Re: [Qemu-riscv] [Qemu-devel] [PATCH v1 1/1] riscv: virt: Cast the initrd start address to target bit length

[Alistair Francis]

On Tue, Nov 27, 2018 at 2:24 PM Alistair Francis <address@hidden> wrote:
>
> On Tue, Nov 27, 2018 at 2:12 PM Alexander Graf <address@hidden> wrote:
> >
> >
> >
> > On 27.11.18 23:05, Alistair Francis wrote:
> > > On Mon, Nov 26, 2018 at 11:02 AM Palmer Dabbelt <address@hidden> wrote:
> > >>
> > >> On Wed, 21 Nov 2018 18:09:27 PST (-0800), address@hidden wrote:
> > >>> On Wed, Nov 21, 2018 at 5:58 PM Palmer Dabbelt <address@hidden> wrote:
> > >>>>
> > >>>> On Wed, 21 Nov 2018 14:34:44 PST (-0800), Alistair Francis wrote:
> > >>>>> Ensure that the calculate initrd offset points to a valid address for
> > >>>>> the architecture.
> > >>>>>
> > >>>>> Signed-off-by: Alistair Francis <address@hidden>
> > >>>>> Suggested-by: Alexander Graf <address@hidden>
> > >>>>> Reported-by: Alexander Graf <address@hidden>
> > >>>>> ---
> > >>>>>  hw/riscv/virt.c | 7 ++++++-
> > >>>>>  1 file changed, 6 insertions(+), 1 deletion(-)
> > >>>>>
> > >>>>> diff --git a/hw/riscv/virt.c b/hw/riscv/virt.c
> > >>>>> index 2b38f89070..4467195fac 100644
> > >>>>> --- a/hw/riscv/virt.c
> > >>>>> +++ b/hw/riscv/virt.c
> > >>>>> @@ -85,7 +85,12 @@ static hwaddr load_initrd(const char *filename, 
> > >>>>> uint64_t mem_size,
> > >>>>>       * halfway into RAM, and for boards with 256MB of RAM or more we 
> > >>>>> put
> > >>>>>       * the initrd at 128MB.
> > >>>>>       */
> > >>>>> -    *start = kernel_entry + MIN(mem_size / 2, 128 * MiB);
> > >>>>> +    /* As hwaddr is a 64-bit number we need to cast it for 32-bit */
> > >>>>> +#if defined(TARGET_RISCV32)
> > >>>>> +    *start = (uint32_t) (kernel_entry + MIN(mem_size / 2, 128ULL * 
> > >>>>> MiB));
> > >>>>> +#elif defined(TARGET_RISCV64)
> > >>>>> +    *start = (uint64_t) (kernel_entry + MIN(mem_size / 2, 128 * 
> > >>>>> MiB));
> > >>>>> +#endif
> > >>>>>
> > >>>>>      size = load_ramdisk(filename, *start, mem_size - *start);
> > >>>>>      if (size == -1) {
> > >>>>> --
> > >>>>> 2.19.1
> > >>>>
> > >>>> Maybe I'm missing something, but wouldn't something like
> > >>>>
> > >>>>     uint64_t start_unclobbered = kernel_entry + MIN(mem_size / 2, 
> > >>>> 128ULL * MiB);
> > >>>>     assert(start_unclobbered == (hwaddr)start_unclobbered);
> > >>>>     *start = (hwaddr)start_unclobbered;
> > >>>>
> > >>>> work better?  That should actually check this all lines up, as opposed 
> > >>>> to just
> > >>>> silently truncating a bad address.
> > >>>
> > >>> The problem is that hwaddr is always 64-bit.
> > >>>
> > >>> Stupidly I forgot about target_ulong, which should work basically the
> > >>> same as above. An assert() seems a little harsh though, Alex reported
> > >>> problem was fixed with just a cast.
> > >>
> > >> OK, I must be misunderstanding the problem, then.  With the current 
> > >> code, isn't
> > >> it possible to end up causing start to overflow a 32-bit address?  This 
> > >> would
> > >> happen if kernel_entry is high, with IIUC is coming from the ELF to load 
> > >> and is
> > >> therefor user input.  I think that's worth some sort of assertion, as 
> > >> it'll
> > >> definitely blow up trying to enter the kernel (and possible before that,
> > >> assuming there's no target memory where it overflows into).
> > >
> > > I don't have a setup to reproduce this unfortunately, but Alex
> > > reported that he saw start being excessively high (0xffffffff88000000)
> > > when loading an initrd.
> >
> > Sorry for only jumping in so late.
> >
> > I didn't actually propose the cast as a solution. The problem must be
> > sign extension of some variable in the mix. I didn't find out quickly
> > which one it was, so I figured I'd leave it for someone else to debug :).
> >
> > The issue is very easy to reproduce: Build U-Boot for the virt machine
> > for riscv32. Then run it with
> >
> >   $ qemu-system-riscv32 -M virt -kernel u-boot -nographic -initrd <a file>
>
> Ah, cool I can reproduce it now.
>
> It happens in load_elf32() (which is actually glue(load_elf, SZ)).
>
> This line ends up sign extending the value:
>     *pentry = (uint64_t)(elf_sword)ehdr.e_entry;
>
> and we just continue it from there.
>
> So I think this diff is a much better solution:
>
> diff --git a/hw/riscv/virt.c b/hw/riscv/virt.c
> index e7f0716fb6..348ecc39d5 100644
> --- a/hw/riscv/virt.c
> +++ b/hw/riscv/virt.c
> @@ -62,7 +62,7 @@ static const struct MemmapEntry {
>      [VIRT_PCIE_ECAM] =   { 0x30000000,    0x10000000 },
>  };
>
> -static uint64_t load_kernel(const char *kernel_filename)
> +static target_ulong load_kernel(const char *kernel_filename)
>  {
>      uint64_t kernel_entry, kernel_high;
>
>
>
> Alistair
>
> >
> > You can find the initrd address with
> >
> >   U-Boot# fdt addr $fdtcontroladdr
> >   U-Boot# fdt ls /chosen
> >
> > Then take a peek at that address:
> >
> >   U-Boot# md.b <addr>
> >
> > and you will see that there is nothing there without this patch. The
> > reason is that the binary was loaded to a negative address. Again,
> > probably some misguided sign extension.
> >
> > > It looks like the kernel entry address ends up being converted to
> > > 0xffffffff80000000 incorrectly instead of being a real overflow.
> >
> > Yeah, so we seem to cast from int32_t to int64_t somewhere. Can you spot
> > where exactly? That's where the bug is :).

Actually this diff looks like a better more generic fix:

diff --git a/include/hw/elf_ops.h b/include/hw/elf_ops.h
index 81cecaf27e..7c1930e7a3 100644
--- a/include/hw/elf_ops.h
+++ b/include/hw/elf_ops.h
@@ -342,8 +342,9 @@ static int glue(load_elf, SZ)(const char *name, int fd,
             }
     }

-    if (pentry)
-       *pentry = (uint64_t)(elf_sword)ehdr.e_entry;
+    if (pentry) {
+        *pentry = (uint64_t)(elf_word)ehdr.e_entry;
+    }

     glue(load_symbols, SZ)(&ehdr, fd, must_swab, clear_lsb, sym_cb);

My only concern is that it will break some other 32-bit guest. It
seems odd that no one else has seen this before.

Alistair

> >
> >
> > Alex