[SeaBIOS] [PATCH] smbios: ensure comparison SMBIOS string can't be paragraph aligned

[Kevin O'Connor]

On Tue, Mar 31, 2015 at 12:37:30PM -0400, Gabriel L. Somlo wrote:
> On Tue, Mar 31, 2015 at 08:49:58AM -0600, Bruce Rogers wrote:
> > >>> On 3/30/2015 at 10:02 PM, Kevin O'Connor <kevin at koconnor.net> wrote: 
> > > On Mon, Mar 30, 2015 at 05:06:30PM -0600, Bruce Rogers wrote:
> > >> The SMBIOS anchor string _SM_ is stored within SeaBIOS to validate
> > >> an SMBIOS entry point structure. There is the possibility (observed)
> > >> that this comparison string ends up paragraph aligned and mistakenly
> > >> found during a search for the real SMBIOS entry point. Ensure it will
> > >> never end up on a paragraph boundary by storing it at odd alignment.
> > > 
> > > Thanks.
> > > 
> > > What OS was this on?  It's really an OS bug as the OS needs to check
> > > both the signature and the checksum.
> > > 
> > > My preferred approach to addressing this would be to turn
> > > p->anchor_string into a u32 and do an integer compare instead of a
> > > string compare.  Although technically this can lead to the same
> > > potential issue, in practice it should not happen because SeaBIOS'
> > > init code is relocated out of the f-segment during startup (while
> > > static strings are generally not).
> > > 
> > > -Kevin
> > 
> > This was actually flagged by the QEMU make check test in
> > tests/bios-tables-test.c, where the code has asserts based on the
> > first find of the _SM_ string found on a paragraph boundary in the
> > f segment. It assumes that the string found is sufficient to identify
> > the smbios entry point structure.
> > 
> > I read in http://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.0.0.pdf
> > where there are number of steps needed to verify the entry-point, beyond
> > finding the anchor string.  So I assume that the make check test needs to
> > be fixed.
> > 
> > But I wonder if bios creators are also supposed to ensure that there is no
> > such string findable on a paragraph boundary so as to avoid any potential
> > confusion? I don't have expereince writing bios's so I am only guessing
> > at that.
> 
> I guess on physical hardware that would be mitigated by the BIOS's
> .rodata (or whatever the equivalent thing is called in BIOS-speak)
> NOT falling within 0xf0000..0xfffff :)
> 
> If there's no way to guarantee that with SeaBIOS, I suppose the
> qemu test could be rewritten to account for the possibility of
> "false positive" signatures...

I would do both - change SeaBIOS to (in practice) not put "_SM_" in
the f-segment and change the QEMU test to not stop on the first "_SM_"
it finds.  SeaBIOS patch below (untested).

-Kevin


diff --git a/src/fw/biostables.c b/src/fw/biostables.c
index 50a891b..450aca2 100644
--- a/src/fw/biostables.c
+++ b/src/fw/biostables.c
@@ -271,7 +271,7 @@ copy_smbios(void *pos)
     if (SMBiosAddr)
         return;
     struct smbios_entry_point *p = pos;
-    if (memcmp(p->anchor_string, "_SM_", 4))
+    if (p->signature != SMBIOS_SIGNATURE)
         return;
     if (checksum(pos, 0x10) != 0)
         return;
diff --git a/src/fw/smbios.c b/src/fw/smbios.c
index dba0541..f3b5ad9 100644
--- a/src/fw/smbios.c
+++ b/src/fw/smbios.c
@@ -37,7 +37,7 @@ smbios_entry_point_setup(u16 max_structure_size,
 
     struct smbios_entry_point ep;
     memset(&ep, 0, sizeof(ep));
-    memcpy(ep.anchor_string, "_SM_", 4);
+    ep.signature = SMBIOS_SIGNATURE;
     ep.length = 0x1f;
     ep.smbios_major_version = 2;
     ep.smbios_minor_version = 4;
diff --git a/src/std/smbios.h b/src/std/smbios.h
index 0513716..4ccf2ea 100644
--- a/src/std/smbios.h
+++ b/src/std/smbios.h
@@ -3,11 +3,13 @@
 
 #include "types.h" // u32
 
+#define SMBIOS_SIGNATURE 0x5f4d535f // "_SM_"
+
 /* SMBIOS entry point -- must be written to a 16-bit aligned address
    between 0xf0000 and 0xfffff.
  */
 struct smbios_entry_point {
-    char anchor_string[4];
+    u32 signature;
     u8 checksum;
     u8 length;
     u8 smbios_major_version;