What value gets stored in a3 when -S execve?

[Vishnu Srinivasa Ramaprasad]

Thank You so much Steve for the clarification.
Have a Nice day :)

On Thu, May 28, 2020 at 1:52 AM Steve Grubb <sgrubb at redhat.com> wrote:

> Hello,
>
> On Wednesday, May 27, 2020 3:44:13 PM EDT Vishnu Srinivasa Ramaprasad
> wrote:
> > I'm Vish and I am a newbie to auditd. My requirement is to log only
> > shell/bash commands and custom commands executed by administrator users.
> > I have created these rules in /etc/audit/rules.d/audit.rules, to ensure
> > SYSCALL, EXECVE are being added to audit.log for administrator users with
> > auid greater than 1000:
> > -a exit,always -F arch=b64 -S execve -F auid>=1000 -F auid!=-1 -k log_cmd
> > -a exit,always -F arch=b32 -S execve -F auid>=1000 -F auid!=-1 -k log_cmd
> >
> > After restarting auditd service, I had executed strace on a simple
> command
> > which will display version of my project's toolkit:
> >
> > ~# strace -e trace=execve toolkit-version-show
> > execve("/var/tmp/toolkit-version-show", ["toolkit-version-show"],
> > 0x7ffef1fa38b0 /* 30 vars */) = 0
> > Toolkit Version: 1.01
> > +++ exited with 0 +++
> >
> > Later, I executed the ausearch command to check the log entry:
> > ~# ausearch -i --start recent
> > ----
> > type=EXECVE msg=audit(05/27/2020 19:01:26.605:12725) : argc=2
> > a0=/usr/bin/perl a1=/var/tmp/toolkit-version-show
> > type=SYSCALL msg=audit(05/27/2020 19:01:26.605:12725) : arch=x86_64
> > syscall=execve success=yes exit=0 a0=0x7ffef1fa2450 a1=0x7ffef1fa38a0
> > a2=0x7ffef1fa38b0 a3=0x7f47f8669740 items=3 ppid=3641 pid=3643
> > auid=administrator uid=root gid=root euid=root suid=root fsuid=root
> > egid=root sgid=root fsgid=root tty=pts0 ses=8936 comm=toolkit-version-sho
> > exe=/usr/bin/perl key=log_cmd
> > ----
> > type=EXECVE msg=audit(05/27/2020 19:01:26.601:12724) : argc=4 a0=strace
> > a1=-e a2=trace=execve a3=toolkit-version-show
> > type=SYSCALL msg=audit(05/27/2020 19:01:26.601:12724) : arch=x86_64
> > syscall=execve success=yes exit=0 a0=0x55a2d44c9010 a1=0x55a2d449fe80
> > a2=0x55a2d4389490 a3=0x8 items=2 ppid=3099 pid=3641 auid=administrator
> > uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root
> > fsgid=root tty=pts0 ses=8936 comm=strace exe=/usr/bin/strace key=log_cmd
> > ----
> >
> > My understanding of a0 - a2: Please refer the syntax of execve() :
> >
> > int execve(const char*filename, char *const argv[], char *const envp[]);
> >
> > Based on syntax of execve() and output from strace, I could understand
> that
> > in audit.log event entry:
> > a0=0x7ffef1fa2450 represents filename argument of execve
>
> It a pointer to the string.
>
> > a1=0x7ffef1fa38a0 represents argv[] argument of execve
> > a2=0x7ffef1fa38b0 represents envp[] argument of execve
> >
> > Question  1:  What does the a3=0x7f47f8669740  value represent?
>
> whatever is on the stack in that position.
>
> > As execve() has only 3 arguments (represented by a0,a1,a2), what value
> gets
> > stored in a3?
> > I have noticed a3 values to be varying from:
> > a3=0x0
> > a3=0x7
> > a3=0x55a2d4389490
> > a3=0x56a2d44adc00
> > a3=0x8
> >
> > Question 2: a3=0x8 seems to be the value assigned for a majority of
> > execve() syscalls.
> > Is this a standard value set in case of main/primary system call, such as
> > toolkit-version-show?
>
> Nope. It depends entirely on what the previous syscalls or system activity
> were and where on the stack it currently is.
>
> > Could I use this in a rule filter *-F a3=8* to log only primary (custom
> > script) command executed by user and not internal commands executed by
> > custom script like ls, cat, grep.. etc.:
> > -a exit,always -F arch=b64 -S execve *-F a3=8* -F auid>=1000 -F auid!=-1
> -k
> > log_cmd
> > -a exit,always -F arch=b32 -S execve *-F a3=8* -F auid>=1000 -F auid!=-1
> -k
> > log_cmd
> >
> > Would I miss logging a few primary syscalls, if a3 is not 0x80 in some
> > cases?
>
> Nope. you can't count on anything in a3 since it is not valid for that
> syscall.
>
> > Question 3: If a3=0x8 is not a standard value, Is it possible to identify
> > primary custom command and log only that command, and not internal
> commands
> > with a3=0x0 or a3=0x55a2d4389490?
>
> You can always place a watch on the custom commands.
> -a exit,always -F path=/path-to/custom-command -F perm=x -F key=command
>
>
> > Question 4: Is it possible to filter out and not log syscalls with
> > tty=(none)?
>
> No. But there is a loose correlation with auid or sessionid being -1. That
> means it is a daemon. And if you only want commands run by people, then
> you
> want -F auid>=1000 -F auid!=-1.
>
> -Steve
>
>
>
>
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