Re: [PATCH] x86: add phys addr validity check for /dev/mem mmap

From: Will Huck
Date: Sat Apr 27 2013 - 03:00:24 EST

On 04/26/2013 11:35 PM, Frantisek Hrbata wrote:
On Fri, Apr 26, 2013 at 01:21:28PM +0800, Will Huck wrote:
Hi Peter,
On 04/02/2013 08:28 PM, Frantisek Hrbata wrote:
When CR4.PAE is set, the 64b PTE's are used(ARCH_PHYS_ADDR_T_64BIT is set for
X86_64 || X86_PAE). According to [1] Chapter 4 Paging, some higher bits in 64b
PTE are reserved and have to be set to zero. For example, for IA-32e and 4KB
page [1] 4.5 IA-32e Paging: Table 4-19, bits 51-M(MAXPHYADDR) are reserved. So
for a CPU with e.g. 48bit phys addr width, bits 51-48 have to be zero. If one of
the reserved bits is set, [1] 4.7 Page-Fault Exceptions, the #PF is generated
with RSVD error code.

RSVD flag (bit 3).
This flag is 1 if there is no valid translation for the linear address because a
reserved bit was set in one of the paging-structure entries used to translate
that address. (Because reserved bits are not checked in a paging-structure entry
whose P flag is 0, bit 3 of the error code can be set only if bit 0 is also

In mmap_mem() the first check is valid_mmap_phys_addr_range(), but it always
returns 1 on x86. So it's possible to use any pgoff we want and to set the PTE's
reserved bits in remap_pfn_range(). Meaning there is a possibility to use mmap
In this case, remap_pfn_range() setup the map and reserved bits for
mmio memory, so the mmio memory is already populated, why trigger

I think this is described in the quote above for the RSVD flag.

remap_pfn_range() => page present => touch page => tlb miss =>
walk through paging structures => reserved bit set => #pf with rsvd flag

Page present can also trigger #PF? why?

I hope I didn't misunderstand your question.


on /dev/mem and cause system panic. It's probably not that serious, because
access to /dev/mem is limited and the system has to have panic_on_oops set, but
still I think we should check this and return error.

This patch adds check for x86 when ARCH_PHYS_ADDR_T_64BIT is set, the same way
as it is already done in e.g. ioremap. With this fix mmap returns -EINVAL if the
requested phys addr is bigger then the supported phys addr width.

[1] Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A

Signed-off-by: Frantisek Hrbata <fhrbata@xxxxxxxxxx>
arch/x86/include/asm/io.h | 4 ++++
arch/x86/mm/mmap.c | 13 +++++++++++++
2 files changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index d8e8eef..39607c6 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -242,6 +242,10 @@ static inline void flush_write_buffers(void)
+extern int valid_phys_addr_range(phys_addr_t addr, size_t count);
+extern int valid_mmap_phys_addr_range(unsigned long pfn, size_t count);
#endif /* __KERNEL__ */
extern void native_io_delay(void);
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 845df68..92ec31c 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -31,6 +31,8 @@
#include <linux/sched.h>
#include <asm/elf.h>
+#include "physaddr.h"
struct __read_mostly va_alignment va_align = {
.flags = -1,
@@ -122,3 +124,14 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
mm->unmap_area = arch_unmap_area_topdown;
+int valid_phys_addr_range(phys_addr_t addr, size_t count)
+ return addr + count <= __pa(high_memory);
+int valid_mmap_phys_addr_range(unsigned long pfn, size_t count)
+ resource_size_t addr = (pfn << PAGE_SHIFT) + count;
+ return phys_addr_valid(addr);
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