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

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Date:	Fri, 26 Apr 2013 17:35:02 +0200
From:	Frantisek Hrbata <fhrbata@...hat.com>
To:	Will Huck <will.huckk@...il.com>
Cc:	hpa@...or.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	oleg@...hat.com, kamaleshb@...ibm.com, hechjie@...ibm.com
Subject: Re: [PATCH] x86: add phys addr validity check for /dev/mem mmap

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.
> >
> ><quote>
> >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
> >set.)
> ></quote>
> >
> >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
> #PF?

Hi,

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

I hope I didn't misunderstand your question.

Thanks

> 
> >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@...hat.com>
> >---
> >  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)
> >  #endif
> >  }
> >+#define ARCH_HAS_VALID_PHYS_ADDR_RANGE
> >+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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-- 
Frantisek Hrbata
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