Re: [PATCH 3/3] x86/ptrace: Fix 32-bit PTRACE_SETREGS vs fsbase and gsbase

[Andy Lutomirski]

On Wed, Jun 24, 2020 at 3:50 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> Debuggers expect that doing PTRACE_GETREGS, then poking at a tracee
> and maybe letting it run for a while, then doing PTRACE_SETREGS will
> put the tracee back where it was.  In the specific case of a 32-bit
> tracer and tracee, the PTRACE_GETREGS/SETREGS data structure doesn't
> have fs_base or gs_base fields, so FSBASE and GSBASE fields are
> never stored anywhere.  Everything used to still work because
> nonzero FS or GS would result full reloads of the segment registers
> when the tracee resumes, and the bases associated with FS==0 or
> GS==0 are irrelevant to 32-bit code.
>
> Adding FSGSBASE support broke this: when FSGSBASE is enabled, FSBASE
> and GSBASE are now restored independently of FS and GS for all tasks
> when context-switched in.  This means that, if a 32-bit tracer
> restores a previous state using PTRACE_SETREGS but the tracee's
> pre-restore and post-restore bases don't match, then the tracee is
> resumed with the wrong base.
>
> Fix it by explicitly loading the base when a 32-bit tracer pokes FS
> or GS on a 64-bit kernel.

> diff --git a/tools/testing/selftests/x86/fsgsbase_restore.c b/tools/testing/selftests/x86/fsgsbase_restore.c
> new file mode 100644
> index 000000000000..70502a708dee
> --- /dev/null
> +++ b/tools/testing/selftests/x86/fsgsbase_restore.c

> +       if (false && syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {

Whoops.  That 'false &&' shouldn't be there.  Want a v2?