On 02/04/2018 10:35 AM, Arnd Bergmann wrote:
On Sat, Feb 3, 2018 at 6:08 PM, Boris Ostrovsky
<boris.ostrov...@oracle.com> wrote:

On 02/03/2018 10:12 AM, Arnd Bergmann wrote:
On Sat, Feb 3, 2018 at 12:33 AM, Boris Ostrovsky
<boris.ostrov...@oracle.com> wrote:
On 02/02/2018 10:32 AM, Arnd Bergmann wrote:
The legacy hypercall handlers were originally added with
a comment explaining that "copying the argument structures in
HYPERVISOR_event_channel_op() and HYPERVISOR_physdev_op() into the local
variable is sufficiently safe" and only made sure to not write
past the end of the argument structure, the checks in linux/string.h
disagree with that, when link-time optimizations are used:

In function 'memcpy',
      inlined from 'pirq_query_unmask' at drivers/xen/fallback.c:53:2,
      inlined from '__startup_pirq' at
drivers/xen/events/events_base.c:529:2,
      inlined from 'restore_pirqs' at
drivers/xen/events/events_base.c:1439:3,
      inlined from 'xen_irq_resume' at
drivers/xen/events/events_base.c:1581:2:
include/linux/string.h:350:3: error: call to '__read_overflow2' declared
with attribute error: detected read beyond size of object passed as 2nd
parameter
     __read_overflow2();
     ^
make[3]: *** [ccLujFNx.ltrans15.ltrans.o] Error 1
make[3]: Target 'all' not remade because of errors.
lto-wrapper: fatal error: make returned 2 exit status
compilation terminated.
ld: error: lto-wrapper failed

This changes the functions so that each argument is accessed with
exactly the correct length based on the command code.

Fixes: cf47a83fb06e ("xen/hypercall: fix hypercall fallback code for
very old hypervisors")
Signed-off-by: Arnd Bergmann <a...@arndb.de>
---
   drivers/xen/fallback.c | 94
++++++++++++++++++++++++++++----------------------
   1 file changed, 53 insertions(+), 41 deletions(-)

        default:
-             WARN_ON(rc != -ENOSYS);
-             break;
+             return -ENOSYS;
        }

+     memcpy(&op.u, arg, len);
+     rc = _hypercall1(int, event_channel_op_compat, &op);
+     memcpy(arg, &op.u, len);


We don't copy back for all commands, only those that are COPY_BACK.

Not sure what you mean. Is it harmful to copy back the data for the others
in any way? Otherwise I wouldn't micro-optimize this.


I should have checked the original commit for fallback.c --- the code that
it replaced was doing copybacks for all hypercalls and selective copybacks
is an optimization introduced in that commit.
It was not an optimization but a correctness fix to avoid overflowing
the caller stack on the copy-back operation. What I tried to explain
in my commit message is that the same fix is also needed on
the copy-out before it. It's only a read access beyond the end
of a local variable, but not both the static checks and kasan-stack
get alarmed about it.


Yes, I understand that. I was referring to:

    Move the fallback code into out-of-line functions, and handle all of
    the operations known by this old a hypervisor individually: *Some don't
    require copying back anything at all*, and for the rest use the
    individual argument structures' sizes rather than the container's

in the original commit. I.e. prior to that commit we *were* copying back for all commands (although possibly with potentially incorrect size). So not copying back for some commands was an optimization.

In any case,

Reviewed-by: Boris Ostrovsky <boris.ostrov...@oracle.com>

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