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

[patchbot@xxxxxxx]

commit 6065a05adf152a556fb9f11a5218c89e41b62893
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Oct 12 13:24:31 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Oct 16 11:55:33 2020 +0100

    x86/traps: 'Fix' safety of read_registers() in #DF path
    
    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.
    
    show_registers() unconditionally reads these fields, but the content is
    discarded before use.  This is benign right now, as all parts of the stack 
are
    readable, including the guard pages.
    
    However, read_registers() in the #DF handler writes to these fields as part 
of
    preparing the state dump, and being IST, hits 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>
---
 xen/arch/x86/cpu/common.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index da74172776..a684519a20 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -770,7 +770,13 @@ void load_system_tables(void)
        tss->ist[IST_MCE - 1] = stack_top + (1 + IST_MCE) * PAGE_SIZE;
        tss->ist[IST_NMI - 1] = stack_top + (1 + IST_NMI) * PAGE_SIZE;
        tss->ist[IST_DB  - 1] = stack_top + (1 + IST_DB)  * PAGE_SIZE;
-       tss->ist[IST_DF  - 1] = stack_top + (1 + IST_DF)  * PAGE_SIZE;
+       /*
+        * Gross bodge.  The #DF handler uses the vm86 fields of cpu_user_regs
+        * beyond the hardware frame.  Adjust the stack entrypoint so this
+        * doesn't manifest as an OoB write which hits the guard page.
+        */
+       tss->ist[IST_DF  - 1] = stack_top + (1 + IST_DF)  * PAGE_SIZE -
+               (sizeof(struct cpu_user_regs) - offsetof(struct cpu_user_regs, 
es));
        tss->bitmap = IOBMP_INVALID_OFFSET;
 
        /* All other stack pointers poisioned. */
--
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