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.coop...@citrix.com>
> Reviewed-by: Jan Beulich <jbeul...@suse.com>
>
> 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.coop...@citrix.com/
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
