linux-kernel - Re: [patch -next] x86, microcode, AMD: signedness bug in generic_load

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 20 Feb 2011 10:50:11 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Borislav Petkov <bp@...64.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Dan Carpenter <error27@...il.com>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"maintainer:X86 ARCHITECTURE..." <x86@...nel.org>,
	"open list:AMD MICROCODE UPD..." <amd64-microcode@...64.org>,
	open list <linux-kernel@...r.kernel.org>,
	"kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: [patch -next] x86, microcode, AMD: signedness bug in
	generic_load_microcode()

On Sun, Feb 20, 2011 at 03:14:52PM +0100, Borislav Petkov wrote:
> int f() {
>         return 0xa5a5a5a5;
> }
> 
> int main()
> {
> 
>         char ret = f();
> 
>         printf("ret = 0x%016x\n", ret);
> 
>         return 0;
> } 
> --
> 
> doesn't cause a warning and prints a sign extended 0x00000000ffffffa5
> which is cast to the return type of the function. If ret is an unsigned
> char, then we return a 0x00000000000000a5.
> 
> I found something about it in the C99 standard??, section "6.5.16.1 Simple
> assignment":
> 
> 4.  EXAMPLE 1       In the program fragment
> 
>            int f(void);
>            char c;
>            /* ... */
>            if ((c = f()) == -1)
>                     /* ... */
> 
> the int value returned by the function may be truncated when stored in
> the char, and then converted back to int width prior to the comparison.
> In an implementation in which ??????plain?????? char has the same range
> of values as unsigned char (and char is narrower than int), the result
> of the conversion cannot be negative, so the operands of the comparison
> can never compare equal. Therefore, for full portability, the variable c
> should be declared as int."
> 
> so the whole "... may be truncated.. " could mean a lot of things. From
> my example above, gcc does truncate the int return type to a byte-sized
> char only when they differ in signedness.

No, that's not what's going on.  GCC _is_ truncating to a byte, 0xa5,
whether it's signed or not.  Then at the time of the call to printf,
the 0xa5 is cast to int.  If the char is signed, 0xa5 is sign-extended;
if unsigned, it's zero-extended.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/