote:
> > 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, the values used
> > didn't include the 'shareable' attribute which means that for later
> > non-early fixmap use, when multiple CPUs are running, any cache entries
> > allocated for fixmap memory aren't kept consistent between CPUs. This
> > can result in different CPUs seeing different memory contents.
> 
> This also likely causes unpredictable behaviour (aliased attributes).
> 
> > This issue was discovered on a dual cluster system by failures with
> > kprobes, which uses fixmap to modify the kernel image if
> > CONFIG_DEBUG_RODATA is enabled. It will also affect kgdb and jump_labels
> > which also make use of the same code to modify the kernel, and any other
> > uses of fixmap after secondary CPUs are brought online.
> > 
> > To fix this issue, and to help avoid other potential problems where
> > pagetable attributes are incorrect, we change the fixmap code to use the
> > same generated value in pgprot_kernel that the rest of the kernel uses,
> > and only fall back to a hard coded value if this isn't set - which will
> > be early on in boot before other CPUs are brought online.
> 
> I'm not happy with this - if we need to create early fixmaps, then
> we need to know the correct attributes to use, so let's move the
> attribute initialisation earlier.  This solution feels too much like
> hacking around the problem.

I must admit to having similar thoughts and plead guilty to ignoring
them. Not knowing how early fixmaps are being used I let myself think
'it must have been originally done this way for a reason'.

It looks to me that build_mem_type_table doesn't have much in the way of
dependencies so can probably be just called earlier. So, is the correct
solution to

a) call build_mem_type_table from setup_arch before early_fixmap_init
b) move call to build_mem_type_table into early_fixmap_init
c) call build_mem_type_table from early_fixmap_init as well as
paging_init and have a test in build_mem_type_table so it only exectutes
once
d) something else

a) seems simplest, b) seems wrong, c) seems over the top

Anyway, below is an alternative to $subject patch that works for my
kprobes test cases. Note, the removal of FIXMAP_PAGE_{NORMAL,RO} means
the generic fixmap code will define these from PAGE_KERNEL{,_RO}.

Not knowing how early fixmap is used, I hope Stefan and/or Ard have
testcases they could run. 

I'm also wondering if the existing definition of FIXMAP_PAGE_IO is
correc
t and should not also be based on some platform specific value
calculated
 in build_mem_type_table?


diff --git a/arch/arm/include/asm/fixmap.h
b/arch/arm/include/asm/fixmap.h
index 5c17d2dec777..30871fb269f0 100644
--- a/arch/arm/include/asm/fixmap.h
+++ b/arch/arm/include/asm/fixmap.h
@@ -41,9 +41,6 @@ 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_RO		(FIXMAP_PAGE_NORMAL |
L_PTE_RDONLY)
-
 /* Used by set_fixmap_(io|nocache), both meant for mapping a device */
 #define FIXMAP_PAGE_IO		(FIXMAP_PAGE_COMMON |
L_PTE_MT_DEV_SHARED | L_PTE_SHARED)
 #define FIXMAP_PAGE_NOCACHE	FIXMAP_PAGE_IO
diff --git a/arch/arm/include/asm/pgtable.h
b/arch/arm/include/asm/pgtable.h
index 1c462381c225..6a98856a8fa9 100644
--- a/arch/arm/include/asm/pgtable.h
+++ b/arch/arm/include/asm/pgtable.h
@@ -98,6 +98,7 @@ extern pgprot_t		pgprot_s2_device;
 #define PAGE_READONLY_EXEC	_MOD_PROT(pgprot_user, L_PTE_USER |
L_PTE_RDONLY)
 #define PAGE_KERNEL		_MOD_PROT(pgprot_kernel, L_PTE_XN)
 #define PAGE_KERNEL_EXEC	pgprot_kernel
+#define PAGE_KERNEL_RO		_MOD_PROT(pgprot_kernel, L_PTE_XN
| L_PTE_RDONLY)
 #define PAGE_HYP		_MOD_PROT(pgprot_kernel, L_PTE_HYP |
L_PTE_XN)
 #define PAGE_HYP_EXEC		_MOD_PROT(pgprot_kernel, L_PTE_HYP
| L_PTE_RDONLY)
 #define PAGE_HYP_RO		_MOD_PROT(pgprot_kernel, L_PTE_HYP |
L_PTE_RDONLY | L_PTE_XN)
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index f4e54503afa9..fc4782fa5071 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -79,6 +79,7 @@ __setup("fpe=", fpe_setup);
 #endif
 
 extern void init_default_cache_policy(unsigned long);
+extern void build_mem_type_table(void);
 extern void paging_init(const struct machine_desc *desc);
 extern void early_paging_init(const struct machine_desc *);
 extern void adjust_lowmem_bounds(void);
@@ -1082,6 +1083,8 @@ void __init setup_arch(char **cmdline_p)
 	strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
 	*cmdline_p = cmd_line;
 
+	build_mem_type_table();
+
 	early_fixmap_init();
 	early_ioremap_init();
 
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 4e016d7f37b3..c8e1de3ceb02 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -425,7 +425,7 @@ void __set_fixmap(enum fixed_addresses idx,
phys_addr_t phys, pgprot_t prot)
 /*
  * Adjust the PMD section entries according to the CPU in use.
  */
-static void __init build_mem_type_table(void)
+void __init build_mem_type_table(void)
 {
 	struct cachepolicy *cp;
 	unsigned int cr = get_cr();
@@ -1616,7 +1616,6 @@ 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);
-- 
2.11.0