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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.)

Jan