Why mov [es:rbx],ax complies

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l4m2

Joined: 15 Jan 2015
Posts: 674

l4m2 14 Dec 2017, 07:40

Code:

use64
mov [es:rbx],ax

3 bytes

14 Dec 2017, 07:40

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20404
Location: In your JS exploiting you and your system

revolution 14 Dec 2017, 07:41

l4m2 wrote:

Why mov [es:rbx],ax complies?
Code:
use64
mov [es:rbx],ax    
3 bytes

Because it is a valid instruction?

14 Dec 2017, 07:41

l4m2

Joined: 15 Jan 2015
Posts: 674

l4m2 14 Dec 2017, 08:10

Document say es: is invalid in 64 bit mode. Is there a time when it still work?(like POP CS)

14 Dec 2017, 08:10

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20404
Location: In your JS exploiting you and your system

revolution 14 Dec 2017, 08:28

The segment registers are still valid and must point to valid descriptors but the base and limit fields are ignored, only the access rights are used.

14 Dec 2017, 08:28

l4m2

Joined: 15 Jan 2015
Posts: 674

l4m2 14 Dec 2017, 08:35

Quote:

(Opcode)07 POP ES (Op/En)ZO (64b)Invalid (compat/leg)Valid (description)Pop top of stack into ES; increment stack
pointer.

Some say Invalid mean ignored and some say throw #UD, not sure which is correct

14 Dec 2017, 08:35

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20404
Location: In your JS exploiting you and your system

revolution 14 Dec 2017, 08:49

You posted an ES override. That is different from POP ES. The POP ES instruction might not be valid, but the ES register is valid.

14 Dec 2017, 08:49

l4m2

Joined: 15 Jan 2015
Posts: 674

l4m2 14 Dec 2017, 08:59

Quote:

If an instruction uses base registers RSP/RBP and uses a segment override prefix to specify a non-SS segment, a
canonical fault generates a #GP (instead of an #SS). In 64-bit mode, only FS and GS segment-overrides are applicable
in this situation. Other segment override prefixes (CS, DS, ES and SS) are ignored. Note that this also means
that an SS segment-override applied to a “non-stack” register reference is ignored. Such a sequence still produces
a #GP for a canonical fault (and not an #SS)

So here treat ignore as allowed

Oh it seems the ES: is also not complied out

14 Dec 2017, 08:59

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20404
Location: In your JS exploiting you and your system

revolution 14 Dec 2017, 09:11

That is for the stack registers RBP/RSP. For ordinary registers like RBX that paragraph does not apply.

14 Dec 2017, 09:11

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