Re: [PATCH] vsprintf: protect kernel from panic due to non-canonical pointer dereference

[Jane Chu]

On 10/18/2022 5:45 AM, Petr Mladek wrote:
> On Mon 2022-10-17 19:31:53, Jane Chu wrote:
>> On 10/17/2022 12:25 PM, Andy Shevchenko wrote:
>>> On Mon, Oct 17, 2022 at 01:16:11PM -0600, Jane Chu wrote:
>>>> While debugging a separate issue, it was found that an invalid string
>>>> pointer could very well contain a non-canical address, such as
>>>> 0x7665645f63616465. In that case, this line of defense isn't enough
>>>> to protect the kernel from crashing due to general protection fault
>>>>
>>>> 	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
>>>>                   return "(efault)";
>>>>
>>>> So instead, use kern_addr_valid() to validate the string pointer.
>>>
>>> How did you check that value of the (invalid string) pointer?
>>>
>>
>> In the bug scenario, the invalid string pointer was an out-of-bound
>> string pointer. While the OOB referencing is fixed,
> 
> Could you please provide more details about the fixed OOB?
> What exact vsprintf()/printk() call was broken and eventually
> how it was fixed, please?

For sensitive reason, I'd like to avoid mentioning the specific name of 
the sysfs attribute in the bug, instead, just call it "devX_attrY[]",
and describe the precise nature of the issue.

devX_attrY[] is a string array, declared and filled at compile time,
like
   const char const devX_attrY[] = {
	[ATTRY_A] = "Dev X AttributeY A",
	[ATTRY_B] = "Dev X AttributeY B",
	...
	[ATTRY_G] = "Dev X AttributeY G",
   }
such that, when user "cat /sys/devices/systems/.../attry_1",
"Dev X AttributeY B" will show up in the terminal.
That's it, no more reference to the pointer devX_attrY[ATTRY_B] after that.

The bug was that the index to the array was wrongfully produced,
leading up to OOB, e.g. devX_attrY[11].  The fix was to fix the 
calculation and that is not an upstream fix.

> 
>> the lingering issue
>> is that the kernel ought to be able to protect itself, as the pointer
>> contains a non-canonical address.
> 
> Was the pointer used only by the vsprintf()?
> Or was it accessed also by another code, please?

The OOB pointer was used only by vsprintf() for the "cat" sysfs case.
No other code uses the OOB pointer, verified both by code examination 
and test.

Here is a snippet of the crash backtrace from an instrumented kernel, 
scratched one line for sensitive reason -

crash> bt
PID: 3250   TASK: ffff9cb50fe23d80  CPU: 18  COMMAND: "cat"
  #0 [ffffc0bacf377998] machine_kexec at ffffffff9b06c7c1
  #1 [ffffc0bacf3779f8] __crash_kexec at ffffffff9b13bb52
  #2 [ffffc0bacf377ac8] crash_kexec at ffffffff9b13cdac
  #3 [ffffc0bacf377ae8] oops_end at ffffffff9b03357a
  #4 [ffffc0bacf377b10] die at ffffffff9b033c32
  #5 [ffffc0bacf377b40] do_general_protection at ffffffff9b030c52
  #6 [ffffc0bacf377b70] general_protection at ffffffff9ba03db4
     [exception RIP: string_nocheck+19]
     RIP: ffffffff9b87cc73  RSP: ffffc0bacf377c20  RFLAGS: 00010286
     RAX: 0000000000000000  RBX: ffff9da13fc17fff  RCX: ffff0a00ffffff04
     RDX: 726f635f63616465  RSI: ffff9da13fc17fff  RDI: ffffffffffffffff
     RBP: ffffc0bacf377c20   R8: ffff9da0bfd2f010   R9: ffff9da0bfc18000
     R10: 0000000000001000  R11: 0000000000000000  R12: 726f635f63616465
     R13: ffff0a00ffffff04  R14: ffffffff9c1a6a4f  R15: ffffffff9c1a6a4f
     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
  #7 [ffffc0bacf377c28] string at ffffffff9b87ce98
  #8 [ffffc0bacf377c58] vsnprintf at ffffffff9b87efe3
  #9 [ffffc0bacf377cb8] sprintf at ffffffff9b87f506
#10 [ffffc0bacf377d18] <------------------------------>
#11 [ffffc0bacf377d28] dev_attr_show at ffffffff9b56d183
#12 [ffffc0bacf377d48] sysfs_kf_seq_show at ffffffff9b3272dc
#13 [ffffc0bacf377d68] kernfs_seq_show at ffffffff9b32576c
#14 [ffffc0bacf377d78] seq_read at ffffffff9b2be407
#15 [ffffc0bacf377de8] kernfs_fop_read at ffffffff9b325ffe
#16 [ffffc0bacf377e28] __vfs_read at ffffffff9b2940ea
#17 [ffffc0bacf377eb0] vfs_read at ffffffff9b2942ac
#18 [ffffc0bacf377ee0] sys_read at ffffffff9b29485c
#19 [ffffc0bacf377f28] do_syscall_64 at ffffffff9b003ca9
#20 [ffffc0bacf377f50] entry_SYSCALL_64_after_hwframe at ffffffff9ba001b1

crash> dis ffffffff9b87cc73
0xffffffff9b87cc73 <string_nocheck+19>: movzbl (%rdx),%r8d

and RDX: 726f635f63616465 was a non-canonical address.

After applying this patch to the instrumented kernel, instead of panic, 
the "cat" command produced  "(efault)"

> 
> I wonder if this patch would prevent the crash or if the broken
> kernel would crash later anyway.

A broken kernel has a different issue to be fixed, the upstream kernel 
isn't broken, it could just offer better protect in case a bug was 
introduced in future.

> 
>> That said, I realized that not all
>> architecture implement meaningful kern_addr_valid(), so this line
>>      if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
>> is still need.  I'll send v2.
> 
> Please, add linux-mm@xxxxxxxxx into CC. 

Will do.

> I wonder if kern_addr_valid()
> is safe to use anywhere, especially during early boot. I wonder if
> it would make sense to implement it on all architectures.

On x86 architecture, kern_addr_valid() looks safe to me though, on 
several other architectures, it's defined (1).

> 
> Best Regards,
> Petr

Thanks!
-jane