Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH v2] xen/pv: Fix a boot up hang revealed by int3 self test | From | Zhenzhong Duan <> | Date | Mon, 15 Jul 2019 13:05:13 +0800 |
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On 2019/7/12 22:06, Andrew Cooper wrote: > On 11/07/2019 03:15, Zhenzhong Duan wrote: >> Commit 7457c0da024b ("x86/alternatives: Add int3_emulate_call() >> selftest") is used to ensure there is a gap setup in exception stack >> which could be used for inserting call return address. >> >> This gap is missed in XEN PV int3 exception entry path, then below panic >> triggered: >> >> [ 0.772876] general protection fault: 0000 [#1] SMP NOPTI >> [ 0.772886] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0+ #11 >> [ 0.772893] RIP: e030:int3_magic+0x0/0x7 >> [ 0.772905] RSP: 3507:ffffffff82203e98 EFLAGS: 00000246 >> [ 0.773334] Call Trace: >> [ 0.773334] alternative_instructions+0x3d/0x12e >> [ 0.773334] check_bugs+0x7c9/0x887 >> [ 0.773334] ? __get_locked_pte+0x178/0x1f0 >> [ 0.773334] start_kernel+0x4ff/0x535 >> [ 0.773334] ? set_init_arg+0x55/0x55 >> [ 0.773334] xen_start_kernel+0x571/0x57a >> >> As xenint3 and int3 entry code are same except xenint3 doesn't generate >> a gap, we can fix it by using int3 and drop useless xenint3. > For 64bit PV guests, Xen's ABI enters the kernel with using SYSRET, with > %rcx/%r11 on the stack. > > To convert back to "normal" looking exceptions, the xen thunks do `pop > %rcx; pop %r11; jmp do_*`... I will add this to commit message. > >> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S >> index 0ea4831..35a66fc 100644 >> --- a/arch/x86/entry/entry_64.S >> +++ b/arch/x86/entry/entry_64.S >> @@ -1176,7 +1176,6 @@ idtentry stack_segment do_stack_segment has_error_code=1 >> #ifdef CONFIG_XEN_PV >> idtentry xennmi do_nmi has_error_code=0 >> idtentry xendebug do_debug has_error_code=0 >> -idtentry xenint3 do_int3 has_error_code=0 >> #endif > What is confusing is why there are 3 extra magic versions here. At a > guess, I'd say something to do with IST settings (given the vectors), > but I don't see why #DB/#BP would need to be different in the first > place. (NMI sure, but that is more to do with the crazy hoops needing > to be jumped through to keep native functioning safely.)
xenint3 will be removed in this patch safely as it don't use IST now.
But debug and nmi need paranoid_entry which will read MSR_GS_BASE to determine
if swapgs is needed. I guess PV guesting running in ring3 will #GP with swapgs?
Zhenzhong
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