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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew