From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
"Julien Grall" <julien@xen.org>
Subject: Re: [PATCH] x86/traps: 'Fix' safety of read_registers() in #DF path
Date: Thu, 15 Oct 2020 09:27:37 +0200 [thread overview]
Message-ID: <948f0753-561b-15e8-bf8c-52ff507133d2@suse.com> (raw)
In-Reply-To: <307753b0-fef8-658d-f897-8c0eb99ce3e5@citrix.com>
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@citrix.com>
>> Reviewed-by: Jan Beulich <jbeulich@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.cooper3@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.
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
next prev parent reply other threads:[~2020-10-15 7:28 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-12 13:49 [PATCH] x86/traps: 'Fix' safety of read_registers() in #DF path Andrew Cooper
2020-10-13 15:51 ` Jan Beulich
2020-10-14 18:00 ` Andrew Cooper
2020-10-15 7:27 ` Jan Beulich [this message]
2020-10-16 10:58 ` Andrew Cooper
2020-10-16 11:03 ` Jan Beulich
2020-10-16 11:24 ` Andrew Cooper
2020-10-16 11:55 ` Jan Beulich
2020-10-16 12:07 ` Andrew Cooper
2020-10-16 12:14 ` Jan Beulich
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