softmmu: Always initialize xlat in address_space_translate_for_iotlb

The bug is an uninitialized memory read, along the translate_fail
path, which results in garbage being read from iotlb_to_section,
which can lead to a crash in io_readx/io_writex.

The bug may be fixed by writing any value with zero
in ~TARGET_PAGE_MASK, so that the call to iotlb_to_section using
the xlat'ed address returns io_mem_unassigned, as desired by the
translate_fail path.

It is most useful to record the original physical page address,
which will eventually be logged by memory_region_access_valid
when the access is rejected by unassigned_mem_accepts.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1065
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220621153829.366423-1-richard.henderson@linaro.org>

Author: tabudz

qemu/exec.c

@@ -487,7 +487,7 @@ MemoryRegion *flatview_translate(struct uc_struct *uc, FlatView *fv, hwaddr addr
 
 /* Called from RCU critical section */
 MemoryRegionSection *
-address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr addr,
+address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr orig_addr,
                                   hwaddr *xlat, hwaddr *plen,
                                   MemTxAttrs attrs, int *prot)
 {
@@ -496,6 +496,7 @@ address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr addr,
     IOMMUMemoryRegionClass *imrc;
     IOMMUTLBEntry iotlb;
     int iommu_idx;
+    hwaddr addr = orig_addr;
     AddressSpaceDispatch *d = cpu->cpu_ases[asidx].memory_dispatch;
 
     for (;;) {
@@ -551,6 +552,16 @@ address_space_translate_for_iotlb(CPUState *cpu, int asidx, hwaddr addr,
     return section;
 
 translate_fail:
+    /*
+     * We should be given a page-aligned address -- certainly
+     * tlb_set_page_with_attrs() does so.  The page offset of xlat
+     * is used to index sections[], and PHYS_SECTION_UNASSIGNED = 0.
+     * The page portion of xlat will be logged by memory_region_access_valid()
+     * when this memory access is rejected, so use the original untranslated
+     * physical address.
+     */
+    assert((orig_addr & ~TARGET_PAGE_MASK) == 0);
+    *xlat = orig_addr;
     return &d->map.sections[PHYS_SECTION_UNASSIGNED];
 }