Re: Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Andrei Borzenkov]

03.11.2015 19:28, Vladimir 'phcoder' Serbinenko пишет:
> The code itself looks good but I'd like more details. Reading 0xffffffff
> shouldn't cause reboot. Why does it?

That I do not know nor do I have access to system in question myself. I 
sent user patch that modified validate_header to do each comparison as 
individual statement and did line by line debug print (fortunately it 
was possible to connect serial port and capture output) and the last 
line printed was immediately before the very first

head->magic == grub_cpu_to_be32_compile_time (CBFS_HEADER_MAGIC

I suppose reading *one* byte from 0xffffffff should not cause issues but 
here we are reading 4 bytes which are beyond 0xffffffff. Who knows what 
memory controller in this system does in this case.

> Le 1 nov. 2015 3:53 PM, "Andrei Borzenkov" <address@hidden> a écrit :
>
> > I was debugging problem reported by user on Dell Dimension 8300 - it
> > rebooted when doing "ls -l". It turned out, the problem was triggered by
> > loading cbfs which probed for header. System has 2GB memory, and attempt to
> > read from address 0xffffffff caused instant reboot. 0xffffffff was returned
> > by read from non-existing address 0xfffffffc.
> >
> > The proof of concept patch below avoids it, but I wonder what the proper
> > fix is.
> >
> > diff --git a/grub-core/fs/cbfs.c b/grub-core/fs/cbfs.c
> > index a34eb88..a5a2fde 100644
> > --- a/grub-core/fs/cbfs.c
> > +++ b/grub-core/fs/cbfs.c
> > @@ -344,8 +344,9 @@ init_cbfsdisk (void)
> >
> >     ptr = *(grub_uint32_t *) 0xfffffffc;
> >     head = (struct cbfs_header *) (grub_addr_t) ptr;
> > +  grub_dprintf ("cbfs", "head=%p\n", head);
> >
> > -  if (!validate_head (head))
> > +  if (0xffffffff - ptr < sizeof (*head) || !validate_head (head))
> >       return;
> >
> >     cbfsdisk_size = ALIGN_UP (grub_be_to_cpu32 (head->romsize),
> >
> >
> > _______________________________________________
> > Grub-devel mailing list
> > address@hidden
> > https://lists.gnu.org/mailman/listinfo/grub-devel
>
>
>
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