[PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

[PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

[Naotaka Hamaguchi]

This patch fixes two bugs of mmap():
 1. mmap() succeeds even if "offset" argument is a negative value, although
    it should return EINVAL in such case. Currently I have only checked
    it on x86_64 because (a) x86 seems to OK to accept a negative offset
    for mapping 2GB-4GB regions, and (b) I don't know about other
    architectures at all (I'll make it if needed).

 2. mmap() would succeed if "offset" + "length" get overflow, although
    it should return EOVERFLOW.

The detail of these problems is as follows:

1. mmap() succeeds even if "offset" argument is a negative value, although
   it should return EINVAL in such case.

POSIX says the type of the argument "off" is "off_t", which
is equivalent to "long" for all architecture, so it is allowed to
give a negative "off" to mmap().

In such case, it is actually regarded as big positive value
because the type of "off" is "unsigned long" in the kernel. 
For example, off=-4096 (-0x1000) is regarded as 
off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
It results in mapping too big offset region.

2. mmap() would succeed if "offset" + "length" get overflow, although
   it should return EOVERFLOW.

The overflow check of mmap() almost doesn't work.

In do_mmap_pgoff(file, addr, len, prot, flags, pgoff),
the existing overflow check logic is as follows.

------------------------------------------------------------------------
do_mmap_pgoff(struct file *file, unsigned long addr,
		unsigned long len, unsigned long prot,
		unsigned long flags, unsigned long pgoff)
{
	if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
		return -EOVERFLOW;
}
------------------------------------------------------------------------

However, for example on x86_64, if we give off=0x1000 and
len=0xfffffffffffff000, but EOVERFLOW is not returned.
It is because the checking is based on the page offset,
not on the byte offset.

To fix this bug, I convert this overflow check from page
offset base to byte offset base. 

Signed-off-by: Naotaka Hamaguchi <n.hamaguchi@jp.fujitsu.com>
---
 arch/x86/kernel/sys_x86_64.c |    3 +++
 mm/mmap.c                    |    3 ++-
 2 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index 0514890..ddefd6c 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -90,6 +90,9 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
        if (off & ~PAGE_MASK)
                goto out;

+       if ((off_t) off < 0)
+               goto out;
+
        error = sys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
 out:
        return error;
diff --git a/mm/mmap.c b/mm/mmap.c
index 3f758c7..2fa99cd 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -948,6 +948,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
        vm_flags_t vm_flags;
        int error;
        unsigned long reqprot = prot;
+       unsigned long off = pgoff << PAGE_SHIFT;

        /*
         * Does the application expect PROT_READ to imply PROT_EXEC?
@@ -971,7 +972,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
                return -ENOMEM;

        /* offset overflow? */
-       if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
+       if ((off + len) < off)
                return -EOVERFLOW;

        /* Too many mappings? */
--
1.7.7.4

Best Regards,
Naotaka Hamaguchi

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RE: [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

[Venu Byravarasu]

> The detail of these problems is as follows:

> 1. mmap() succeeds even if "offset" argument is a negative value, although
>    it should return EINVAL in such case.

> In such case, it is actually regarded as big positive value
> because the type of "off" is "unsigned long" in the kernel.
> For example, off=-4096 (-0x1000) is regarded as
> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
> It results in mapping too big offset region.

It is not true always.

Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
hence (off & ~PAGE_MASK) will be true and " -EINVAL" will be returned.

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Re: [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

[Naotaka Hamaguchi]

Hi Venu,

>> 1. mmap() succeeds even if "offset" argument is a negative value, although
>>     it should return EINVAL in such case.
>
>> In such case, it is actually regarded as big positive value
>> because the type of "off" is "unsigned long" in the kernel.
>> For example, off=-4096 (-0x1000) is regarded as
>> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
>> It results in mapping too big offset region.
>
> It is not true always.
>
> Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
> hence (off&  ~PAGE_MASK) will be true and " -EINVAL" will be returned.

Is PAGE_MASK 0xfffffffffffff000 (x86_64) and 0xfffff000 (x86), isn't it?
Or am I missing something?

arch/x86/include/asm/page_types.h
=================================================
...
#define PAGE_SHIFT      12
#define PAGE_SIZE       (_AC(1,UL) << PAGE_SHIFT)
#define PAGE_MASK       (~(PAGE_SIZE-1))
...
=================================================

Thanks,
Naotaka Hamaguchi

(2012/02/17 18:04), Venu Byravarasu wrote:
>> The detail of these problems is as follows:
>
>> 1. mmap() succeeds even if "offset" argument is a negative value, although
>>     it should return EINVAL in such case.
>
>> In such case, it is actually regarded as big positive value
>> because the type of "off" is "unsigned long" in the kernel.
>> For example, off=-4096 (-0x1000) is regarded as
>> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
>> It results in mapping too big offset region.
>
> It is not true always.
>
> Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
> hence (off&  ~PAGE_MASK) will be true and " -EINVAL" will be returned.
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
>
>


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Re: [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

[Hugh Dickins]

On Fri, 17 Feb 2012, Naotaka Hamaguchi wrote:
> This patch fixes two bugs of mmap():
>  1. mmap() succeeds even if "offset" argument is a negative value, although
>     it should return EINVAL in such case. Currently I have only checked
>     it on x86_64 because (a) x86 seems to OK to accept a negative offset
>     for mapping 2GB-4GB regions, and (b) I don't know about other
>     architectures at all (I'll make it if needed).
> 
>  2. mmap() would succeed if "offset" + "length" get overflow, although
>     it should return EOVERFLOW.

I'm not convinced that either of these is a problem.  Do you see an
actual bug arising from these, or is it just that you think the Linux
mmap() permits more than you expect from your reading of POSIX?

1. Should a negative offset necessarily return -EINVAL?  At present I
   can mmap() /dev/kmem on x86_64 and see what's at 0xffff880000000000:
   why should that say -EINVAL?  (I admit that my example wanted to say
   0xffffffff81000000, where /proc/kallsyms locates _text, but that did
   disappoint me with -EINVAL, because mmap_kmem() only understands the
   direct map, not the further layouts which architectures may use.)

2. We will have bugs if you manage to mmap an area crossing from pgoff
   -1 to pgoff 0, but I thought the existing checks prevented that.

mmap() should be permitting as far as it safely can; but it's a bug
if a fault on an offset beyond (page-rounded-up) end-of-file does not
then give SIGBUS.

> 
> The detail of these problems is as follows:
> 
> 1. mmap() succeeds even if "offset" argument is a negative value, although
>    it should return EINVAL in such case.
> 
> POSIX says the type of the argument "off" is "off_t", which
> is equivalent to "long" for all architecture, so it is allowed to
> give a negative "off" to mmap().
> 
> In such case, it is actually regarded as big positive value
> because the type of "off" is "unsigned long" in the kernel. 
> For example, off=-4096 (-0x1000) is regarded as 
> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
> It results in mapping too big offset region.
> 
> 2. mmap() would succeed if "offset" + "length" get overflow, although
>    it should return EOVERFLOW.
> 
> The overflow check of mmap() almost doesn't work.
> 
> In do_mmap_pgoff(file, addr, len, prot, flags, pgoff),
> the existing overflow check logic is as follows.
> 
> ------------------------------------------------------------------------
> do_mmap_pgoff(struct file *file, unsigned long addr,
> 		unsigned long len, unsigned long prot,
> 		unsigned long flags, unsigned long pgoff)
> {
> 	if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
> 		return -EOVERFLOW;
> }
> ------------------------------------------------------------------------
> 
> However, for example on x86_64, if we give off=0x1000 and
> len=0xfffffffffffff000, but EOVERFLOW is not returned.
> It is because the checking is based on the page offset,
> not on the byte offset.
> 
> To fix this bug, I convert this overflow check from page
> offset base to byte offset base. 
> 
> Signed-off-by: Naotaka Hamaguchi <n.hamaguchi@jp.fujitsu.com>
> ---
>  arch/x86/kernel/sys_x86_64.c |    3 +++
>  mm/mmap.c                    |    3 ++-
>  2 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
> index 0514890..ddefd6c 100644
> --- a/arch/x86/kernel/sys_x86_64.c
> +++ b/arch/x86/kernel/sys_x86_64.c
> @@ -90,6 +90,9 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
>         if (off & ~PAGE_MASK)
>                 goto out;
> 
> +       if ((off_t) off < 0)
> +               goto out;
> +
>         error = sys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
>  out:
>         return error;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 3f758c7..2fa99cd 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -948,6 +948,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>         vm_flags_t vm_flags;
>         int error;
>         unsigned long reqprot = prot;
> +       unsigned long off = pgoff << PAGE_SHIFT;
> 
>         /*
>          * Does the application expect PROT_READ to imply PROT_EXEC?
> @@ -971,7 +972,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>                 return -ENOMEM;
> 
>         /* offset overflow? */
> -       if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
> +       if ((off + len) < off)
>                 return -EOVERFLOW;

I think you are taking away the 32-bit kernel's ability to mmap() files
up to MAX_LFS_FILESIZE.

Hugh

> 
>         /* Too many mappings? */
> --
> 1.7.7.4
> 
> Best Regards,
> Naotaka Hamaguchi

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Re: [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

[Naotaka Hamaguchi]

Hi Hugh,

> 1. Should a negative offset necessarily return -EINVAL?  At present I
>     can mmap() /dev/kmem on x86_64 and see what's at 0xffff880000000000:
>     why should that say -EINVAL?  (I admit that my example wanted to say
>     0xffffffff81000000, where /proc/kallsyms locates _text, but that did
>     disappoint me with -EINVAL, because mmap_kmem() only understands the
>     direct map, not the further layouts which architectures may use.)
>
> 2. We will have bugs if you manage to mmap an area crossing from pgoff
>     -1 to pgoff 0, but I thought the existing checks prevented that.

>> -       if ((pgoff + (len>>  PAGE_SHIFT))<  pgoff)
>> +       if ((off + len)<  off)
>>                  return -EOVERFLOW;
>
> I think you are taking away the 32-bit kernel's ability to mmap() files
> up to MAX_LFS_FILESIZE.

Thanks, I see. I drop this patch.

BTW, I think the current error check of EOVERFLOW is meaningless, isn't it?

mm/mmap.c
===================================================================
unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
                         unsigned long len, unsigned long prot,
                         unsigned long flags, unsigned long pgoff)
{
...
        /* offset overflow? */
         if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
                return -EOVERFLOW;
...
===================================================================

Thanks,
Naotaka Hamaguchi

(2012/02/18 11:00), Hugh Dickins wrote:
> On Fri, 17 Feb 2012, Naotaka Hamaguchi wrote:
>> This patch fixes two bugs of mmap():
>>   1. mmap() succeeds even if "offset" argument is a negative value, although
>>      it should return EINVAL in such case. Currently I have only checked
>>      it on x86_64 because (a) x86 seems to OK to accept a negative offset
>>      for mapping 2GB-4GB regions, and (b) I don't know about other
>>      architectures at all (I'll make it if needed).
>>
>>   2. mmap() would succeed if "offset" + "length" get overflow, although
>>      it should return EOVERFLOW.
>
> I'm not convinced that either of these is a problem.  Do you see an
> actual bug arising from these, or is it just that you think the Linux
> mmap() permits more than you expect from your reading of POSIX?
>
> 1. Should a negative offset necessarily return -EINVAL?  At present I
>     can mmap() /dev/kmem on x86_64 and see what's at 0xffff880000000000:
>     why should that say -EINVAL?  (I admit that my example wanted to say
>     0xffffffff81000000, where /proc/kallsyms locates _text, but that did
>     disappoint me with -EINVAL, because mmap_kmem() only understands the
>     direct map, not the further layouts which architectures may use.)
>
> 2. We will have bugs if you manage to mmap an area crossing from pgoff
>     -1 to pgoff 0, but I thought the existing checks prevented that.
>
> mmap() should be permitting as far as it safely can; but it's a bug
> if a fault on an offset beyond (page-rounded-up) end-of-file does not
> then give SIGBUS.
>
>>
>> The detail of these problems is as follows:
>>
>> 1. mmap() succeeds even if "offset" argument is a negative value, although
>>     it should return EINVAL in such case.
>>
>> POSIX says the type of the argument "off" is "off_t", which
>> is equivalent to "long" for all architecture, so it is allowed to
>> give a negative "off" to mmap().
>>
>> In such case, it is actually regarded as big positive value
>> because the type of "off" is "unsigned long" in the kernel.
>> For example, off=-4096 (-0x1000) is regarded as
>> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
>> It results in mapping too big offset region.
>>
>> 2. mmap() would succeed if "offset" + "length" get overflow, although
>>     it should return EOVERFLOW.
>>
>> The overflow check of mmap() almost doesn't work.
>>
>> In do_mmap_pgoff(file, addr, len, prot, flags, pgoff),
>> the existing overflow check logic is as follows.
>>
>> ------------------------------------------------------------------------
>> do_mmap_pgoff(struct file *file, unsigned long addr,
>> 		unsigned long len, unsigned long prot,
>> 		unsigned long flags, unsigned long pgoff)
>> {
>> 	if ((pgoff + (len>>  PAGE_SHIFT))<  pgoff)
>> 		return -EOVERFLOW;
>> }
>> ------------------------------------------------------------------------
>>
>> However, for example on x86_64, if we give off=0x1000 and
>> len=0xfffffffffffff000, but EOVERFLOW is not returned.
>> It is because the checking is based on the page offset,
>> not on the byte offset.
>>
>> To fix this bug, I convert this overflow check from page
>> offset base to byte offset base.
>>
>> Signed-off-by: Naotaka Hamaguchi<n.hamaguchi@jp.fujitsu.com>
>> ---
>>   arch/x86/kernel/sys_x86_64.c |    3 +++
>>   mm/mmap.c                    |    3 ++-
>>   2 files changed, 5 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
>> index 0514890..ddefd6c 100644
>> --- a/arch/x86/kernel/sys_x86_64.c
>> +++ b/arch/x86/kernel/sys_x86_64.c
>> @@ -90,6 +90,9 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
>>          if (off&  ~PAGE_MASK)
>>                  goto out;
>>
>> +       if ((off_t) off<  0)
>> +               goto out;
>> +
>>          error = sys_mmap_pgoff(addr, len, prot, flags, fd, off>>  PAGE_SHIFT);
>>   out:
>>          return error;
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 3f758c7..2fa99cd 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -948,6 +948,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>>          vm_flags_t vm_flags;
>>          int error;
>>          unsigned long reqprot = prot;
>> +       unsigned long off = pgoff<<  PAGE_SHIFT;
>>
>>          /*
>>           * Does the application expect PROT_READ to imply PROT_EXEC?
>> @@ -971,7 +972,7 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>>                  return -ENOMEM;
>>
>>          /* offset overflow? */
>> -       if ((pgoff + (len>>  PAGE_SHIFT))<  pgoff)
>> +       if ((off + len)<  off)
>>                  return -EOVERFLOW;
>
> I think you are taking away the 32-bit kernel's ability to mmap() files
> up to MAX_LFS_FILESIZE.
>
> Hugh
>
>>
>>          /* Too many mappings? */
>> --
>> 1.7.7.4
>>
>> Best Regards,
>> Naotaka Hamaguchi
> --
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>
>


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