On Mon, 20 Jun 2022 at 17:54, Richard Henderson <[email protected]> wrote: > > On 6/20/22 05:52, Peter Maydell wrote: > > On Wed, 15 Jun 2022 at 17:43, Richard Henderson > > <[email protected]> wrote: > >> > >> 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 <[email protected]> > >> --- > >> softmmu/physmem.c | 3 +++ > >> 1 file changed, 3 insertions(+) > >> > >> diff --git a/softmmu/physmem.c b/softmmu/physmem.c > >> index 657841eed0..fb0f0709b5 100644 > >> --- a/softmmu/physmem.c > >> +++ b/softmmu/physmem.c > >> @@ -681,6 +681,9 @@ address_space_translate_for_iotlb(CPUState *cpu, int > >> asidx, hwaddr addr, > >> AddressSpaceDispatch *d = > >> qatomic_rcu_read(&cpu->cpu_ases[asidx].memory_dispatch); > >> > >> + /* Record the original phys page for use by the translate_fail path. > >> */ > >> + *xlat = addr; > > > > There's no doc comment for address_space_translate_for_iotlb(), > > so there's nothing that says explicitly that addr is obliged > > to be page aligned, although it happens that its only caller > > does pass a page-aligned address. Were we already implicitly > > requiring a page-aligned address here, or does not masking > > addr before assigning to *xlat impose a new requirement ? > > I have no idea. The whole lookup process is both undocumented and twistedly > complex. I'm > willing to add an extra masking operation here, if it seems necessary?
I think we should do one of: * document that we assume the address is page-aligned * assert that the address is page-aligned * mask to force it to page-alignedness but I much don't care which one of those we do. Maybe we should assert((*xlat & ~TARGET_PAGE_MASK) == 0) at the translate_fail label, with a suitable comment ? thanks -- PMM
