Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Andrei Borzenkov]

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),

Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Martin Roth]

Thanks Andrei,

Vladimir, what do you think?

Martin

On Sun, Nov 1, 2015 at 7:53 AM, Andrei Borzenkov <[e-mail]> wrote:
> 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),

Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Vladimir 'phcoder' Serbinenko]

The code itself looks good but I'd like more details. Reading 0xffffffff
shouldn't cause reboot. Why does it?
Le 1 nov. 2015 3:53 PM, "Andrei Borzenkov" <[e-mail]> 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
> [e-mail]
> https://lists.gnu.org/mailman/listinfo/grub-devel

Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Nico Huber]

Hi Andrei,

your patch looks good generally, but the check is off by one. It's
obvious, we want to have sane checking there. Reading from a random
address can cause trouble and 0xffffffff is not the only offending
address.

On x86, the cbfs is mapped right below the 4GiB line. Current machines
don't have more than 16MiB space for cbfs, FWIW. So maybe it's best to
check if the ptr points somewhere between 0xff000000 and (0x100000000 -
sizeof(*head)).

Nico

On 01.11.2015 15:53, Andrei Borzenkov wrote:
> 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),

Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Andrei Borzenkov]

04.11.2015 00:41, Nico Huber пишет:
> Hi Andrei,
>
> your patch looks good generally, but the check is off by one. It's

Yes, but it should not matter in real life. Last 4 bytes are pointer, so 
header cannot start after 0xffffffdc anyway. I'll change it.
> obvious, we want to have sane checking there. Reading from a random
> address can cause trouble and 0xffffffff is not the only offending
> address.
>
> On x86, the cbfs is mapped right below the 4GiB line. Current machines
> don't have more than 16MiB space for cbfs, FWIW. So maybe it's best to
> check if the ptr points somewhere between 0xff000000 and (0x100000000 -
> sizeof(*head)).

That's far exceeds my intention and knowledge, sorry.
> Nico
>
> On 01.11.2015 15:53, Andrei Borzenkov wrote:
> > 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),