Re: [PATCH for-8.1 v10 04/14] linux-user: Use MAP_FIXED_NOREPLACE for initial image mmap

["Alex Bennée" <alex.bennee@linaro.org>]

Akihiko Odaki <akihiko.odaki@daynix.com> writes:

> On 2023/08/08 18:43, Alex Bennée wrote:
>> Richard Henderson <richard.henderson@linaro.org> writes:
>> 
>>> Use this as extra protection for the guest mapping over
>>> any qemu host mappings.
>>>
>>> Tested-by: Helge Deller <deller@gmx.de>
>>> Reviewed-by: Helge Deller <deller@gmx.de>
>>> Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>>> ---
>>>   linux-user/elfload.c | 9 ++++++---
>>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
>>> index 36e4026f05..1b4bb2d5af 100644
>>> --- a/linux-user/elfload.c
>>> +++ b/linux-user/elfload.c
>>> @@ -3147,8 +3147,11 @@ static void load_elf_image(const char *image_name, int image_fd,
>>>       /*
>>>        * Reserve address space for all of this.
>>>        *
>>> -     * In the case of ET_EXEC, we supply MAP_FIXED so that we get
>>> -     * exactly the address range that is required.
>>> +     * In the case of ET_EXEC, we supply MAP_FIXED_NOREPLACE so that we get
>>> +     * exactly the address range that is required.  Without reserved_va,
>>> +     * the guest address space is not isolated.  We have attempted to avoid
>>> +     * conflict with the host program itself via probe_guest_base, but using
>>> +     * MAP_FIXED_NOREPLACE instead of MAP_FIXED provides an extra check.
>>>        *
>>>        * Otherwise this is ET_DYN, and we are searching for a location
>>>        * that can hold the memory space required.  If the image is
>>> @@ -3160,7 +3163,7 @@ static void load_elf_image(const char *image_name, int image_fd,
>>>        */
>>>       load_addr = target_mmap(loaddr, (size_t)hiaddr - loaddr + 1, PROT_NONE,
>>>                               MAP_PRIVATE | MAP_ANON | MAP_NORESERVE |
>>> -                            (ehdr->e_type == ET_EXEC ? MAP_FIXED : 0),
>>> +                            (ehdr->e_type == ET_EXEC ? MAP_FIXED_NOREPLACE : 0),
>>>                               -1, 0);
>> We should probably also check the result == load_addr for the places
>> where MAP_FIXED_NOREPLACE isn't supported as we have this in osdep.h:
>>    #ifndef MAP_FIXED_NOREPLACE
>>    #define MAP_FIXED_NOREPLACE 0
>>    #endif
>> See 2667e069e7 (linux-user: don't use MAP_FIXED in
>> pgd_find_hole_fallback)
>
> It assumes target_mmap() emulates MAP_FIXED_NOREPLACE when the host
> does not support it as commit e69e032d1a ("linux-user: Use
> MAP_FIXED_NOREPLACE for do_brk()") already does, but defining
> MAP_FIXED_NOREPLACE zero breaks such emulation. I wrote a fix:
> https://patchew.org/QEMU/20230808115242.73025-1-akihiko.odaki@daynix.com/

Hmm doesn't that push the problem to real mmap() calls to a host system
that doesn't support MAP_FIXED_NOREPLACE?

I wonder if we need an internal flag rather than overloading the host
flags?

>
>> 
>>>       if (load_addr == -1) {
>>>           goto exit_mmap;
>> 


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro