Re: [PATCH] printk: fix buffer overflow potential for print_text()

[Sven Schnelle <svens@linux.ibm.com>]

John Ogness <john.ogness@linutronix.de> writes:

> On 2021-01-22, Sven Schnelle <svens@linux.ibm.com> wrote:
>>
>> So somehow the pointer for console_drivers changes.
>>
>> I can't provide the normal kernel crash output as printk is no longer
>> working,
>
> I don't understand what you mean here. The crash tool can dump the
> printk buffer.
>
> I would be curious to see what the messages look like. It would also be
> helpful to know the last message you saw on the console.
>

The last message is:

[ 1845.640466] User process fault: interruption code 0007 ilc:2 in libm.so[7d78c000+a3000]
[ 1845.640474] CPU: 9 PID: 859915 Comm: ld.so.1 Not tainted 5.11.0-20210122.rc4.git0.9f29bd8b2e71.300.fc33.s390x+git #1
[ 1845.640476] Hardware name: IBM 8561 T01 703 (LPAR)
[ 1845.640478] User PSW : 0705300080000000 000000007d7983b2
[ 1845.640480]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:1 AS:0 CC:3 PM:0 RI:0 EA:1
[ 1845.640483] User GPRS: 0000000000000000 000000007d798360 0000000000000010 000003ff00000004
[ 1845.640485]            000000007f854dd0 000000007d7e9788 000003ff00401b30 000000007f854ff8
[ 1845.640487]            000003ff00000000 000003ff7d5b4da0 000003ff00000000 000003ff00000010
[ 1845.640489]            000003ff004022c6 000003ff004024b0 0000000080401796 000000007f854de8

That's also what's written to the console. Please note the the output
above is expected. It's only reporting that a user space program
performed an invalid FPU operation. The kernel crash is neither written
to the console nor the dmesg buffer.

>> kmsg_dump_rewind+290 is:
>>
>>               if (!(con->flags & CON_ENABLED))
>>                         continue;
>>
>> in kernel/printk/printk.c:1845
>>
>> I haven't dived into whether our show_code() is doing something wrong
>> which was covered in the past or whether that's because of the patch
>> above but wanted to make you aware of that in case you have an idea.
>> Otherwise i have to dig into the code.
>
> Unless we are dealing with very long printk messages that normally get
> truncated (800+ characters) this patch simply adds a string
> terminator. I do not see how that could possibly cause this kind of
> damage.
>
> When this triggers, there is nothing happening with consoles registering
> or deregistering, right?

That's correct. No registering/unregistering taking place.

> The string terminator (kernel/printk/printk.c:1402) is only added for
> paranoia. If you comment that out, this patch will have no effect (for
> "normal" length lines). I would be curious if that somehow makes a
> difference for you.

I was able to reproduce it in a virtual machine where i have a few more
ways to debug. What i got was:

01:  -> 00000000001B8814"  MVI     92001000 >> 000000000163F1CD     CC 2

That's a watchpoint telling me that the code at 0x1b8814 wants to store
one byte to 0x163f1cd, which is the second byte of console_drivers.

gdb tells me about 0x1b8814:

(gdb) list *(0x1b8814)
0x1b8814 is in record_print_text (/home/svens/ibmgit/linux/kernel/printk/printk.c:1402).
1397		 * If a buffer was provided, it will be terminated. Space for the
1398		 * string terminator is guaranteed to be available. The terminator is
1399		 * not counted in the return value.
1400		 */
1401		if (buf_size > 0)
1402			text[len] = 0;
1403	
1404		return len;
1405	}
1406

In s390 assembly, this is the store:

   0x00000000001b8810 <+0x130>:	la	%r1,0(%r7,%r9)
   0x00000000001b8814 <+0x134>:	mvi	0(%r1),0

The cpu registers at the time of write:

01: GRG  0 =  0000000000000020  000000000163F1CD
01: GRG  2 =  0000000000000042  000003E000623A98
01: GRG  4 =  000000000000000E  000000000087BB70
01: GRG  6 =  0000000000000400  *0000000000000224*
01: GRG  8 =  000000000000000F  *000000000163EFA9*
01: GRG 10 =  0000000000000033  0000000000000000
01: GRG 12 =  00000000809AE000  000003E000623A98
01: GRG 14 =  00000000001B884C  000003E0006239E8

So r9 is 000000000163EFA9 - seems to be the start of the current message:

0163EFA9:  *[   24.069514]            000000007d7af3c2: b30d0002..debr.%f0,%f2.debr.%f0,%f2

while r7 is the offset, and that one is way to big:

0000000000000224

If you add that to 0163EFA9, you'll see that we're outside of the buffer
and overwriting space after, which is console_drivers.

The message it's trying to print is rather long, so the 0x224 could be
the size of the whole line:

This is not from the same crash, but this is how the message looks like
it's trying to print:

[   23.960773] User Code: 000000007d7af3ba: 78005000            le      %f0,0(%r5) 
[   23.960773]            000000007d7af3be: 78205004            le      %f2,4(%r5) 
[   23.960773]           #000000007d7af3c2: b30d0002            debr    %f0,%f2 
[   23.960773]           >000000007d7af3c6: a7280000            lhi     %r2,0 
[   23.960773]            000000007d7af3ca: 07fe                bcr     15,%r14 
[   23.960773]            000000007d7af3cc: 0707                bcr     0,%r7 
[   23.960773]            000000007d7af3ce: 0707                bcr     0,%r7 
[   23.960773]            000000007d7af3d0: c050000289ec        larl    %r5,000000007d8007a8 

I'm pretty sure it's related to the way how the s390 disassembler
prints the buffer, it uses several pr_cont() calls, and it's crashing in the
last pr_cont("\n") (in arch/s390/kernel/dis.c - show_code());

I'm trying to get the content of the log buffer and other state, if
there's anything else i can provide let me know.