Messages in this thread Patch in this message | | | Date | Wed, 17 Aug 2022 12:56:15 +0200 | From | Peter Zijlstra <> | Subject | [PATCH] x86/kvm, objtool: Avoid fastop ENDBR from being sealed |
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On Tue, Aug 09, 2022 at 02:44:28AM +0000, Xu, Pengfei wrote: > Hi Peter, > > Greeting!
You again forgot to Cc LKML and relevant people...
> Platform: ADL-P (I tried that TGL-H could reproduce this issue also) > Kernel: 5.19 mainline kernel with kernel IBT enabled. > > Boot up ADL-P, and then run syzkaller fuzzing tests, syzkaller will start up guests(Guest kernel is 5.19 mainline also but doesn't enable kernel IBT) to do fuzzing tests. > After about 1 hour later, there was "traps: Missing ENDBR: andw_ax_dx+0x0/0x10 [kvm]" info generated. > > [ 5048.057266] traps: Missing ENDBR: andw_ax_dx+0x0/0x10 [kvm] > [ 5048.057440] ------------[ cut here ]------------ > [ 5048.057457] kernel BUG at arch/x86/kernel/traps.c:253! > [ 5048.057482] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI > [ 5048.057908] <TASK> > [ 5048.057919] asm_exc_control_protection+0x2b/0x30 > [ 5048.057940] RIP: 0010:andw_ax_dx+0x0/0x10 [kvm] > [ 5048.057999] Code: c3 cc cc cc cc 0f 1f 44 00 00 66 0f 1f 00 48 19 d0 c3 cc cc cc cc 0f 1f 40 00 f3 0f 1e fa 20 d0 c3 cc cc cc cc 0f 1f 44 00 00 <66> 0f 1f 00 66 21 d0 c3 cc cc cc cc 0f 1f 40 00 66 0f 1f 00 21 d0 > ... > [ 5048.058209] ? andb_al_dl+0x10/0x10 [kvm] > [ 5048.058265] ? fastop+0x5d/0xa0 [kvm] > [ 5048.058320] x86_emulate_insn+0x822/0x1060 [kvm] > [ 5048.058376] x86_emulate_instruction+0x46f/0x750 [kvm] > [ 5048.058433] complete_emulated_mmio+0x216/0x2c0 [kvm] > [ 5048.058488] kvm_arch_vcpu_ioctl_run+0x604/0x650 [kvm] > [ 5048.058543] kvm_vcpu_ioctl+0x2f4/0x6b0 [kvm] > [ 5048.058590] ? wake_up_q+0xa0/0xa0 > ... > [ 6324.778942] traps: Missing ENDBR: andw_ax_dx+0x0/0x10 [kvm] > ... > [ 8760.430810] traps: Missing ENDBR: andw_ax_dx+0x0/0x10 [kvm]
Urgh, fastops again :/
> Dmesg and all the ko files in "/lib/modules/5.19.0-m2/kernel/arch/x86" are in attached.
From your kvm.ko:
00000000000376f0 <andw_ax_dx>: 376f0: f3 0f 1e fa endbr64 376f4: 66 21 d0 and %dx,%ax 376f7: e9 00 00 00 00 jmp 376fc <andw_ax_dx+0xc> 376f8: R_X86_64_PLT32 __x86_return_thunk-0x4 376fc: 0f 1f 40 00 nopl 0x0(%rax)
However, the Code: thing above gives:
2a:* 66 0f 1f 00 nopw (%rax) <-- trapping instruction 2e: 66 21 d0 and %dx,%ax 31: c3 ret 32: cc int3 33: cc int3 34: cc int3 35: cc int3 36: 0f 1f 40 00 nopl 0x0(%rax)
From that we can tell gen_endbr_poison() has been used to scribble the endbr -- so clearly objtool didn't manage to find code references here.
And indeed, kvm.ko's .rela.ibt_endbr_seal section includes:
00000000000002c0 0000000100000002 R_X86_64_PC32 0000000000000000 .text + 376f0
--- Subject: x86/kvm, objtool: Avoid fastop ENDBR from being sealed
Xu reported a number of "Missing ENDBR" triggers for the KVM fastop emulation code. It turns out that because of how the fastops are set up, only the first of a series -- the 8 byte variants that overlap with the em_ symbols -- is found referenced.
Specifically:
.pushsection .text, "ax" .global em_and .align 16 em_and: .align 16 .type andb_al_dl, @function andb_al_dl: endbr64 andb %dl, %al 11: jmp __x86_return_thunk .size andb_al_dl, .-andb_al_dl .align 16 .type andw_ax_dx, @function andw_ax_dx: endbr64 andw %dx, %ax 11: jmp __x86_return_thunk .size andw_ax_dx, .-andw_ax_dx .align 16 .type andl_eax_edx, @function andl_eax_edx: endbr64 andl %edx, %eax 11: jmp __x86_return_thunk .size andl_eax_edx, .-andl_eax_edx .align 16 .type andq_rax_rdx, @function andq_rax_rdx: endbr64 andq %rdx, %rax 11: jmp __x86_return_thunk .size andq_rax_rdx, .-andq_rax_rdx .popsection
Only has the em_and symbol referenced, resulting in and{w,l,q}_* getting sealed.
Add (yet another) annotation to inhibit objtool from sealing a specific ENDBR instance.
Fixes: 6649fa876da4 ("x86/ibt,kvm: Add ENDBR to fastops") Reported-by: "Xu, Pengfei" <pengfei.xu@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/x86/include/asm/ibt.h | 5 +++++ arch/x86/kvm/emulate.c | 4 ++-- include/linux/objtool.h | 6 ++++++ tools/objtool/check.c | 44 ++++++++++++++++++++++++++++++++++++++------ 4 files changed, 51 insertions(+), 8 deletions(-)
diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h index 689880eca9ba..f32ba1c4e619 100644 --- a/arch/x86/include/asm/ibt.h +++ b/arch/x86/include/asm/ibt.h @@ -29,6 +29,10 @@ #define ASM_ENDBR "endbr32\n\t" #endif +#define ASM_ENDBR_NOSEAL \ + ANNOTATE_NOSEAL \ + ASM_ENDBR + #define __noendbr __attribute__((nocf_check)) static inline __attribute_const__ u32 gen_endbr(void) @@ -84,6 +88,7 @@ extern __noendbr void ibt_restore(u64 save); #ifndef __ASSEMBLY__ #define ASM_ENDBR +#define ASM_ENDBR_NOSEAL #define __noendbr diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index b4eeb7c75dfa..d51ee8a3bcae 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -326,7 +326,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop); ".align " __stringify(FASTOP_SIZE) " \n\t" \ ".type " name ", @function \n\t" \ name ":\n\t" \ - ASM_ENDBR + ASM_ENDBR_NOSEAL #define FOP_FUNC(name) \ __FOP_FUNC(#name) @@ -461,7 +461,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop); ".align " __stringify(SETCC_ALIGN) " \n\t" \ ".type " #op ", @function \n\t" \ #op ": \n\t" \ - ASM_ENDBR \ + ASM_ENDBR_NOSEAL \ #op " %al \n\t" \ __FOP_RET(#op) \ ".skip " __stringify(SETCC_ALIGN) " - (.-" #op "), 0xcc \n\t" diff --git a/include/linux/objtool.h b/include/linux/objtool.h index 62c54ffbeeaa..ad752f8b3b36 100644 --- a/include/linux/objtool.h +++ b/include/linux/objtool.h @@ -90,6 +90,12 @@ struct unwind_hint { _ASM_PTR " 986b\n\t" \ ".popsection\n\t" +#define ANNOTATE_NOSEAL \ + "986: \n\t" \ + ".pushsection .discard.noseal\n\t" \ + _ASM_PTR " 986b\n\t" \ + ".popsection\n\t" + #define ASM_REACHABLE \ "998:\n\t" \ ".pushsection .discard.reachable\n\t" \ diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 0cec74da7ffe..0d04d0a707f4 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -2111,6 +2111,40 @@ static int read_noendbr_hints(struct objtool_file *file) return 0; } +static void mark_endbr_used(struct instruction *insn) +{ + if (!list_empty(&insn->call_node)) + list_del_init(&insn->call_node); +} + +static int read_noseal_hints(struct objtool_file *file) +{ + struct section *sec; + struct instruction *insn; + struct reloc *reloc; + + sec = find_section_by_name(file->elf, ".rela.discard.noseal"); + if (!sec) + return 0; + + list_for_each_entry(reloc, &sec->reloc_list, list) { + insn = find_insn(file, reloc->sym->sec, reloc->sym->offset + reloc->addend); + if (!insn) { + WARN("bad .discard.noseal entry"); + return -1; + } + + if (insn->type != INSN_ENDBR) { + WARN_FUNC("ANNOTATE_NOSEAL not on ENDBR", insn->sec, insn->offset); + continue; + } + + mark_endbr_used(insn); + } + + return 0; +} + static int read_retpoline_hints(struct objtool_file *file) { struct section *sec; @@ -2356,6 +2390,10 @@ static int decode_sections(struct objtool_file *file) if (ret) return ret; + ret = read_noseal_hints(file); + if (ret) + return ret; + /* * Must be before add_{jump_call}_destination. */ @@ -3952,12 +3990,6 @@ static int validate_functions(struct objtool_file *file) return warnings; } -static void mark_endbr_used(struct instruction *insn) -{ - if (!list_empty(&insn->call_node)) - list_del_init(&insn->call_node); -} - static int validate_ibt_insn(struct objtool_file *file, struct instruction *insn) { struct instruction *dest;
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