Bug in mov to/from control register

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CandyMan

Joined: 04 Sep 2009
Posts: 414
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop

CandyMan 02 Mar 2020, 21:23

Code:

        use32
        mov     eax,cr8         ;\ fasm 1.73.22 generates the same code
        mov     eax,cr9         ;/
        mov     cr8,eax         ;\ same as above
        mov     cr9,eax         ;/

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02 Mar 2020, 21:23

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8428
Location: Kraków, Poland

Tomasz Grysztar 02 Mar 2020, 21:38

Probably CR9 should not be allowed at all here. Looking at recent AMD manuals I see that only CR8 has ever been documented for F0-prefixed variant.

02 Mar 2020, 21:38

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20666
Location: In your JS exploiting you and your system

revolution 02 Feb 2023, 10:20

Intel manuals mention that cr8 is not encodable in 32-bit mode.

https://www.felixcloutier.com/x86/mov-1

02 Feb 2023, 10:20

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8428
Location: Kraków, Poland

Tomasz Grysztar 02 Feb 2023, 10:29

That's because this is an AMD-specific feature.

AMD64 Architecture Programmer’s Manual Volume 3 wrote:

CR8 can be read and written in 64-bit mode, using a REX prefix. CR8 can be read and written in all modes using a LOCK prefix instead of a REX prefix to specify the additional opcode bit. To verify whether the LOCK prefix can be used in this way, check for support of this feature. CPUID Fn8000_0001_ECX[AltMovCr8] = 1, indicates that this feature is supported.

02 Feb 2023, 10:29

Roman

Joined: 21 Apr 2012
Posts: 1956

Roman 02 Feb 2023, 12:05

Windows 10 CPU AMD Ryzen 3500
I try this. Fasmw 1.73 compiled ok.

Code:

lock mov rax,cr15 ;this crash. IDA Pro show priveleged instruction
mov rax,cr15 ;this crash.
mov     rax,cr0  ;funny IDA Pro show mov     rax,cr8d

02 Feb 2023, 12:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20666
Location: In your JS exploiting you and your system

revolution 02 Feb 2023, 12:43

All the CR0...CR15 require PL=0.

You need kernel code to execute them. And CR15 doesn't exist anyway, so even PL=0 isn't going to work for that.

02 Feb 2023, 12:43

Roman

Joined: 21 Apr 2012
Posts: 1956

Roman 02 Feb 2023, 17:27

What is PL = 0?
How set PL to 0 ?

02 Feb 2023, 17:27

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20666
Location: In your JS exploiting you and your system

revolution 02 Feb 2023, 17:36

Privilege level 0. The kernel code.

If you use Windows then you need a kernel mode driver. Or hack the kernel binaries.

You can't get there from user code, unless you can find a vulnerability.

02 Feb 2023, 17:36

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