Date:   Thu, 4 Aug 2022 10:39:52 +0200
From:   Helge Deller <deller@....de>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] Dump command line of faulting process to syslog

On 8/4/22 10:09, Josh Triplett wrote:
> On Tue, Aug 02, 2022 at 09:40:50PM +0200, Helge Deller wrote:
>> On 8/1/22 18:57, Josh Triplett wrote:
>>> On Mon, Aug 01, 2022 at 05:20:13PM +0200, Helge Deller wrote:
>>>> This patch series allows the arch-specific kernel fault handlers to dump
>>>> in addition to the typical info (IP address, fault type, backtrace and so on)
>>>> the command line of the faulting process.
>>>>
>>>> The motivation for this patch is that it's sometimes quite hard to find out and
>>>> annoying to not know which program *exactly* faulted when looking at the syslog.
>>>>
>>>> Some examples from the syslog are:
>>>>
>>>> On parisc:
>>>>    do_page_fault() command='cc1' type=15 address=0x00000000 in libc-2.33.so[f6abb000+184000]
>>>>    CPU: 1 PID: 13472 Comm: cc1 Tainted: G            E     5.10.133+ #45
>>>>    Hardware name: 9000/785/C8000
>>>>
>>>> -> We see the "cc1" compiler crashed, but it would be useful to know which file was compiled.
>>>>
>>>> With this patch series, the kernel now prints in addition:
>>>>    cc1[13472] cmdline: /usr/lib/gcc/hppa-linux-gnu/12/cc1 -quiet @/tmp/ccRkFSfY -imultilib . -imultiarch hppa-linux-gnu -D USE_MINIINTERPRETER -D NO_REGS -D _HPUX_SOURCE -D NOSMP -D THREADED_RTS -include /build/ghc/ghc-9.0.2/includes/dist-install/build/ghcversion.h -iquote compiler/GHC/Iface -quiet -dumpdir /tmp/ghc13413_0/ -dumpbase ghc_5.hc -dumpbase-ext .hc -O -Wimplicit -fno-PIC -fwrapv -fno-builtin -fno-strict-aliasing -o /tmp/ghc13413_0/ghc_5.s
>>>>
>>>> -> now we know that cc1 crashed while compiling some haskell code.
>>>
>>> This does seem really useful for debugging.
>>
>> Yes.
>>
>>> However, it's also an information disclosure in various ways. The
>>> arguments of a program are often more sensitive than the name, and logs
>>> have a tendency to end up in various places, such as bug reports.
>>>
>>> An example of how this can be an issue:
>>> - You receive an email or other message with a sensitive link to follow
>>> - You open the link, which launches `firefox https://...`
>>> - You continue browsing from that window
>>> - Firefox crashes (and recovers and restarts, so you don't think
>>>   anything of it)
>>> - Later, you report a bug on a different piece of software, and the bug
>>>   reporting process includes a copy of the kernel log
>>
>> Yes, that's a possible way how such information can leak.
>>
>>> I am *not* saying that we shouldn't do this; it seems quite helpful.
>>> However, I think we need to arrange to treat this as sensitive
>>> information, similar to kptr_restrict.
>>
>> I wonder what the best solution could be.
>>
>> A somewhat trivial solution is to combine it with the dmesg_restrict sysctl, e.g.:
>>
>> * When ``dmesg_restrict`` is set to 0 there are no restrictions for users to read
>> dmesg. In this case my patch would limit the information (based on example above):
>>     cc1[13472] cmdline: /usr/lib/gcc/hppa-linux-gnu/12/cc1 [note: other parameters hidden due to dmesg_restrict=0 sysctl]
>> So it would show the full argv[0] with a hint that people would need to change dmesg_restrict.
>>
>> * When ``dmesg_restrict`` is set to 1, users must have ``CAP_SYSLOG`` to use dmesg(8)
>> and the patch could output all parameters:
>>      cc1[13472] cmdline: /usr/lib/gcc/hppa-linux-gnu/12/cc1 -quiet @/tmp/ccRkFSfY -imultilib . -imultiarch hppa-linux-gnu ....
>>
>> That would of course still leave few possible corner-cases where information
>> could leak, but since usually programs shouldn't crash and that
>> people usually shouldn't put sensitive information into the parameter
>> list directly, it's somewhat unlikely to happen.
>>
>> Another different solution would be to add another sysctl.
>>
>> Any other ideas?
>
> I don't think we should overload the meaning of dmesg_restrict. But
> overloading kptr_restrict seems reasonable to me. (Including respecting
> kptr_restrict==2 by not showing this at all.)

I'm fine with kptr_restrict, but I'm puzzled for which value of kptr_restrict
the command line should be shown then.
By looking at the meaning of kptr_restrict, I think the command line should be
hidden for values 0-2.
Do you suggest to add a new value "3" or am I missing something?

Helge

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