Re: 2.6.34 rex64 sysret instruction setup not preserving flags (r11 contents and eflags do not match)

[Jeffrey Merkey]

On Fri, Jul 2, 2010 at 4:18 PM, Jeffrey Merkey <jeffmerkey@xxxxxxxxx> wrote:
> On an AMD64 based system executing int 1 exceptions across a rex64
> sysret, if the debugger sets the trap flag, r11 which holds the eflags
> values for the
> sysret return from syscall, the flags do not appear to get set
> resutling in the int exception nesting by calling sysret over and over
> again until the kernel stack
> runs off the end.  Looks like the resume did not get set on this instruction.
>
> sysret on AMD requires the flags be saved into r11 and what I am
> seeing is the flags not matching what has been set in the pt_regs
> struct.
>
> Jeff
>

The specific function to look at is in entry_64.S sysret_check.  The
sequence goes;

swapgs
rex64 sysret

After swapgs the eflags in r11 do not match the actual flags passed.
The resume flag gets cleared when the sysret instruction completes,
and int 1 keeps firing on that processor until the stack runs out of
space.

Jeff
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