Re: [PATCH v2 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

[Andy Lutomirski]

[cc: Oleg, Borislav]

On Tue, Mar 10, 2015 at 7:03 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> The comment in the signal code says that apps can save/restore other
> segments on their own.  It's true that apps can *save* SS on their
> own, but there's no way for apps to restore it: SYSCALL effectively
> resets SS to __USER_DS, so any value that user code tries to load
> into SS gets lost on entry to sigreturn.
>
> This recycles two padding bytes in the segment selector area for SS.
>
> While we're at it, we need a second change to make this useful.  If
> the signal we're delivering is caused by a bad SS value, saving that
> value isn't enough.  We need to remove that bad value from the regs
> before we try to deliver the signal.  Oddly, x32 already got this
> right.
>
> I suspect that 64-bit programs that try to run 16-bit code and uses
> signals will have a lot of trouble without this.
>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/sigcontext.h      |  2 +-
>  arch/x86/include/uapi/asm/sigcontext.h |  2 +-
>  arch/x86/kernel/signal.c               | 20 +++++++++++---------
>  3 files changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h
> index 9dfce4e0417d..f910cdcb71fd 100644
> --- a/arch/x86/include/asm/sigcontext.h
> +++ b/arch/x86/include/asm/sigcontext.h
> @@ -59,7 +59,7 @@ struct sigcontext {
>         unsigned short cs;
>         unsigned short gs;
>         unsigned short fs;
> -       unsigned short __pad0;
> +       unsigned short ss;
>         unsigned long err;
>         unsigned long trapno;
>         unsigned long oldmask;
> diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
> index d8b9f9081e86..076b11fd6fa1 100644
> --- a/arch/x86/include/uapi/asm/sigcontext.h
> +++ b/arch/x86/include/uapi/asm/sigcontext.h
> @@ -179,7 +179,7 @@ struct sigcontext {
>         __u16 cs;
>         __u16 gs;
>         __u16 fs;
> -       __u16 __pad0;
> +       __u16 ss;
>         __u64 err;
>         __u64 trapno;
>         __u64 oldmask;
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index ed37a768d0fc..40f34574fb36 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -94,15 +94,8 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc,
>                 COPY(r15);
>  #endif /* CONFIG_X86_64 */
>
> -#ifdef CONFIG_X86_32
>                 COPY_SEG_CPL3(cs);
>                 COPY_SEG_CPL3(ss);
> -#else /* !CONFIG_X86_32 */
> -               /* Kernel saves and restores only the CS segment register on signals,
> -                * which is the bare minimum needed to allow mixed 32/64-bit code.
> -                * App's signal handler can save/restore other segments if needed. */
> -               COPY_SEG_CPL3(cs);
> -#endif /* CONFIG_X86_32 */
>
>                 get_user_ex(tmpflags, &sc->flags);
>                 regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS);
> @@ -164,6 +157,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
>                 put_user_ex(regs->cs, &sc->cs);
>                 put_user_ex(0, &sc->gs);
>                 put_user_ex(0, &sc->fs);
> +               put_user_ex(regs->ss, &sc->ss);
>  #endif /* CONFIG_X86_32 */
>
>                 put_user_ex(fpstate, &sc->fpstate);
> @@ -457,9 +451,17 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
>
>         regs->sp = (unsigned long)frame;
>
> -       /* Set up the CS register to run signal handlers in 64-bit mode,
> -          even if the handler happens to be interrupting 32-bit code. */
> +       /* Set up the CS and SS registers to run signal handlers in
> +          64-bit mode, even if the handler happens to be interrupting
> +          32-bit or 16-bit code.
> +
> +          SS is subtle.  In 64-bit mode, we don't need any particular
> +          SS descriptor, but we do need SS to be valid.  It's possible
> +          that the old SS is entirely bogus -- this can happen if the
> +          signal we're trying to deliver is #GP or #SS caused by a bad
> +          SS value. */
>         regs->cs = __USER_CS;
> +       regs->ss = __USER_DS;
>
>         return 0;
>  }
> --
> 2.3.0
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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