Since commit a4d5613c4dc6 ("arm: extend pfn_valid to take into account
freed memory map alignment") changes the semantics of pfn_valid() to check
presence of the memory map for a PFN. A valid page for an address which
is reserved but not mapped by the kernel[1], the system crashed during
some uio test with the following memory layout:

 node   0: [mem 0x00000000c0a00000-0x00000000cc8fffff]
 node   0: [mem 0x00000000d0000000-0x00000000da1fffff]
 the uio layout is：0xc0900000, 0x100000

the crash backtrace like:

  Unable to handle kernel paging request at virtual address bff00000
  [...]
  CPU: 1 PID: 465 Comm: startapp.bin Tainted: G           O      5.10.0 #1
  Hardware name: Generic DT based system
  PC is at b15_flush_kern_dcache_area+0x24/0x3c
  LR is at __sync_icache_dcache+0x6c/0x98
  [...]
   (b15_flush_kern_dcache_area) from (__sync_icache_dcache+0x6c/0x98)
   (__sync_icache_dcache) from (set_pte_at+0x28/0x54)
   (set_pte_at) from (remap_pfn_range+0x1a0/0x274)
   (remap_pfn_range) from (uio_mmap+0x184/0x1b8 [uio])
   (uio_mmap [uio]) from (__mmap_region+0x264/0x5f4)
   (__mmap_region) from (__do_mmap_mm+0x3ec/0x440)
   (__do_mmap_mm) from (do_mmap+0x50/0x58)
   (do_mmap) from (vm_mmap_pgoff+0xfc/0x188)
   (vm_mmap_pgoff) from (ksys_mmap_pgoff+0xac/0xc4)
   (ksys_mmap_pgoff) from (ret_fast_syscall+0x0/0x5c)
  Code: e0801001 e2423001 e1c00003 f57ff04f (ee070f3e)
  ---[ end trace 09cf0734c3805d52 ]---
  Kernel panic - not syncing: Fatal exception

So check if PG_reserved was set to solve this issue.

[1]: https://lore.kernel.org/lkml/Zbtdue57RO0QScJM@linux.ibm.com/

Fixes: a4d5613c4dc6 ("arm: extend pfn_valid to take into account freed memory map alignment")
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
---
v1 -> v2
 - Improve commit message
 - Use helpers instead of PG_reserved
 - drop the unnecessary include headers

 arch/arm/mm/flush.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm/mm/flush.c b/arch/arm/mm/flush.c
index d19d140a10c7..0749cf8a6637 100644
--- a/arch/arm/mm/flush.c
+++ b/arch/arm/mm/flush.c
@@ -296,6 +296,9 @@ void __sync_icache_dcache(pte_t pteval)
 		return;
 
 	folio = page_folio(pfn_to_page(pfn));
+	if (folio_test_reserved(folio))
+		return;
+
 	if (cache_is_vipt_aliasing())
 		mapping = folio_flush_mapping(folio);
 	else
-- 
2.25.1

Since some abuses of pfn_valid() have been reported, I check all the use 
of pfn_valid(), and find some suspicious cases.

phys_mem_access_prot() defined in arch/arm/mm/mmu.c returns 
pgprot_noncached() when pfn_valid() returns false.
I think it’s purpose is to return pgprot_noncached() when the pfn is not 
in RAM, and the use of pfn_valid() is incorrect.
Notably, phys_mem_access_prot() defined in arm64 uses 
pfn_is_map_memory() instead of pfn_valid() since commit
873ba463914c (arm64: decouple check whether pfn is in linear map from 
pfn_valid()).

Similarly, virt_addr_valid() defined in arm64 uses pfn_is_map_memory() 
instead of pfn_valid() since commit
873ba463914c (arm64: decouple check whether pfn is in linear map from 
pfn_valid()), But virt_addr_valid() still
uses pfn_valid(). Besides, the implementation of x86 also uses pfn_valid().

update_mmu_cache_range() defined in arch/arm/mm/fault-armv.c checks 
pfn_valid() and then calls __flush_dcache_folio().
This case is similar to the case reported by Yongqiang Liu, the pfn may 
not be a RAM pfn, and the system will crash in
__flush_dcache_folio() due to the kernel linear mapping is not 
established. virt_addr_valid() is used to check whether a
vrtual address is valid linear mapping. Are these uses of pfn_valid() 
incorrect?

pfn_modify_allowed() defined in arch/x86/mm/mmap.c checks pfn_valid(), 
and the comment says it is intended to check
whether the pfn is in real memory. So the use of pfn_valid() should be 
incorrent. This case is only involved when the cpu
is affected by X86_BUG_L1TF.

try_ram_remap() defined in kernel/iomem.c returns the linear address 
when three checks are passed. One of the checks is
pfn_valid(). The only caller memremap() guarantees the pfn passed to 
try_ram_remap() is in RAM, but the pfn may be in
NOMAP memory regions and is not mapped in linear mapping. commit 
260364d112bc (arm[64]/memremap: don't abuse
pfn_valid() to ensure presence of linear map) solves it by checking in 
arch_memremap_can_ram_remap(), However, if other
architectures involve this issue?

Do these suspicious case abuse pfn_valid() really? Thanks