Re: [PATCH] x86/traps: 'Fix' safety of read_registers() in #DF path

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.10.2020 20:00, Andrew Cooper wrote:
> On 13/10/2020 16:51, Jan Beulich wrote:
>> On 12.10.2020 15:49, Andrew Cooper wrote:
>>> All interrupts and exceptions pass a struct cpu_user_regs up into C.  This
>>> contains the legacy vm86 fields from 32bit days, which are beyond the
>>> hardware-pushed frame.
>>>
>>> Accessing these fields is generally illegal, as they are logically out of
>>> bounds for anything other than an interrupt/exception hitting ring1/3 code.
>>>
>>> Unfortunately, the #DF handler uses these fields as part of preparing the
>>> state dump, and being IST, accesses the adjacent stack frame.
>>>
>>> This has been broken forever, but c/s 6001660473 "x86/shstk: Rework the 
>>> stack
>>> layout to support shadow stacks" repositioned the #DF stack to be adjacent 
>>> to
>>> the guard page, which turns this OoB write into a fatal pagefault:
>>>
>>>   (XEN) *** DOUBLE FAULT ***
>>>   (XEN) ----[ Xen-4.15-unstable  x86_64  debug=y   Tainted:  C   ]----
>>>   (XEN) ----[ Xen-4.15-unstable  x86_64  debug=y   Tainted:  C   ]----
>>>   (XEN) CPU:    4
>>>   (XEN) RIP:    e008:[<ffff82d04031fd4f>] traps.c#read_registers+0x29/0xc1
>>>   (XEN) RFLAGS: 0000000000050086   CONTEXT: hypervisor (d1v0)
>>>   ...
>>>   (XEN) Xen call trace:
>>>   (XEN)    [<ffff82d04031fd4f>] R traps.c#read_registers+0x29/0xc1
>>>   (XEN)    [<ffff82d0403207b3>] F do_double_fault+0x3d/0x7e
>>>   (XEN)    [<ffff82d04039acd7>] F double_fault+0x107/0x110
>>>   (XEN)
>>>   (XEN) Pagetable walk from ffff830236f6d008:
>>>   (XEN)  L4[0x106] = 80000000bfa9b063 ffffffffffffffff
>>>   (XEN)  L3[0x008] = 0000000236ffd063 ffffffffffffffff
>>>   (XEN)  L2[0x1b7] = 0000000236ffc063 ffffffffffffffff
>>>   (XEN)  L1[0x16d] = 8000000236f6d161 ffffffffffffffff
>>>   (XEN)
>>>   (XEN) ****************************************
>>>   (XEN) Panic on CPU 4:
>>>   (XEN) FATAL PAGE FAULT
>>>   (XEN) [error_code=0003]
>>>   (XEN) Faulting linear address: ffff830236f6d008
>>>   (XEN) ****************************************
>>>   (XEN)
>>>
>>> and rendering the main #DF analysis broken.
>>>
>>> The proper fix is to delete cpu_user_regs.es and later, so no
>>> interrupt/exception path can access OoB, but this needs disentangling from 
>>> the
>>> PV ABI first.
>>>
>>> Not-really-fixes: 6001660473 ("x86/shstk: Rework the stack layout to 
>>> support shadow stacks")
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> Is it perhaps worth also saying explicitly that the other IST
>> stacks don't suffer the same problem because show_registers()
>> makes an local copy of the passed in struct? (Doing so also
>> for #DF would apparently be an alternative solution.)
> 
> They're not safe.  They merely don't explode.
> 
> https://lore.kernel.org/xen-devel/1532546157-5974-1-git-send-email-andrew.cooper3@xxxxxxxxxx/
> was "fixed" by CET-SS turning the guard page from not present to
> read-only, but the same CET-SS series swapped #DB for #DF when it comes
> to the single OoB write problem case.

I see. While indeed I didn't pay attention to the OoB read aspect,
me saying "the other IST stacks don't suffer the same problem" was
still correct, wasn't it? Anyway - not a big deal.

Jan