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Subject[PATCH] ARM: Mark the FDT_FIXED sections as shareable
Date
commit 7a1be318f579 ("ARM: 9012/1: move device tree mapping out of linear
region") use FDT_FIXED_BASE to map the whole FDT_FIXED_SIZE memory area
which contains fdt. But it only reserves the exact physical memory that
fdt occupied. Unfortunately, this mapping is non-shareable. An illegal or
speculative read access can bring the RAM content from non-fdt zone into
cache, PIPT makes it to be hit by subsequently read access through
shareable mapping(such as linear mapping), and the cache consistency
between cores is lost due to non-shareable property.

|<---------FDT_FIXED_SIZE------>|
| |
-------------------------------
| <non-fdt> | <fdt> | <non-fdt> |
-------------------------------

1. CoreA read <non-fdt> through MT_ROM mapping, the old data is loaded
into the cache.
2. CoreB write <non-fdt> to update data through linear mapping. CoreA
received the notification to invalid the corresponding cachelines, but
the property non-shareable makes it to be ignored.
3. CoreA read <non-fdt> through linear mapping, cache hit, the old data
is read.

To eliminate this risk, mark the MT_ROM sections as shareable.

The other user of MT_ROM is XIP_KERNEL. XIP allows the kernel to run from
flash to save RAM space. Not sure if anyone is still using XIP in order to
save a little memory and not care about performance degradation. Add a new
memory type MT_ROM_XIP to be compatible with it.

BTW: Another solution is to memblock_reserve() all the sections that fdt
spans, but this will waste 2-4MiB memory.

Here's an example:
list_del corruption. prev->next should be c0ecbf74, but was c08410dc
kernel BUG at lib/list_debug.c:53!
... ...
PC is at __list_del_entry_valid+0x58/0x98
LR is at __list_del_entry_valid+0x58/0x98
psr: 60000093
sp : c0ecbf30 ip : 00000000 fp : 00000001
r10: c08410d0 r9 : 00000001 r8 : c0825e0c
r7 : 20000013 r6 : c08410d0 r5 : c0ecbf74 r4 : c0ecbf74
r3 : c0825d08 r2 : 00000000 r1 : df7ce6f4 r0 : 00000044
... ...
Stack: (0xc0ecbf30 to 0xc0ecc000)
bf20: c0ecbf74 c0164fd0 c0ecbf70 c0165170
bf40: c0eca000 c0840c00 c0840c00 c0824500 c0825e0c c0189bbc c088f404 60000013
bf60: 60000013 c0e85100 000004ec 00000000 c0ebcdc0 c0ecbf74 c0ecbf74 c0825d08
... ... < next prev >
(__list_del_entry_valid) from (__list_del_entry+0xc/0x20)
(__list_del_entry) from (finish_swait+0x60/0x7c)
(finish_swait) from (rcu_gp_kthread+0x560/0xa20)
(rcu_gp_kthread) from (kthread+0x14c/0x15c)
(kthread) from (ret_from_fork+0x14/0x24)

The faulty list node to be deleted is a local variable, its address is
c0ecbf74. The dumped stack shows that 'prev' = c0ecbf74, but its value
before lib/list_debug.c:53 is c08410dc. A large amount of printing results
in swapping out the cacheline containing the old data(MT_ROM mapping is
read only, so the cacheline cannot be dirty), and the subsequent dump
operation obtains new data from the DDR.

Fixes: 7a1be318f579 ("ARM: 9012/1: move device tree mapping out of linear region")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
arch/arm/include/asm/mach/map.h | 1 +
arch/arm/mm/mmu.c | 9 ++++++++-
2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/mach/map.h b/arch/arm/include/asm/mach/map.h
index 92282558caf7cdb..0100f95c8104b3c 100644
--- a/arch/arm/include/asm/mach/map.h
+++ b/arch/arm/include/asm/mach/map.h
@@ -28,6 +28,7 @@ enum {
MT_MEMORY_RWX,
MT_MEMORY_RW,
MT_ROM,
+ MT_ROM_XIP,
MT_MEMORY_RWX_NONCACHED,
MT_MEMORY_RW_DTCM,
MT_MEMORY_RWX_ITCM,
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 5e2be37a198e29e..a707eac189b1d6d 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -300,6 +300,10 @@ static struct mem_type mem_types[] __ro_after_init = {
.prot_sect = PMD_TYPE_SECT,
.domain = DOMAIN_KERNEL,
},
+ [MT_ROM_XIP] = {
+ .prot_sect = PMD_TYPE_SECT,
+ .domain = DOMAIN_KERNEL,
+ },
[MT_MEMORY_RWX_NONCACHED] = {
.prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY |
L_PTE_MT_BUFFERABLE,
@@ -566,6 +570,7 @@ static void __init build_mem_type_table(void)
* from SVC mode and no access from userspace.
*/
mem_types[MT_ROM].prot_sect |= PMD_SECT_APX|PMD_SECT_AP_WRITE;
+ mem_types[MT_ROM_XIP].prot_sect |= PMD_SECT_APX|PMD_SECT_AP_WRITE;
mem_types[MT_MINICLEAN].prot_sect |= PMD_SECT_APX|PMD_SECT_AP_WRITE;
mem_types[MT_CACHECLEAN].prot_sect |= PMD_SECT_APX|PMD_SECT_AP_WRITE;
#endif
@@ -590,6 +595,7 @@ static void __init build_mem_type_table(void)
mem_types[MT_MEMORY_DMA_READY].prot_pte |= L_PTE_SHARED;
mem_types[MT_MEMORY_RWX_NONCACHED].prot_sect |= PMD_SECT_S;
mem_types[MT_MEMORY_RWX_NONCACHED].prot_pte |= L_PTE_SHARED;
+ mem_types[MT_ROM].prot_sect |= PMD_SECT_S;
}
}

@@ -650,6 +656,7 @@ static void __init build_mem_type_table(void)
mem_types[MT_MEMORY_DMA_READY].prot_pte |= kern_pgprot;
mem_types[MT_MEMORY_RWX_NONCACHED].prot_sect |= ecc_mask;
mem_types[MT_ROM].prot_sect |= cp->pmd;
+ mem_types[MT_ROM_XIP].prot_sect |= cp->pmd;

switch (cp->pmd) {
case PMD_SECT_WT:
@@ -1372,7 +1379,7 @@ static void __init devicemaps_init(const struct machine_desc *mdesc)
map.pfn = __phys_to_pfn(CONFIG_XIP_PHYS_ADDR & SECTION_MASK);
map.virtual = MODULES_VADDR;
map.length = ((unsigned long)_exiprom - map.virtual + ~SECTION_MASK) & SECTION_MASK;
- map.type = MT_ROM;
+ map.type = MT_ROM_XIP;
create_mapping(&map);
#endif

--
2.25.1

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