Re: Crashes in linux-next on powerpc with CONFIG_PPC_KUAP and CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG

From: Michael Ellerman
Date: Thu May 09 2019 - 10:44:08 EST

Petr Mladek <pmladek@xxxxxxxx> writes:
> On Wed 2019-05-08 00:54:51, Michael Ellerman wrote:
>> Hi folks,
>> Just an FYI in case anyone else is seeing crashes very early in boot in
>> linux-next with the above config options.
>> 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.
> Sigh, the check_pointer() stuff is in Linus's tree now, see
> the commit 3e5903eb9cff707301712 ("vsprintf: Prevent crash when
> dereferencing invalid pointers").

No worries.

>> 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
>> The simple fix is to use early_mmu_has_feature() in allow_user_access(),
>> but we'd rather not do that because it penalises all
>> copy_to/from_users() for the life of the system with the cost of the
>> runtime check vs the jump label. The irony is probe_kernel_read()
>> shouldn't be allowing user access at all, because we're reading the
>> kernel not userspace.
> I have tried to find a lightweight way for a safe reading of unknown
> kernel pointer. But I have not succeeded so far. I see only variants
> with user access. The user access is handled in arch-specific code
> and I do not see any variant without it.
> I am not sure on which level it should get fixed.

I sent a fix in powerpc code (sorry might have forgot to Cc you):

I've merged that into the powerpc tree. I think it's too subtle for us
to have an ordering requirement that deep in the user copy code, it was
just a matter of time before it caused a problem, you were just unlucky
it was your patch that did :)

We'll eventually switch it back to using a jump label but make it safe
to call early in boot before we've detected features.

> Could you please send it to lkml to get a wider audience?

I see you also sent a fix, that looks like a safe default to me.

But as I said I'm happy with the powerpc fix, so there's no requirement
from us that your fix get merged.