Re: [PATCH] firmware: dmi_scan: Fix ordering of product_uuid

[Jean Delvare]

Hi zduan,

Thanks for your reply.

Le Thursday 16 April 2015 Ã 14:22 +0800, Zhenzhong Duan a Ãcrit :
> On 2015/4/15 17:02, Jean Delvare wrote:
> > In function dmi_present(), dmi_walk_early() calls dmi_table(), which
> > calls dmi_decode(), which ultimately calls dmi_save_uuid(). This last
> > function makes a decision based on the value of global variable
> > dmi_ver. The problem is that this variable is set right _after_
> > dmi_walk_early() returns. So dmi_save_uuid() always sees dmi_ver == 0
> > regardless of the actual version implemented.
> >
> > This causes /sys/class/dmi/id/product_uuid to always use the old
> > ordering even on systems implementing DMI/SMBIOS 2.6 or later, which
> > should use the new ordering.
> >
> > This is broken since kernel v3.8 for legacy DMI implementations and
> > since kernel v3.10 for SMBIOS 2 implementations. SMBIOS 3
> > implementations with the 64-bit entry point are not affected.
> >
> > The first breakage does not matter much as in practice legacy DMI
> > implementations are always for versions older than 2.6, which is when
> > the UUID ordering changed. The second breakage is more problematic as
> > it affects the vast majority of x86 systems manufactured since 2009.
> >
> > Signed-off-by: Jean Delvare <jdelvare@xxxxxxx>
> > Fixes: 9f9c9cbb6057 ("drivers/firmware/dmi_scan.c: fetch dmi version from SMBIOS if it exists")
> I think above line should be removed as dmi_ver is set before 
> dmi_walk_early with the commit, see below clip.
> We did get right UUID order with SMBIOS 2.6 per customer test.

I bet your customers tested only with recent SMBIOS implementations that
have the _SM_ entry point. They did not test on systems with only legacy
_DMI_ entry points. As I said above, odds are that such systems would
implement a version of the specification older than 2.6 anyway, so the
bug wouldn't trigger.

I agree that 9f9c9cbb6057 is not problematic in practice and this is why
I wrote that the fix is only needed for kernels v3.10+, not v3.8+. But I
think it is still interesting to document the first commit which
introduced the bug. I'm pretty sure that the second faulty commit would
not have been faulty if the first commit had been correct. After all,
that second commit aligned the _SM_ code path on the _DMI_ code path,
without realizing that the latter had a bug.

> +static int __init smbios_present(const char __iomem *p)
> +{
> +    u8 buf[32];
> +    int offset = 0;
> +
> +    memcpy_fromio(buf, p, 32);
> +    if ((buf[5] < 32) && dmi_checksum(buf, buf[5])) {
> +        dmi_ver = (buf[6] << 8) + buf[7];

But look at the _DMI_ code path:

static int __init dmi_present(const char __iomem *p)
{
	(...)
              if (dmi_walk_early(dmi_decode) == 0) {
                      if (dmi_ver)
                              pr_info("SMBIOS %d.%d present.\n",
                                     dmi_ver >> 8, dmi_ver & 0xFF);
                      else {
                              dmi_ver = (buf[14] & 0xF0) << 4 |
                                         (buf[14] & 0x0F);
                              pr_info("Legacy DMI %d.%d present.\n",
                                     dmi_ver >> 8, dmi_ver & 0xFF);
                      }
                      dmi_dump_ids();
                      return 0;
              }

Here dmi_ver may be set _after_ dmi_walk_early is called.

> > Fixes: 79bae42d51a5 ("dmi_scan: refactor dmi_scan_machine(), {smbios,dmi}_present()")
> > Cc: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> > Cc: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> > Cc: Artem Savkov <artem.savkov@xxxxxxxxx>
> > Cc: Ivan Khoronzhuk <ivan.khoronzhuk@xxxxxxxxxx>
> > Cc: Matt Fleming <matt.fleming@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx [v3.10+]
> > ---
> >   drivers/firmware/dmi_scan.c |    7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > --- linux-4.0.orig/drivers/firmware/dmi_scan.c	2015-04-13 00:12:50.000000000 +0200
> > +++ linux-4.0/drivers/firmware/dmi_scan.c	2015-04-15 10:24:37.556994240 +0200
> > @@ -499,18 +499,19 @@ static int __init dmi_present(const u8 *
> >   	buf += 16;
> >   
> >   	if (memcmp(buf, "_DMI_", 5) == 0 && dmi_checksum(buf, 15)) {
> > +		if (smbios_ver)
> > +			dmi_ver = smbios_ver;
> > +		else
> > +			dmi_ver = (buf[14] & 0xF0) << 4 | (buf[14] & 0x0F);
> >   		dmi_num = get_unaligned_le16(buf + 12);
> >   		dmi_len = get_unaligned_le16(buf + 6);
> >   		dmi_base = get_unaligned_le32(buf + 8);
> >   
> >   		if (dmi_walk_early(dmi_decode) == 0) {
> >   			if (smbios_ver) {
> > -				dmi_ver = smbios_ver;
> >   				pr_info("SMBIOS %d.%d present.\n",
> >   				       dmi_ver >> 8, dmi_ver & 0xFF);
> >   			} else {
> > -				dmi_ver = (buf[14] & 0xF0) << 4 |
> > -					   (buf[14] & 0x0F);
> >   				pr_info("Legacy DMI %d.%d present.\n",
> >   				       dmi_ver >> 8, dmi_ver & 0xFF);
> >   			}
> >
> >
> The basic idea is right, but you ignore the case dmi_walk_early may 
> fail, though looks impossible when bootup.
> 
> Better to add below for robust.
> 
> @@ -521,6 +521,6 @@ static int __init dmi_present(const u8 *
> 
>               return 0;
>           }
>       }
> +    dmi_ver = 0;
>       return 1;
>   }
> 

What is the value of this? dmi_ver will never be accessed after this
point anyway, as far as I can see.

-- 
Jean Delvare
SUSE L3 Support

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