Re: mm/vmalloc.c:3689 vread_iter() error: we previously assumed 'vm' could be null (see line 3667)

[Baoquan He <bhe@redhat.com>]

On 10/18/23 at 08:52am, Andrew Morton wrote:
> On Wed, 18 Oct 2023 23:15:31 +0800 Baoquan He <bhe@redhat.com> wrote:
> 
> > From: Baoquan He <bhe@redhat.com>
> > Date: Wed, 18 Oct 2023 22:50:14 +0800
> > Subject: [PATCH] mm/vmalloc: fix the unchecked dereference warning in vread_iter()
> > Content-type: text/plain
> > 
> > LKP reported smatch warning as below:
> > 
> > ===================
> > smatch warnings:
> > mm/vmalloc.c:3689 vread_iter() error: we previously assumed 'vm' could be null (see line 3667)
> > ......
> > 06c8994626d1b7  @3667 size = vm ? get_vm_area_size(vm) : va_size(va);
> > ......
> > 06c8994626d1b7  @3689 else if (!(vm->flags & VM_IOREMAP))
> >                                  ^^^^^^^^^
> > Unchecked dereference
> > =====================
> > 
> > So add checking on whether 'vm' is not null when dereferencing it in
> > vread_iter(). This mutes smatch complaint.
> > 
> > ...
> >
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3813,7 +3813,7 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
> >  
> >  		if (flags & VMAP_RAM)
> >  			copied = vmap_ram_vread_iter(iter, addr, n, flags);
> > -		else if (!(vm->flags & VM_IOREMAP))
> > +		else if (!(vm && (vm->flags & VM_IOREMAP)))
> >  			copied = aligned_vread_iter(iter, addr, n);
> >  		else /* IOREMAP area is treated as memory hole */
> >  			copied = zero_iter(iter, n);
> 
> So is this not a real runtime bug?  We're only doing this to suppress a
> smatch warning?
> 
> If so, can we please include a description of *why* this wasn't a bug? 
> What conditions ensure that vm!=NULL at this point?

I think this is not a real runtime bug. The only chance it can hapen is
when (flags == VMAP_BLOCK) is true. That has been warned and could never
happen. I updated patch log and paste v2 here. 

                /*
                 * VMAP_BLOCK indicates a sub-type of vm_map_ram area, need
                 * be set together with VMAP_RAM.
                 */
                WARN_ON(flags == VMAP_BLOCK);
 
                if (!vm && !flags)
                        continue;


From 89cc02302766ab7a67cc668390c24079b4a9406b Mon Sep 17 00:00:00 2001
From: Baoquan He <bhe@redhat.com>
Date: Wed, 18 Oct 2023 22:50:14 +0800
Subject: [PATCH v2] mm/vmalloc: fix the unchecked dereference warning in vread_iter()
Content-type: text/plain

LKP reported smatch warning as below:

===================
smatch warnings:
mm/vmalloc.c:3689 vread_iter() error: we previously assumed 'vm' could be null (see line 3667)
......
06c8994626d1b7  @3667 size = vm ? get_vm_area_size(vm) : va_size(va);
......
06c8994626d1b7  @3689 else if (!(vm->flags & VM_IOREMAP))
                                 ^^^^^^^^^
Unchecked dereference
=====================

This is not a real-time bug because the possible null 'vm' in the pointed
place could only happen when flags == VMAP_BLOCK. However, the case
'flags == VMAP_BLOCK' should never happen and has been detected with WARN_ON.
Please check vm_map_ram() implementation and the earlier checking in
vread_iter() at below:

                ~~~~~~~~~~~~~~~~~~~~~~~~~~
                /*
                 * VMAP_BLOCK indicates a sub-type of vm_map_ram area, need
                 * be set together with VMAP_RAM.
                 */
                WARN_ON(flags == VMAP_BLOCK);

                if (!vm && !flags)
                        continue;
                ~~~~~~~~~~~~~~~~~~~~~~~~~~

So add checking on whether 'vm' could be null when dereferencing it in
vread_iter(). This mutes smatch complaint.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202310171600.WCrsOwFj-lkp@intel.com/
Signed-off-by: Baoquan He <bhe@redhat.com>
---
v1->v2:
- Update patch log to state that it's not a realtime bug as Andrew
  suggested.

 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index aad48ed8d86b..2cc992392db7 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3813,7 +3813,7 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 
 		if (flags & VMAP_RAM)
 			copied = vmap_ram_vread_iter(iter, addr, n, flags);
-		else if (!(vm->flags & VM_IOREMAP))
+		else if (!(vm && (vm->flags & VM_IOREMAP)))
 			copied = aligned_vread_iter(iter, addr, n);
 		else /* IOREMAP area is treated as memory hole */
 			copied = zero_iter(iter, n);
-- 
2.41.0