[PATCH] vsprintf: Do not break early boot with probing addresses

[Petr Mladek]

On Thu 2019-05-09 09:13:57, Steven Rostedt wrote:
> On Thu,  9 May 2019 14:19:23 +0200
> Petr Mladek <pmladek at suse.com> wrote:
> 
> > The commit 3e5903eb9cff70730 ("vsprintf: Prevent crash when dereferencing
> > invalid pointers") broke boot on several architectures. The common
> > pattern is that probe_kernel_read() is not working during early
> > boot because userspace access framework is not ready.
> > 
> > The check is only the best effort. Let's not rush with it during
> > the early boot.
> > 
> > Details:
> > 
> > 1. Report on Power:
> > 
> > Kernel crashes very early during boot with with CONFIG_PPC_KUAP and
> > CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
> > 
> > The problem is the combination of some new code called via printk(),
> > check_pointer() which calls probe_kernel_read(). That then calls
> > allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early
> > (before we've patched features). With the JUMP_LABEL debug enabled that
> > causes us to call printk() & dump_stack() and we end up recursing and
> > overflowing the stack.
> > 
> > Because it happens so early you don't get any output, just an apparently
> > dead system.
> > 
> > The stack trace (which you don't see) is something like:
> > 
> >   ...
> >   dump_stack+0xdc
> >   probe_kernel_read+0x1a4
> >   check_pointer+0x58
> >   string+0x3c
> >   vsnprintf+0x1bc
> >   vscnprintf+0x20
> >   printk_safe_log_store+0x7c
> >   printk+0x40
> >   dump_stack_print_info+0xbc
> >   dump_stack+0x8
> >   probe_kernel_read+0x1a4
> >   probe_kernel_read+0x19c
> >   check_pointer+0x58
> >   string+0x3c
> >   vsnprintf+0x1bc
> >   vscnprintf+0x20
> >   vprintk_store+0x6c
> >   vprintk_emit+0xec
> >   vprintk_func+0xd4
> >   printk+0x40
> >   cpufeatures_process_feature+0xc8
> >   scan_cpufeatures_subnodes+0x380
> >   of_scan_flat_dt_subnodes+0xb4
> >   dt_cpu_ftrs_scan_callback+0x158
> >   of_scan_flat_dt+0xf0
> >   dt_cpu_ftrs_scan+0x3c
> >   early_init_devtree+0x360
> >   early_setup+0x9c
> > 
> > 2. Report on s390:
> > 
> > vsnprintf invocations, are broken on s390. For example, the early boot
> > output now looks like this where the first (efault) should be
> > the linux_banner:
> > 
> > [    0.099985] (efault)
> > [    0.099985] setup: Linux is running as a z/VM guest operating system in 64-bit mode
> > [    0.100066] setup: The maximum memory size is 8192MB
> > [    0.100070] cma: Reserved 4 MiB at (efault)
> > [    0.100100] numa: NUMA mode: (efault)
> > 
> > The reason for this, is that the code assumes that
> > probe_kernel_address() works very early. This however is not true on
> > at least s390. Uaccess on KERNEL_DS works only after page tables have
> > been setup on s390, which happens with setup_arch()->paging_init().
> > 
> > Any probe_kernel_address() invocation before that will return -EFAULT.
> 
> Hmm, this sounds to me that probe_kernel_address() is broken for these
> architectures. Perhaps the system_state check should be in
> probe_kernel_address() for those architectures?

Yeah. Well, these problems are hard to debug. It left a dead power
system with a blank screen. I am not sure if the added check is
worth the pain.

I hope that the check would help to debug problems. But it is yet
another complexity in printk() path. I think that it is fine
to keep it enabled only on the booted system for a while
and get some more feedback.

Best Regards,
Petr