Messages in this thread Patch in this message | | | Date | Tue, 10 Jan 2023 16:19:35 +0100 | From | "Jason A. Donenfeld" <> | Subject | Re: [PATCH v2] x86: Disable kernel stack offset randomization for !TSC |
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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
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