KVM: PPC: Book3S HV: Fix copy_tofrom_guest routines

KVM: PPC: Book3S HV: Fix copy_tofrom_guest routines

[ Upstream commit 5d7d6dac8fe99ed59eee2300e4a03370f94d5222 ]

The __kvmhv_copy_tofrom_guest_radix function was introduced along with
nested HV guest support. It uses the platform's Radix MMU quadrants to
provide a nested hypervisor with fast access to its nested guests
memory (H_COPY_TOFROM_GUEST hypercall). It has also since been added
as a fast path for the kvmppc_ld/st routines which are used during
instruction emulation.

The commit def0bfdbd603 ("powerpc: use probe_user_read() and
probe_user_write()") changed the low level copy function from
raw_copy_from_user to probe_user_read, which adds a check to
access_ok. In powerpc that is:

 static inline bool __access_ok(unsigned long addr, unsigned long size)
 {
         return addr < TASK_SIZE_MAX && size <= TASK_SIZE_MAX - addr;
 }

and TASK_SIZE_MAX is 0x0010000000000000UL for 64-bit, which means that
setting the two MSBs of the effective address (which correspond to the
quadrant) now cause access_ok to reject the access.

This was not caught earlier because the most common code path via
kvmppc_ld/st contains a fallback (kvm_read_guest) that is likely to
succeed for L1 guests. For nested guests there is no fallback.

Another issue is that probe_user_read (now __copy_from_user_nofault)
does not return the number of bytes not copied in case of failure, so
the destination memory is not being cleared anymore in
kvmhv_copy_from_guest_radix:

 ret = kvmhv_copy_tofrom_guest_radix(vcpu, eaddr, to, NULL, n);
 if (ret > 0)                            <-- always false!
         memset(to + (n - ret), 0, ret);

This patch fixes both issues by skipping access_ok and open-coding the
low level __copy_to/from_user_inatomic.

Fixes: def0bfdbd603 ("powerpc: use probe_user_read() and probe_user_write()")
Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210805212616.2641017-2-farosas@linux.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>