diff mbox series

[v3] Fixes: null pointer dereference in pfnmap_lockdep_assert

Message ID 20241004-fix-null-deref-v3-1-f9459b1cc95f@iiitd.ac.in (mailing list archive)
State New
Headers show
Series [v3] Fixes: null pointer dereference in pfnmap_lockdep_assert | expand

Commit Message

Manas via B4 Relay Oct. 4, 2024, 1:45 p.m. UTC 
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 assigns mapping only if vm_file is not 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 assigns mapping only if vm_file is not NULL.

[1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b

Signed-off-by: Manas <manas18244@iiitd.ac.in>
---
Changes in v3:
- v3: use assigned var instead of accessing member again 
- Link to v2: https://lore.kernel.org/r/20241004-fix-null-deref-v2-1-23ad90999cd1@iiitd.ac.in

Changes in v2:
- v2: use ternary operator according to feedback
- Link to v1: https://lore.kernel.org/r/20241003-fix-null-deref-v1-1-0a45df9d016a@iiitd.ac.in
---
 mm/memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)


---
base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
change-id: 20241003-fix-null-deref-6bfa0337efc3

Best regards,

Comments

Manas Oct. 4, 2024, 1:47 p.m. UTC | #1 
On 04.10.2024 19:15, 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 assigns mapping only if vm_file is not NULL.
I also edited the commit message (and cover letter) slightly to tell about the
newer fix, instead of the v1 fix of returning.

I hope this is okay.
Matthew Wilcox Oct. 4, 2024, 3:17 p.m. UTC | #2 
On Fri, Oct 04, 2024 at 07:15:48PM +0530, Manas via B4 Relay wrote:
> +++ b/mm/memory.c
> @@ -6346,10 +6346,10 @@ 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
> -	struct address_space *mapping = vma->vm_file->f_mapping;
> +	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;

Overly long and complex line.  Much simpler to write:

	struct address_space *mapping = NULL;

	if (vma->vm_file)
		mapping = vma->vm_file->f_mapping;

>  	if (mapping)
> -		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
> +		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));

This one should have been lockdep_assert_held(&vma->vm_mm->mmap_lock).

I'm not sure that the previous one is correct.  The
lockdep_assert_held() macro is pretty careful about checking
LOCK_STATE_NOT_HELD to avoid the LOCK_STATE_UNKNOWN possibility.
But I'll leave that for Peter to fix.
Manas Oct. 4, 2024, 5:40 p.m. UTC | #3 
Hi Matthew,

On 04.10.2024 16:17, Matthew Wilcox wrote:
>On Fri, Oct 04, 2024 at 07:15:48PM +0530, Manas via B4 Relay wrote:
>> +++ b/mm/memory.c
>> @@ -6346,10 +6346,10 @@ 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
>> -	struct address_space *mapping = vma->vm_file->f_mapping;
>> +	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
>
>Overly long and complex line.  Much simpler to write:
>
>	struct address_space *mapping = NULL;
>
>	if (vma->vm_file)
>		mapping = vma->vm_file->f_mapping;
>
Thank you for reviewing! I am sending v4.

>>  	if (mapping)
>> -		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
>> +		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));
>
>This one should have been lockdep_assert_held(&vma->vm_mm->mmap_lock).
>
>I'm not sure that the previous one is correct.  The
>lockdep_assert_held() macro is pretty careful about checking
>LOCK_STATE_NOT_HELD to avoid the LOCK_STATE_UNKNOWN possibility.
>But I'll leave that for Peter to fix.
Peter Xu Oct. 8, 2024, 7:27 p.m. UTC | #4 
On Mon, Oct 07, 2024 at 09:23:47AM -0400, Peter Xu wrote:
> On Fri, Oct 04, 2024 at 04:17:42PM +0100, Matthew Wilcox wrote:
> > On Fri, Oct 04, 2024 at 07:15:48PM +0530, Manas via B4 Relay wrote:
> > > +++ b/mm/memory.c
> > > @@ -6346,10 +6346,10 @@ 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
> > > -	struct address_space *mapping = vma->vm_file->f_mapping;
> > > +	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
> > 
> > Overly long and complex line.  Much simpler to write:
> > 
> > 	struct address_space *mapping = NULL;
> > 
> > 	if (vma->vm_file)
> > 		mapping = vma->vm_file->f_mapping;
> > 
> > >  	if (mapping)
> > > -		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
> > > +		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));
> > 
> > This one should have been lockdep_assert_held(&vma->vm_mm->mmap_lock).
> > 
> > I'm not sure that the previous one is correct.  The
> > lockdep_assert_held() macro is pretty careful about checking
> > LOCK_STATE_NOT_HELD to avoid the LOCK_STATE_UNKNOWN possibility.
> > But I'll leave that for Peter to fix.
> 
> Indeed..
> 
> Then looks like we could have quite a few other places in Linux that can
> have used this wrong.. when the assert wants to check against either of the
> two locks (one mutex or rcu read lock, for example) is held.
> 
> I'll send a patch after this one lands.

Just to follow this up and leave a record: I had a closer look today and
then quickly I found above should be all fine (similar to all kernel usages
like this, for example, rcu_dereference_check()).

The trick is LOCK_STATE_NOT_HELD is defined as 0:

#define LOCK_STATE_UNKNOWN	-1
#define LOCK_STATE_NOT_HELD	0
#define LOCK_STATE_HELD		1

So this:

#define lockdep_assert_held(l)		\
	lockdep_assert(lockdep_is_held(l) != LOCK_STATE_NOT_HELD)

Is the same to:

#define lockdep_assert_held(l)		\
	lockdep_assert(lockdep_is_held(l))

The lockdep_assert() was introduced exactly for such >1 lock assertion use
cases, in this commit:

commit d19c81378829e5d774c951219c5a973965b9202c
Author: Peter Zijlstra <peterz@infradead.org>
Date:   Mon Aug 2 18:59:56 2021 +0800

    locking/lockdep: Provide lockdep_assert{,_once}() helpers

    Extract lockdep_assert{,_once}() helpers to more easily write composite
    assertions like, for example:

            lockdep_assert(lockdep_is_held(&drm_device.master_mutex) ||
                           lockdep_is_held(&drm_file.master_lookup_lock));

Thanks,
diff mbox series

Patch

diff --git a/mm/memory.c b/mm/memory.c
index 2366578015ad..828967a13596 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6346,10 +6346,10 @@  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
-	struct address_space *mapping = vma->vm_file->f_mapping;
+	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
 
 	if (mapping)
-		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
+		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));