Re: [PATCH v2] x86/entry_32: Move CLEAR_CPU_BUFFERS before restoring segments

[Pawan Gupta]

On Sat, Jun 29, 2024 at 11:41:07AM -0400, Brian Gerst wrote:
> >   #GP(0) - If a memory operand effective address is outside the CS, DS, ES,
> >            FS, or GS segment limit.
> 
> This isn't limited to just VM86 mode, since any user DS that isn't a
> flat segment can also cause problems.

Right.

> > CLEAR_CPU_BUFFERS macro executes VERW instruction before returning to
> > user space. Add CLEAR_CPU_BUFFERS to the macro RESTORE_REGS before it
> > restores segment registers. In vm86 mode kernel does not support SYSCALL
> > and SYSENTER instructions, so the problem is only limited to int80 path
> > in 32-bit mode. In the opportunistic SYSEXIT path use
> > CLEAR_CPU_BUFFERS_SAFE that ensures a sane %ds value.
> 
> The simpler fix is to use an SS segment override (verw
> %ss:mds_verw_sel), since the stack segment is still valid right up to
> the IRET/SYSEXIT.

Thanks a lot, I have verified that your suggestion works. I will send a new
version with your suggestion.