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Date:   Wed, 29 Nov 2017 17:42:25 +0000
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     alexander.levin@...izon.com
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH AUTOSEL for 4.9 04/52] x86/selftests: Add clobbers for
 int80 on x86_64

Hi Sasha,

I would object including this to stable tree:
1. It's selftest fixup
2. I have never saw selftests failing because of it (but it's possible)
    I saw this in CRIU (Checkpoint Restore In Userspace) project
    triggering, so I've fixed the selftests, mostly for documentation
    reasons - as people in userspace can reuse this code and think
    it's good and will just work after copy-paste (which is not the case).

So, this patch doesn't look urgent to include it in -stable kernel.

Thanks,
Dmitry

2017-11-29 17:21 GMT+00:00  <alexander.levin@...izon.com>:
> From: Dmitry Safonov <dsafonov@...tuozzo.com>
>
> [ Upstream commit 2a4d0c627f5374f365a873dea4e10ae0bb437680 ]
>
> Kernel erases R8..R11 registers prior returning to userspace
> from int80:
>
>   https://lkml.org/lkml/2009/10/1/164
>
> GCC can reuse these registers and doesn't expect them to change
> during syscall invocation. I met this kind of bug in CRIU once
> GCC 6.1 and CLANG stored local variables in those registers
> and the kernel zerofied them during syscall:
>
>   https://github.com/xemul/criu/commit/990d33f1a1cdd17bca6c2eb059ab3be2564f7fa2
>
> By that reason I suggest to add those registers to clobbers
> in selftests.  Also, as noted by Andy - removed unneeded clobber
> for flags in INT $0x80 inline asm.
>
> Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>
> Acked-by: Andy Lutomirski <luto@...nel.org>
> Cc: 0x7f454c46@...il.com
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: linux-kselftest@...r.kernel.org
> Link: http://lkml.kernel.org/r/20170213101336.20486-1-dsafonov@virtuozzo.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
> ---
>  tools/testing/selftests/x86/fsgsbase.c            |  2 +-
>  tools/testing/selftests/x86/ldt_gdt.c             | 16 +++++++++++-----
>  tools/testing/selftests/x86/ptrace_syscall.c      |  3 ++-
>  tools/testing/selftests/x86/single_step_syscall.c |  5 ++++-
>  4 files changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
> index 9b4610c6d3fb..f249e042b3b5 100644
> --- a/tools/testing/selftests/x86/fsgsbase.c
> +++ b/tools/testing/selftests/x86/fsgsbase.c
> @@ -245,7 +245,7 @@ void do_unexpected_base(void)
>                 long ret;
>                 asm volatile ("int $0x80"
>                               : "=a" (ret) : "a" (243), "b" (low_desc)
> -                             : "flags");
> +                             : "r8", "r9", "r10", "r11");
>                 memcpy(&desc, low_desc, sizeof(desc));
>                 munmap(low_desc, sizeof(desc));
>
> diff --git a/tools/testing/selftests/x86/ldt_gdt.c b/tools/testing/selftests/x86/ldt_gdt.c
> index e717fed80219..b9a22f18566a 100644
> --- a/tools/testing/selftests/x86/ldt_gdt.c
> +++ b/tools/testing/selftests/x86/ldt_gdt.c
> @@ -45,6 +45,12 @@
>  #define AR_DB                  (1 << 22)
>  #define AR_G                   (1 << 23)
>
> +#ifdef __x86_64__
> +# define INT80_CLOBBERS "r8", "r9", "r10", "r11"
> +#else
> +# define INT80_CLOBBERS
> +#endif
> +
>  static int nerrs;
>
>  /* Points to an array of 1024 ints, each holding its own index. */
> @@ -634,7 +640,7 @@ static int invoke_set_thread_area(void)
>         asm volatile ("int $0x80"
>                       : "=a" (ret), "+m" (low_user_desc) :
>                         "a" (243), "b" (low_user_desc)
> -                     : "flags");
> +                     : INT80_CLOBBERS);
>         return ret;
>  }
>
> @@ -703,7 +709,7 @@ static void test_gdt_invalidation(void)
>                         "+a" (eax)
>                       : "m" (low_user_desc_clear),
>                         [arg1] "r" ((unsigned int)(unsigned long)low_user_desc_clear)
> -                     : "flags");
> +                     : INT80_CLOBBERS);
>
>         if (sel != 0) {
>                 result = "FAIL";
> @@ -734,7 +740,7 @@ static void test_gdt_invalidation(void)
>                         "+a" (eax)
>                       : "m" (low_user_desc_clear),
>                         [arg1] "r" ((unsigned int)(unsigned long)low_user_desc_clear)
> -                     : "flags");
> +                     : INT80_CLOBBERS);
>
>         if (sel != 0) {
>                 result = "FAIL";
> @@ -767,7 +773,7 @@ static void test_gdt_invalidation(void)
>                         "+a" (eax)
>                       : "m" (low_user_desc_clear),
>                         [arg1] "r" ((unsigned int)(unsigned long)low_user_desc_clear)
> -                     : "flags");
> +                     : INT80_CLOBBERS);
>
>  #ifdef __x86_64__
>         syscall(SYS_arch_prctl, ARCH_GET_FS, &new_base);
> @@ -820,7 +826,7 @@ static void test_gdt_invalidation(void)
>                         "+a" (eax)
>                       : "m" (low_user_desc_clear),
>                         [arg1] "r" ((unsigned int)(unsigned long)low_user_desc_clear)
> -                     : "flags");
> +                     : INT80_CLOBBERS);
>
>  #ifdef __x86_64__
>         syscall(SYS_arch_prctl, ARCH_GET_GS, &new_base);
> diff --git a/tools/testing/selftests/x86/ptrace_syscall.c b/tools/testing/selftests/x86/ptrace_syscall.c
> index b037ce9cf116..eaea92439708 100644
> --- a/tools/testing/selftests/x86/ptrace_syscall.c
> +++ b/tools/testing/selftests/x86/ptrace_syscall.c
> @@ -58,7 +58,8 @@ static void do_full_int80(struct syscall_args32 *args)
>         asm volatile ("int $0x80"
>                       : "+a" (args->nr),
>                         "+b" (args->arg0), "+c" (args->arg1), "+d" (args->arg2),
> -                       "+S" (args->arg3), "+D" (args->arg4), "+r" (bp));
> +                       "+S" (args->arg3), "+D" (args->arg4), "+r" (bp)
> +                       : : "r8", "r9", "r10", "r11");
>         args->arg5 = bp;
>  #else
>         sys32_helper(args, int80_and_ret);
> diff --git a/tools/testing/selftests/x86/single_step_syscall.c b/tools/testing/selftests/x86/single_step_syscall.c
> index 50c26358e8b7..a48da95c18fd 100644
> --- a/tools/testing/selftests/x86/single_step_syscall.c
> +++ b/tools/testing/selftests/x86/single_step_syscall.c
> @@ -56,9 +56,11 @@ static volatile sig_atomic_t sig_traps;
>  #ifdef __x86_64__
>  # define REG_IP REG_RIP
>  # define WIDTH "q"
> +# define INT80_CLOBBERS "r8", "r9", "r10", "r11"
>  #else
>  # define REG_IP REG_EIP
>  # define WIDTH "l"
> +# define INT80_CLOBBERS
>  #endif
>
>  static unsigned long get_eflags(void)
> @@ -140,7 +142,8 @@ int main()
>
>         printf("[RUN]\tSet TF and check int80\n");
>         set_eflags(get_eflags() | X86_EFLAGS_TF);
> -       asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid));
> +       asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
> +                       : INT80_CLOBBERS);
>         check_result();
>
>         /*
> --
> 2.11.0

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