Re: [PATCH v5 4/4] x86: re-enable rng seeding via SetupData

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Wed, Dec 28, 2022 at 04:07:13AM +0100, Jason A. Donenfeld wrote:
> On Tue, Dec 27, 2022 at 02:36:54PM +0100, Jason A. Donenfeld wrote:
> > On Mon, Dec 26, 2022 at 05:57:30PM +0100, Jason A. Donenfeld wrote:
> > > On Mon, Dec 26, 2022 at 03:43:04PM +0100, Jason A. Donenfeld wrote:
> > > > On Mon, Dec 26, 2022 at 03:24:07PM +0100, Jason A. Donenfeld wrote:
> > > > > Hi,
> > > > > 
> > > > > I'm currently stumped at the moment, so adding linux-mm@ and x86@. Still
> > > > > working on it though. Details of where I'm at are below the quote below.
> > > > > 
> > > > > On Sat, Dec 24, 2022 at 05:21:46AM +0100, Jason A. Donenfeld wrote:
> > > > > > On Sat, Dec 24, 2022 at 04:09:08AM +0100, Jason A. Donenfeld wrote:
> > > > > > > Hi Eric,
> > > > > > > 
> > > > > > > Replying to you from my telephone, and I'm traveling the next two days,
> > > > > > > but I thought I should mention some preliminary results right away from
> > > > > > > doing some termux compiles:
> > > > > > > 
> > > > > > > On Fri, Dec 23, 2022 at 04:14:00PM -0800, Eric Biggers wrote:
> > > > > > > > Hi Jason,
> > > > > > > > 
> > > > > > > > On Wed, Sep 21, 2022 at 11:31:34AM +0200, Jason A. Donenfeld wrote:
> > > > > > > > > This reverts 3824e25db1 ("x86: disable rng seeding via setup_data"), but
> > > > > > > > > for 7.2 rather than 7.1, now that modifying setup_data is safe to do.
> > > > > > > > > 
> > > > > > > > > Cc: Laurent Vivier <laurent@vivier.eu>
> > > > > > > > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > > > > > > > Cc: Peter Maydell <peter.maydell@linaro.org>
> > > > > > > > > Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
> > > > > > > > > Cc: Richard Henderson <richard.henderson@linaro.org>
> > > > > > > > > Cc: Ard Biesheuvel <ardb@kernel.org>
> > > > > > > > > Acked-by: Gerd Hoffmann <kraxel@redhat.com>
> > > > > > > > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> > > > > > > > > ---
> > > > > > > > >  hw/i386/microvm.c | 2 +-
> > > > > > > > >  hw/i386/pc_piix.c | 3 ++-
> > > > > > > > >  hw/i386/pc_q35.c  | 3 ++-
> > > > > > > > >  3 files changed, 5 insertions(+), 3 deletions(-)
> > > > > > > > > 
> > > > > > > > 
> > > > > > > > After upgrading to QEMU 7.2, Linux 6.1 no longer boots with some configs.  There
> > > > > > > > is no output at all.  I bisected it to this commit, and I verified that the
> > > > > > > > following change to QEMU's master branch makes the problem go away:
> > > > > > > > 
> > > > > > > > diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> > > > > > > > index b48047f50c..42f5b07d2f 100644
> > > > > > > > --- a/hw/i386/pc_piix.c
> > > > > > > > +++ b/hw/i386/pc_piix.c
> > > > > > > > @@ -441,6 +441,7 @@ static void pc_i440fx_8_0_machine_options(MachineClass *m)
> > > > > > > >      pc_i440fx_machine_options(m);
> > > > > > > >      m->alias = "pc";
> > > > > > > >      m->is_default = true;
> > > > > > > > +    PC_MACHINE_CLASS(m)->legacy_no_rng_seed = true;
> > > > > > > >  }
> > > > > > > > 
> > > > > > > > I've attached the kernel config I am seeing the problem on.
> > > > > > > > 
> > > > > > > > For some reason, the problem also goes away if I disable CONFIG_KASAN.
> > > > > > > > 
> > > > > > > > Any idea what is causing this?
> > > > > > > 
> > > > > > > - Commenting out the call to parse_setup_data() doesn't fix the issue.
> > > > > > >   So there's no KASAN issue with the actual parser.
> > > > > > > 
> > > > > > > - Using KASAN_OUTLINE rather than INLINE does fix the issue!
> > > > > > > 
> > > > > > > That makes me suspect that it's file size related, and QEMU or the BIOS
> > > > > > > is placing setup data at an overlapping offset by accident, or something
> > > > > > > similar.
> > > > > > 
> > > > > > I removed the file systems from your config to bring the kernel size
> > > > > > back down, and voila, it works, even with KASAN_INLINE. So perhaps I'm
> > > > > > on the right track here...
> > > > > 
> > > > > QEMU sticks setup_data after the kernel image, the same as kexec-tools
> > > > > and everything else. Apparently, when the kernel image is large, the
> > > > > call to early_memremap(boot_params.hdr.setup_data, ...) returns a value
> > > > > that points some place bogus, and the system crashes or does something
> > > > > weird. I haven't yet determined what this limit is, but in my current
> > > > > test kernel, a value of 0x0000000001327650 is enough to make it point to
> > > > > rubbish.
> > > > > 
> > > > > Is this expected? What's going on here?
> > > > 
> > > > Attaching gdb to QEMU and switching it to physical memory mode
> > > > (`maintenance packet Qqemu.PhyMemMode:1 `) indicates that it
> > > > early_memremap is actually working fine and something *else* is at this
> > > > address? That's kinda weird... Is KASAN populating physical addresses
> > > > immediately after the kernel image extremely early in boot? I'm seeing
> > > > the crash happen from early_reserve_memory()->
> > > > memblock_x86_reserve_range_setup_data(), which should be before
> > > > kasan_init() even runs. Is QEMU calculating kernel_size wrong, when it
> > > > goes to determine where to put the setup_data data? But that's the same
> > > > calculation as used everywhere else, so hmm...
> > > > 
> > > > Jason
> > > 
> > > If bzImage is 15770544 bytes, it does not boot. If bzImage is 15641776
> > > bytes, it does boot. So something is happening somewhat close to the
> > > 16MB mark?
> > > 
> > 
> > Okay, the issue is that it's being decompressed to an area that overlaps
> > the source. So for example in my test kernel:
> > 
> > input_addr: 0x3f112bf
> > output_addr: 0x1000000
> > output_len: 0x3a5d7d8
> > 
> > Since 0x3a5d7d8 + 0x1000000 > 0x3f112bf, eventually this corrupts the
> > setup_data at the end there.
> > 
> > Now digging into what can be done about it.
> 
> Not quite. input_addr doesn't matter, since setup_data still points to
> the old mapping.
> 
> So the actual issue is:
> 
> compressed_size: 	0xf028d4
> decompressed_size:      0x3a5d7d8
> setup_data:      	0x100000 + compressed_size
> output_addr:    	0x1000000 (this is LOAD_PHYSICAL_ADDR)
> 
> Since `output_addr + decompressed_size > setup_data && output_addr <
> setup_data`, then it means the decompressor will write over setup_data.
> 
> Note that this is also a problem for SETUP_DTB, so it's a longstanding
> bug.
> 
> I'm experimenting now with appending lots of zeros between the kernel
> image and setup_data, so that the decompressor doesn't overwrite
> setup_data, but so far it's not working.
> 
> Another option would be to have the build system warn when this is going
> to happen, and suggest that the user increase the value of
> CONFIG_PHYSICAL_START. This might be the best option...

I posted a patch:
https://lore.kernel.org/qemu-devel/20221228143831.396245-1-Jason@zx2c4.com/

We can move discussion on the topic over to that thread now.

Jason