[debbugs-tracker] bug#15366: closed (MBR disk signature not random enough)

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Your message dated Sat, 23 Nov 2013 20:34:26 -0500
with message-id <address@hidden>
and subject line Re: bug#15366: MBR disk signature not random enough
has caused the debbugs.gnu.org bug report #15366,
regarding MBR disk signature not random enough
to be marked as done.

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-- 
15366: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15366
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> --- Begin Message ---
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> Subject: 
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> MBR disk signature not random enough
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> Date: 
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> Fri, 13 Sep 2013 14:06:30 +0100
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> User-agent: 
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> Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20130801 Thunderbird/17.0.8
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> libparted/labels/dos.c:generate_random_id()
> is used to create a new mbr_signature where none exists. The possible 
> return values for generate_random_id() range from 0 to 999999, derived 
> from the tv_usec field of gettimeofday(2).
>
> This limits the actual value written to mbr_signature to 0.023% of the 
> possible uint32 values.
>
> I don't know how many MBRs in the world libparted has written the 
> mbr_signature to, but the chance of collisions are greatly increased by 
> having an artificially restricted value.
>
> Is there a better libc-provided rand(3)/random(3) function which can be 
> used to provide a full 32 bits of randomness?
>
> I haven't checked all the other mklabel types, but it's possible they 
> have similar limitations. gpt.c uses uuid_generate() which delegates the 
> task to libuuid. Perhaps this could be used (with truncated output) for 
> the MBR signature?
>
> Cheers,
> Phil
>
>
>
> --- End Message ---
> --- Begin Message ---
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> Subject: 
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> Re: bug#15366: MBR disk signature not random enough
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> Date: 
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> Sat, 23 Nov 2013 20:34:26 -0500
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> User-agent: 
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> Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.1
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> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA512
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> This was fixed by this commit:
>
> commit 70aa35b2b4d2e723fe82ac3184e5921a52be73ab
> Author: Jonathan Liu <address@hidden>
> Date:   Fri Oct 4 07:32:12 2013 -0700
>
>     dos: improve MBR signature generation
>     
>     Using tv_usec in struct timeval from gettimeofday() doesn't provide
>     enough precision to fill an unsigned 32-bit integer and isn't really
>     random. It it always less than one million when using the GNU C library
>     while an unsigned 32-bit integer ranges between 0 and 4294967295.
>     
>     In FAT filesystem creation, parted already uses a better random
>     generator, so move that code into a common function and use it
>     for MS-DOS MBR signature generation.
>     
>     * libparted/fs/r/fat/fat.c (_gen_new_serial_number): Remove.
>     (fat_create): Use generate_random_uint32 instead of
>     _gen_new_serial_number.
>     * libparted/labels/dos.c (generate_random_id): Remove.
>     (msdos_write): Use generate_random_uint32 instead of
>     generate_random_id.
>     * libparted/labels/misc.h (generate_random_uint32): New function.
>     Created from _gen_new_serial_number in libparted/fs/r/fat/fat.c with
>     additional check to avoid returning zero, which may be interpreted
>     as no FAT serial number or no MBR signature.
>
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