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Re: [Qemu-devel] [PATCH] mmap: add check if requested memory area fits t


From: andrzej zaborowski
Subject: Re: [Qemu-devel] [PATCH] mmap: add check if requested memory area fits target address space
Date: Mon, 10 Nov 2008 04:30:39 +0100

Sorry to resurrect this old thread, I still can't convince myself.

2008/10/27 Kirill A. Shutemov <address@hidden>:
> On Mon, Oct 27, 2008 at 08:37:39PM +0100, andrzej zaborowski wrote:
>> 2008/10/27 Kirill A. Shutemov <address@hidden>:
>> > On Mon, Oct 27, 2008 at 02:08:52PM +0100, andrzej zaborowski wrote:
>> >> On 17/10/2008, Kirill A. Shutemov <address@hidden> wrote:
>> >> > Signed-off-by: Kirill A. Shutemov <address@hidden>
>> >> >  ---
>> >> >   linux-user/mmap.c |    5 +++++
>> >> >   1 files changed, 5 insertions(+), 0 deletions(-)
>> >> >
>> >> >  diff --git a/linux-user/mmap.c b/linux-user/mmap.c
>> >> >  index bc20f4b..9a2f355 100644
>> >> >  --- a/linux-user/mmap.c
>> >> >  +++ b/linux-user/mmap.c
>> >> >  @@ -388,6 +388,11 @@ abi_long target_mmap(abi_ulong start, abi_ulong 
>> >> > len, int prot,
>> >> >          end = start + len;
>> >> >          real_end = HOST_PAGE_ALIGN(end);
>> >> >
>> >> >  +        if ((unsigned long)start + len > (abi_ulong) -1) {
>> >> >  +            errno = EINVAL;
>> >> >  +            goto fail;
>> >> >  +        }
>> >>
>> >> I'm being picky but this would prevent the last byte from being used?
>> >> :p  (or the last page because len is aligned?)
>> >
>> > No, it returns error if start + len is more than 0xFFFFFFFF (32-bit
>> > target).
>> >
>> >>
>> >> I'm not sure unsigned long is the best choice.
>> >
>> > Why?
>>
>> I may be misunderstanding but I think the range of valid addresses
>> should depend on target word size, not host (even if the combination
>> where it matters is not yet supported).
>
> start + len can be more than 0xFFFFFFFF ((abi_ulong) -1) on 32-bit targets,
> so we should use host's long.
>
>> On a 32-bit host the condition is always false.
>
> It's ok. It can be true, only on 64-bit host.

Let's say we have a 32-bit host and target, the call receives start ==
0xffff0050 and len == 0x100000, the check passes, when it shouldn't
(?).  On a 64-bit host it would fail, but this check should be
independent of the host type.
(It'll probably fail later in the host mmap() -- but in the meantime
mmap_frag() might succeed for example)

Cheers




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