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Date:   Tue, 10 Jan 2023 16:19:35 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86: Disable kernel stack offset randomization for
 !TSC
Message-ID: <Y72CByxvewQv78YH@zx2c4.com>
References: <alpine.DEB.2.21.2301082113350.65308@angie.orcam.me.uk>
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On Sun, Jan 08, 2023 at 09:26:11PM +0000, Maciej W. Rozycki wrote:
> For x86 kernel stack offset randomization uses the RDTSC instruction, 
> which causes an invalid opcode exception with hardware that does not 
> implement this instruction:
> 
> process '/sbin/init' started with executable stack
> invalid opcode: 0000 [#1]
> CPU: 0 PID: 1 Comm: init Not tainted 6.1.0-rc4+ #1
> EIP: exit_to_user_mode_prepare+0x90/0xe1
> Code: 30 02 00 75 ad 0f ba e3 16 73 05 e8 a7 a5 fc ff 0f ba e3 0e 73 05 e8 3e af fc ff a1 c4 c6 51 c0 85 c0 7e 13 8b 0d ac 01 53 c0 <0f> 31 0f b6 c0 31 c1 89 0d ac 01 53 c0 83 3d 30 ed 62 c0 00 75 33
> EAX: 00000001 EBX: 00004000 ECX: 00000000 EDX: 000004ff
> ESI: c10253c0 EDI: 00000000 EBP: c1027f98 ESP: c1027f8c
> DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068 EFLAGS: 00010002
> CR0: 80050033 CR2: bfe8659b CR3: 012e0000 CR4: 00000000
> Call Trace:
>  ? rest_init+0x72/0x72
>  syscall_exit_to_user_mode+0x15/0x27
>  ret_from_fork+0x10/0x30
> EIP: 0xb7f74800
> Code: Unable to access opcode bytes at 0xb7f747d6.
> EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000
> ESI: 00000000 EDI: 00000000 EBP: 00000000 ESP: bfe864b0
> DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 007b EFLAGS: 00000200
> ---[ end trace 0000000000000000 ]---
> EIP: exit_to_user_mode_prepare+0x90/0xe1
> Code: 30 02 00 75 ad 0f ba e3 16 73 05 e8 a7 a5 fc ff 0f ba e3 0e 73 05 e8 3e af fc ff a1 c4 c6 51 c0 85 c0 7e 13 8b 0d ac 01 53 c0 <0f> 31 0f b6 c0 31 c1 89 0d ac 01 53 c0 83 3d 30 ed 62 c0 00 75 33
> EAX: 00000001 EBX: 00004000 ECX: 00000000 EDX: 000004ff
> ESI: c10253c0 EDI: 00000000 EBP: c1027f98 ESP: c1027f8c
> DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068 EFLAGS: 00010002
> CR0: 80050033 CR2: b7f747d6 CR3: 012e0000 CR4: 00000000
> Kernel panic - not syncing: Fatal exception
> 
> Therefore do not use randomization where the CPU does not have the TSC 
> feature.
> 
> Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
> ---
> Changes from v1:
> 
> - Disable randomization at run time rather than in configuration.
> ---
>  arch/x86/include/asm/entry-common.h |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> linux-x86-randomize-kstack-offset-tsc.diff
> Index: linux-macro/arch/x86/include/asm/entry-common.h
> ===================================================================
> --- linux-macro.orig/arch/x86/include/asm/entry-common.h
> +++ linux-macro/arch/x86/include/asm/entry-common.h
> @@ -5,6 +5,7 @@
>  #include <linux/randomize_kstack.h>
>  #include <linux/user-return-notifier.h>
>  
> +#include <asm/cpufeature.h>
>  #include <asm/nospec-branch.h>
>  #include <asm/io_bitmap.h>
>  #include <asm/fpu/api.h>
> @@ -85,7 +86,8 @@ static inline void arch_exit_to_user_mod
>  	 * Therefore, final stack offset entropy will be 5 (x86_64) or
>  	 * 6 (ia32) bits.
>  	 */
> -	choose_random_kstack_offset(rdtsc() & 0xFF);
> +	if (cpu_feature_enabled(X86_FEATURE_TSC))
> +		choose_random_kstack_offset(rdtsc() & 0xFF);

What would happen if you just called `get_random_u8()` here?

Jason