Re: [PATCH] efifb: prevent null dereferences by removing unused array indices from dmi_list

Hi

On Wed, Aug 7, 2013 at 1:15 AM, James Bates [off-list ref] wrote:

Hi all,

The dmi_list array is initialized using gnu designated initializers, and therefore
contains fewer explicitly defined entries as there are elements in it. This
is because the enum above with M_blabla constants contains more items than the designated
initializer. Those elements not explicitly initialized are implicitly set to 0.

Now efifb_setup(), L.322 & L.323, loops through all these array elements, and performs
a strcmp o a field (optname) in each item. For non explicitly initialized elements this
will be a null pointer:

                        for (i = 0; i < M_UNKNOWN; i++) {
                                if (!strcmp(this_opt, dmi_list[i].optname) &&

On my macbook6,1 the predefined values are for some reason incorrect, and most parameters
are preset correctly by my efi bootloader (elilo). but stride/line_length is not detected
correctly and so I wish to set it explicitly using a "videoïifb:stride:2048" command-line
argument. Because of the above null dereference, an exception (presumably) occurs before
the parsing code (L.333) is ever reached. I say presumably since the mac hangs on boot
without a console, and I can therefore not see any output.

By removing the unused values from the enum, and thus preventing implicitly initialized items
in the dmi_list array, the null dereference does not occur, my customer command-line arg is
parsed correctly, and my console displays correctly.

Signed-off-by: James Bates <redacted>

(CC'ing hpa)

Yepp, this patch looks good:
Reviewed-by: David Herrmann <redacted>

However, I'd like to see some code to prevent this from happening
again. It isn't really obvious that removing an entry will result in a
NULL-deref. I am not the maintainer of this code, but I'd really like
to see a "(dmi_list[i].optname && !strcmp())" check in efifb_setup().
Otherwise we _will_ run into this again.

Side note: this code got moved to arch/x86/kernel/sysfb_efi.c in the
x86 tree. I am adding hpa here so he will remember this once Linus
gets a merge conflict (iff the sysfb changes get merged through the
x86 tree).

Thanks
David