Re: [PATCH] Fixes: null pointer dereference in pfnmap_lockdep_assert

[Manas <manas18244@iiitd.ac.in>]

Hi Peter, thanks for reviewing.

On 03.10.2024 12:41, Peter Xu wrote:
>On Thu, Oct 03, 2024 at 09:31:06PM +0530, Manas via B4 Relay wrote:
>> From: Manas <manas18244@iiitd.ac.in>
>>
>> syzbot has pointed to a possible null pointer dereference in
>> pfnmap_lockdep_assert. vm_file member of vm_area_struct is being
>> dereferenced without any checks.
>>
>> This fix returns if vm_file member in vm_area_struct is NULL.
>>
>> Reported-by: syzbot+093d096417e7038a689b@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
>> ---
>> This bug[1] triggers a general protection fault in follow_pfnmap_start
>> function. An assertion pfnmap_lockdep_assert inside this function
>> dereferences vm_file member of vm_area_struct. And panic gets triggered
>> when vm_file is NULL.
>>
>> This patch returns from the assertion pfnmap_lockdep_assert if vm_file
>> is found to be NULL.
>>
>> [1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
>>
>> Signed-off-by: Manas <manas18244@iiitd.ac.in>
>
>Thanks for the patch!
>
>> ---
>>  mm/memory.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 2366578015ad..b152a95e543f 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6346,6 +6346,9 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
>>  static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
>>  {
>>  #ifdef CONFIG_LOCKDEP
>> +	if (!vma->vm_file)
>> +		return;
>> +
>
>Hmm I guess I wasn't careful enough here as I was mostly only thinking
>about file mappings, but I just notice we have other pfnmaps like the vvar
>mappings.. the mapping var can also already be reused later when available.
>
>Logically even if !vm_file we can still check against mmap write lock.  So
>would it be better to do this instead:
>
>        struct address_space *mapping = vma->vm_file && vma->vm_file->f_mapping;
>
This will lead to `-Wint-conversion` error in the assignment. We can either do a
cast like the following:

         struct address_space *mapping = (struct address_space *)(vma->vm_file && vma->vm_file->f_mapping);

But I am not sure if it is the canonical way of doing it. It will also lead to
warning about pointer from integer casting.

Or will a conditional like this work here?

         struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
       
>        if (mapping)
>                lockdep_assert(lockdep_is_held(&mapping->i_mmap_rwsem) ||
>                               lockdep_is_held(&vma->vm_mm->mmap_lock));
>        else
>                lockdep_assert(lockdep_is_held(&vma->vm_mm->mmap_lock));
>
>?
>

>>  	struct address_space *mapping = vma->vm_file->f_mapping;
>>
>>  	if (mapping)
>>
>> ---
>> base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
>> change-id: 20241003-fix-null-deref-6bfa0337efc3
>>
>> Best regards,
>> --
>> Manas <manas18244@iiitd.ac.in>
>>
>>
>>
>
>-- 
>Peter Xu
>

-- 
Manas