Re: [PATCH v3] x86: Use `get_random_u8' for kernel stack offset randomization

[Jason A. Donenfeld]

On Mon, Jan 30, 2023 at 09:30:31PM +0000, Maciej W. Rozycki wrote:
> For x86 kernel stack offset randomization uses the RDTSC instruction, 
> which according to H. Peter Anvin is not a secure source of entropy:
> 
> "RDTSC isn't a super fast instruction either, but what is *way* more
> significant is that this use of RDTSC is NOT safe: in certain power states
> it may very well be that stone number of lower bits of TSC contain no
> entropy at all."
> 
> It also 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 switch to our generic entropy source and use `get_random_u8' 
> instead, which according to Jason A. Donenfeld is supposed to be fast 
> enough:
> 
> "Generally it's very very fast, as most cases wind up being only a
> memcpy -- in this case, a single byte copy. So by and large it should
> be suitable. It's fast enough now that most networking things are able
> to use it. And lots of other places where you'd want really high
> performance. So I'd expect it's okay to use here too. And if it is too
> slow, we should figure out how to make it faster. But I don't suspect
> it'll be too slow."
> 
> Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
> Suggested-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> Fixes: fe950f602033 ("x86/entry: Enable random_kstack_offset support")
> Cc: stable@xxxxxxxxxxxxxxx # v5.13+
> ---
> Changes from v2:
> 
> - Use `get_random_u8' rather than `rdtsc', universally; update the heading 
>   (was: "x86: Disable kernel stack offset randomization for !TSC") and the 
>   description accordingly.
> 
> - As a security concern mark for backporting.
> 
> Changes from v1:
> 
> - Disable randomization at run time rather than in configuration.
> ---
>  arch/x86/include/asm/entry-common.h |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> linux-x86-randomize-kstack-offset-random-u8.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
> @@ -2,6 +2,7 @@
>  #ifndef _ASM_X86_ENTRY_COMMON_H
>  #define _ASM_X86_ENTRY_COMMON_H
>  
> +#include <linux/random.h>
>  #include <linux/randomize_kstack.h>
>  #include <linux/user-return-notifier.h>
>  
> @@ -85,7 +86,7 @@ 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);
> +	choose_random_kstack_offset(get_random_u8());
>  }
>  #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare

Acked-by: Jason A. Donenfeld <Jason@xxxxxxxxx>