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Date: Tue, 21 Feb 2012 17:30:23 +0900
From: Naotaka Hamaguchi <n.hamaguchi@...fujitsu.com>
To: Hugh Dickins <hughd@...gle.com>
CC: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
KOSAKI Motohiro <kosaki.motohiro@...il.com>
Subject: Re: [PATCH] mm: mmap() sometimes succeeds even if the region to map
is invalid.
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@...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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