Re: [PATCHv7 2/3] x86/vdso: add mremap hook to vm_special_mapping

[Dmitry Safonov <dsafonov@virtuozzo.com>]

On 04/21/2016 10:52 PM, Andy Lutomirski wrote:
> On Mon, Apr 18, 2016 at 7:23 AM, Dmitry Safonov <dsafonov@virtuozzo.com> wrote:
>> Add possibility for userspace 32-bit applications to move
>> vdso mapping. Previously, when userspace app called
>> mremap for vdso, in return path it would land on previous
>> address of vdso page, resulting in segmentation violation.
>> Now it lands fine and returns to userspace with remapped vdso.
>> This will also fix context.vdso pointer for 64-bit, which does not
>> affect the user of vdso after mremap by now, but this may change.
>>
>> As suggested by Andy, return EINVAL for mremap that splits vdso image.
>>
>> Renamed and moved text_mapping structure declaration inside
>> map_vdso, as it used only there and now it complement
>> vvar_mapping variable.
>>
>> There is still problem for remapping vdso in glibc applications:
>> linker relocates addresses for syscalls on vdso page, so
>> you need to relink with the new addresses. Or the next syscall
>> through glibc may fail:
>>    Program received signal SIGSEGV, Segmentation fault.
>>    #0  0xf7fd9b80 in __kernel_vsyscall ()
>>    #1  0xf7ec8238 in _exit () from /usr/lib32/libc.so.6
>>
>> Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
>> ---
>> v7: that's just not my day: add new_vma parameter to vdso_fix_landing
>>      sorry for the noise
>> v6: moved vdso_image_32 check and fixup code into vdso_fix_landing function
>>      with ifdefs around
>> v5: as Andy suggested, add a check that new_vma->vm_mm and current->mm are
>>      the same, also check not only in_ia32_syscall() but image == &vdso_image_32
>> v4: drop __maybe_unused & use image from mm->context instead vdso_image_32
>> v3: as Andy suggested, return EINVAL in case of splitting vdso blob on mremap;
>>      used is_ia32_task instead of ifdefs
>> v2: added __maybe_unused for pt_regs in vdso_mremap
>>
>>   arch/x86/entry/vdso/vma.c | 50 ++++++++++++++++++++++++++++++++++++++++++-----
>>   include/linux/mm_types.h  |  3 +++
>>   mm/mmap.c                 | 10 ++++++++++
>>   3 files changed, 58 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
>> index 10f704584922..d94291a19b6e 100644
>> --- a/arch/x86/entry/vdso/vma.c
>> +++ b/arch/x86/entry/vdso/vma.c
>> @@ -12,6 +12,7 @@
>>   #include <linux/random.h>
>>   #include <linux/elf.h>
>>   #include <linux/cpu.h>
>> +#include <linux/ptrace.h>
>>   #include <asm/pvclock.h>
>>   #include <asm/vgtod.h>
>>   #include <asm/proto.h>
>> @@ -98,10 +99,43 @@ static int vdso_fault(const struct vm_special_mapping *sm,
>>          return 0;
>>   }
>>
>> -static const struct vm_special_mapping text_mapping = {
>> -       .name = "[vdso]",
>> -       .fault = vdso_fault,
>> -};
>> +#if defined CONFIG_X86_32 || defined CONFIG_COMPAT
> I think you mean defined(CONFIG_IA32_EMULATION).  IIRC CONFIG_COMPAT
> is set for X32, !IA32 kernels, too.

Sure, I just copied it from vdso_image_32 declaration.
Should it be there CONFIG_IA32_EMULATION there instead of
CONFIG_COMPAT? Or I miss something?

>> +static void vdso_fix_landing(const struct vdso_image *image,
>> +               struct vm_area_struct *new_vma)
>> +{
>> +       if (in_ia32_syscall() && image == &vdso_image_32) {
>> +               struct pt_regs *regs = current_pt_regs();
>> +               unsigned long vdso_land = image->sym_int80_landing_pad;
>> +               unsigned long old_land_addr = vdso_land +
>> +                       (unsigned long)current->mm->context.vdso;
>> +
>> +               /* Fixing userspace landing - look at do_fast_syscall_32 */
>> +               if (regs->ip == old_land_addr)
>> +                       regs->ip = new_vma->vm_start + vdso_land;
>> +       }
>> +}
>> +#else
>> +static void vdso_fix_landing(const struct vdso_image *image,
>> +               struct vm_area_struct *new_vma) {}
>> +#endif
> You could also define the function regardless and simply ifdef out the
> body, thus saving a few lines of code.

Yep, will do.

>> +
>> +static int vdso_mremap(const struct vm_special_mapping *sm,
>> +               struct vm_area_struct *new_vma)
>> +{
>> +       unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
>> +       const struct vdso_image *image = current->mm->context.vdso_image;
>> +
>> +       if (image->size != new_size)
>> +               return -EINVAL;
>> +
>> +       if (current->mm != new_vma->vm_mm)
>> +               return -EFAULT;
> Can you make that if (WARN_ON_ONCE(current->mm != new_vma->vm_mm))?
> It should be impossible.

Sure.

>> +
>> +       vdso_fix_landing(image, new_vma);
>> +       current->mm->context.vdso = (void __user *)new_vma->vm_start;
>> +
>> +       return 0;
>> +}
>>
>>   static int vvar_fault(const struct vm_special_mapping *sm,
>>                        struct vm_area_struct *vma, struct vm_fault *vmf)
>> @@ -162,6 +196,12 @@ static int map_vdso(const struct vdso_image *image, bool calculate_addr)
>>          struct vm_area_struct *vma;
>>          unsigned long addr, text_start;
>>          int ret = 0;
>> +
>> +       static const struct vm_special_mapping vdso_mapping = {
>> +               .name = "[vdso]",
>> +               .fault = vdso_fault,
>> +               .mremap = vdso_mremap,
>> +       };
>>          static const struct vm_special_mapping vvar_mapping = {
>>                  .name = "[vvar]",
>>                  .fault = vvar_fault,
>> @@ -195,7 +235,7 @@ static int map_vdso(const struct vdso_image *image, bool calculate_addr)
>>                                         image->size,
>>                                         VM_READ|VM_EXEC|
>>                                         VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
>> -                                      &text_mapping);
>> +                                      &vdso_mapping);
>>
>>          if (IS_ERR(vma)) {
>>                  ret = PTR_ERR(vma);
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index c2d75b4fa86c..4d16ab9287af 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -586,6 +586,9 @@ struct vm_special_mapping {
>>          int (*fault)(const struct vm_special_mapping *sm,
>>                       struct vm_area_struct *vma,
>>                       struct vm_fault *vmf);
>> +
>> +       int (*mremap)(const struct vm_special_mapping *sm,
>> +                    struct vm_area_struct *new_vma);
>>   };
>>
>>   enum tlb_flush_reason {
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index bd2e1a533bc1..ba71658dd1a1 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2930,9 +2930,19 @@ static const char *special_mapping_name(struct vm_area_struct *vma)
>>          return ((struct vm_special_mapping *)vma->vm_private_data)->name;
>>   }
>>
>> +static int special_mapping_mremap(struct vm_area_struct *new_vma)
>> +{
>> +       struct vm_special_mapping *sm = new_vma->vm_private_data;
>> +
>> +       if (sm->mremap)
>> +               return sm->mremap(sm, new_vma);
>> +       return 0;
>> +}
>> +
>>   static const struct vm_operations_struct special_mapping_vmops = {
>>          .close = special_mapping_close,
>>          .fault = special_mapping_fault,
>> +       .mremap = special_mapping_mremap,
>>          .name = special_mapping_name,
>>   };
>>
>> --
>> 2.8.0
>>
>
>


-- 
Regards,
Dmitry Safonov

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