Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS to %ss

[Brian Gerst]

On Tue, Mar 31, 2015 at 8:38 AM, tip-bot for Denys Vlasenko
<tipbot@xxxxxxxxx> wrote:
> Commit-ID:  e7d6eefaaa443130079d73cd05039d90b3db7a4a
> Gitweb:     http://git.kernel.org/tip/e7d6eefaaa443130079d73cd05039d90b3db7a4a
> Author:     Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> AuthorDate: Fri, 27 Mar 2015 11:48:17 -0700
> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> CommitDate: Tue, 31 Mar 2015 10:45:15 +0200
>
> x86/vdso32/syscall.S: Do not load __USER32_DS to %ss
>
> This vDSO code only gets used by 64-bit kernels, not 32-bit ones.
>
> On 64-bit kernels, the data segment is the same for 32-bit and
> 64-bit userspace, and the SYSRET instruction loads %ss with its
> selector.
>
> So there's no need to repeat it by hand. Segment loads are somewhat
> expensive: tens of cycles.
>
> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> [ Removed unnecessary comment. ]
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Will Drewry <wad@xxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/63da6d778f69fd0f1345d9287f6764d58be519fa.1427482099.git.luto@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  arch/x86/vdso/vdso32/syscall.S | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/arch/x86/vdso/vdso32/syscall.S b/arch/x86/vdso/vdso32/syscall.S
> index 5415b56..6b286bb 100644
> --- a/arch/x86/vdso/vdso32/syscall.S
> +++ b/arch/x86/vdso/vdso32/syscall.S
> @@ -19,8 +19,6 @@ __kernel_vsyscall:
>  .Lpush_ebp:
>         movl    %ecx, %ebp
>         syscall
> -       movl    $__USER32_DS, %ecx
> -       movl    %ecx, %ss
>         movl    %ebp, %ecx
>         popl    %ebp
>  .Lpop_ebp:

This patch unfortunately is causing Wine to break on some applications:

Unhandled exception: stack overflow in 32-bit code (0xf779bc07).
Register dump:
 CS:0023 SS:002b DS:002b ES:002b FS:0063 GS:006b
 EIP:f779bc07 ESP:00aed60c EBP:00aed750 EFLAGS:00010216(  R- --  I   -A-P- )
 EAX:00000040 EBX:00000010 ECX:00aed750 EDX:00000040
 ESI:00000040 EDI:7ffd4000
Stack dump:
0x00aed60c:  00aed648 f7575e5b 7bcc8000 00000000
0x00aed61c:  7bc7bc09 00000010 00aed750 00000040
0x00aed62c:  00aed750 00aed650 7bcc8000 7bc7bbdd
0x00aed63c:  7bcc8000 00aed6a0 00aed750 00aed738
0x00aed64c:  7bc7cfa9 00000011 00aed750 00000040
0x00aed65c:  00000020 00000000 00000000 7bc4f141
Backtrace:
=>0 0xf779bc07 __kernel_vsyscall+0x7() in [vdso].so (0x00aed750)
  1 0xf7575e5b __libc_read+0x4a() in libpthread.so.0 (0x00aed648)
  2 0x7bc7bc09 read_reply_data+0x38(buffer=0xaed750, size=0x40)
[/home/bgerst/src/wine/wine32/dlls/ntdll/../../../dlls/ntdll/server.c:239]
in ntdll (0x00aed648)
  3 0x7bc7cfa9 wine_server_call+0x178() in ntdll (0x00aed738)
  4 0x7bc840ec NtSetEvent+0x4b(handle=0x80,
NumberOfThreadsReleased=0x0(nil))
[/home/bgerst/src/wine/wine32/dlls/ntdll/../../../dlls/ntdll/sync.c:361]
in ntdll (0x00aed7c8)
  5 0x7b874afa SetEvent+0x24(handle=<couldn't compute location>)
[/home/bgerst/src/wine/wine32/dlls/kernel32/../../../dlls/kernel32/sync.c:572]
in kernel32 (0x00aed7e8)
  6 0x0044e31a in battle.net launcher (+0x4e319) (0x00aed818)
...

__kernel_vsyscall+0x7 points to "pop %ebp".

This is on an AMD Phenom(tm) II X6 1055T Processor.

It appears that there are some subtle differences in how sysretl works
on AMD vs. Intel.  According to the Intel docs, the SS selector and
descriptor cache is completely reset by sysret to fixed values.  The
AMD docs however are concerning:

AMD's syscall:
SS.sel = MSR_STAR.SYSCALL_CS + 8
SS.attr = 64-bit stack,dpl0
SS.base = 0x00000000
SS.limit = 0xFFFFFFFF

AMD's sysret:
SS.sel = MSR_STAR.SYSRET_CS + 8 // SS selector is changed,
                                // SS base, limit, attributes unchanged.

Not changing base or limit is no big deal, but not changing attributes
could be the problem.  It might be leaving the "64-bit stack"
attribute set, for whatever that means.

Reloading SS from the GDT would obviously reset any bad state left by
sysretl.  Unfortunately we may have to put it back in, and then NOP it
out on Intel.

--
Brian Gerst
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