Messages in this thread Patch in this message | | | Date | Thu, 29 Nov 2018 10:33:42 -0600 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64 |
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On Thu, Nov 29, 2018 at 03:38:53PM +0100, Peter Zijlstra wrote: > On Thu, Nov 29, 2018 at 05:37:39AM -0800, Andy Lutomirski wrote: > > > > > > > On Nov 29, 2018, at 1:42 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > On Wed, Nov 28, 2018 at 10:05:54PM -0800, Andy Lutomirski wrote: > > > > > >>>> +static void static_call_bp_handler(struct pt_regs *regs, void *_data) > > >>>> +{ > > >>>> + struct static_call_bp_data *data = _data; > > >>>> + > > >>>> + /* > > >>>> + * For inline static calls, push the return address on the stack so the > > >>>> + * "called" function will return to the location immediately after the > > >>>> + * call site. > > >>>> + * > > >>>> + * NOTE: This code will need to be revisited when kernel CET gets > > >>>> + * implemented. > > >>>> + */ > > >>>> + if (data->ret) { > > >>>> + regs->sp -= sizeof(long); > > >>>> + *(unsigned long *)regs->sp = data->ret; > > >>>> + } > > >> > > >> You can’t do this. Depending on the alignment of the old RSP, which > > >> is not guaranteed, this overwrites regs->cs. IRET goes boom. > > > > > > I don't get it; can you spell that out? > > > > > > The way I understand it is that we're at a location where a "E8 - Near > > > CALL" instruction should be, and thus RSP should be the regular kernel > > > stack, and the above simply does "PUSH ret", which is what that CALL > > > would've done too. > > > > > > > int3 isn’t IST anymore, so the int3 instruction conditionally > > subtracts 8 from RSP and then pushes SS, etc. So my email was > > obviously wrong wrt “cs”, but you’re still potentially overwriting the > > int3 IRET frame. > > ARGH!.. > > can't we 'fix' that again? The alternative is moving that IRET-frame and > fixing everything up, which is going to be fragile, ugly and such > things more. > > Commit d8ba61ba58c8 ("x86/entry/64: Don't use IST entry for #BP stack") > doesn't list any strong reasons for why it should NOT be an IST.
This seems to work...
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index ce25d84023c0..184523447d35 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -876,7 +876,7 @@ apicinterrupt IRQ_WORK_VECTOR irq_work_interrupt smp_irq_work_interrupt * @paranoid == 2 is special: the stub will never switch stacks. This is for * #DF: if the thread stack is somehow unusable, we'll still get a useful OOPS. */ -.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 +.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 create_gap=0 ENTRY(\sym) UNWIND_HINT_IRET_REGS offset=\has_error_code*8 @@ -891,6 +891,12 @@ ENTRY(\sym) pushq $-1 /* ORIG_RAX: no syscall to restart */ .endif + .if \create_gap == 1 + .rept 6 + pushq 5*8(%rsp) + .endr + .endif + .if \paranoid == 1 testb $3, CS-ORIG_RAX(%rsp) /* If coming from userspace, switch stacks */ jnz .Lfrom_usermode_switch_stack_\@ @@ -1126,7 +1132,7 @@ apicinterrupt3 HYPERV_STIMER0_VECTOR \ #endif /* CONFIG_HYPERV */ idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK -idtentry int3 do_int3 has_error_code=0 +idtentry int3 do_int3 has_error_code=0 create_gap=1 idtentry stack_segment do_stack_segment has_error_code=1 #ifdef CONFIG_XEN_PV
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