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SubjectRe: [REVIEW][PATCH 2/6] signal: Fail sigqueueinfo if si_signo != sig
Andrei Vagin <avagin@gmail.com> writes:

> On Tue, Sep 25, 2018 at 07:19:02PM +0200, Eric W. Biederman wrote:
>> The kernel needs to validate that the contents of struct siginfo make
>> sense as siginfo is copied into the kernel, so that the proper union
>> members can be put in the appropriate locations. The field si_signo
>> is a fundamental part of that validation. As such changing the
>> contents of si_signo after the validation make no sense and can result
>> in nonsense values in the kernel.
>
> Accoding to the man page, the user should not set si_signo, it has to be set
> by kernel.

I wanted to say look at copy_siginfo_from_user32 it uses si_signo and it
has always done so. But before I got to fixing copy_siginfo_from_user32
only looked at si_code. This is one of the areas where we deliberately
slightly changed the KABI. To start looking an signo instead of magic
kernel internal si_code values.

So yes. Looking at si_signo instead of the passed in signo appears to
be a regression all of the way around (except for ptrace) where that
value is not present. So I will see if I can figure out how to refactor
the code to accomplish that.

I am travelling for the next couple of days so I won't be able to get to
this for a bit.

I am thinking I will need two version of copy_siginfo_from user now.
One for ptrace and one for rt_sgiqueueinfo. Sigh.

> $ man 2 rt_sigqueueinfo
>
> The uinfo argument specifies the data to accompany the signal. This
> argument is a pointer to a structure of type siginfo_t, described in
> sigaction(2) (and defined by including <sigaction.h>). The caller
> should set the following fields in this structure:
>
> si_code
> This must be one of the SI_* codes in the Linux kernel source
> file include/asm-generic/siginfo.h, with the restriction that
> the code must be negative (i.e., cannot be SI_USER, which is
> used by the kernel to indicate a signal sent by kill(2)) and
> cannot (since Linux 2.6.39) be SI_TKILL (which is used by the
> kernel to indicate a signal sent using tgkill(2)).
>
> si_pid This should be set to a process ID, typically the process ID of
> the sender.
>
> si_uid This should be set to a user ID, typically the real user ID of
> the sender.
>
> si_value
> This field contains the user data to accompany the signal. For
> more information, see the description of the last (union sigval)
> argument of sigqueue(3).
>
> Internally, the kernel sets the si_signo field to the value specified
> in sig, so that the receiver of the signal can also obtain the signal
> number via that field.
>
>>
>> As such simply fail if someone is silly enough to set si_signo out of
>> sync with the signal number passed to sigqueueinfo.
>>
>> I don't expect a problem as glibc's sigqueue implementation sets
>> "si_signo = sig" and CRIU just returns to the kernel what the kernel
>> gave to it.
>>
>> If there is some application that calls sigqueueinfo directly that has
>> a problem with this added sanity check we can revisit this when we see
>> what kind of crazy that application is doing.
>
>
> I already know two "applications" ;)
>
> https://github.com/torvalds/linux/blob/master/tools/testing/selftests/ptrace/peeksiginfo.c
> https://github.com/checkpoint-restore/criu/blob/master/test/zdtm/static/sigpending.c
>
> Disclaimer: I'm the author of both of them.
>
>>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>> kernel/signal.c | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index 7b49c31d3fdb..e445b0a63faa 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -3306,7 +3306,8 @@ static int do_rt_sigqueueinfo(pid_t pid, int sig, siginfo_t *info)
>> (task_pid_vnr(current) != pid))
>> return -EPERM;
>>
>> - info->si_signo = sig;
>> + if (info->si_signo != sig)
>> + return -EINVAL;
>>
>> /* POSIX.1b doesn't mention process groups. */
>> return kill_proc_info(sig, info, pid);
>> @@ -3354,7 +3355,8 @@ static int do_rt_tgsigqueueinfo(pid_t tgid, pid_t pid, int sig, siginfo_t *info)
>> (task_pid_vnr(current) != pid))
>> return -EPERM;
>>
>> - info->si_signo = sig;
>> + if (info->si_signo != sig)
>> + return -EINVAL;
>>
>> return do_send_specific(tgid, pid, sig, info);
>> }

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