Getting the value of a syscall's memory address argument - setxattr

[Richard Guy Briggs]

On 2021-02-26 22:17, Alan Evangelista wrote:
> Each syscall has some arguments and the Linux Audit framework logs each
> pointer argument as a memory address instead of its values. For instance,
> when tracking the setxattr syscall, I get its arguments in the following
> format:
> 
> "a0":"55f3604ba000"
> "a1":"7f1b0bd342fd"
> "a2":"55f3604d9b20"
> "a3":"38"
> 
> According to https://man7.org/linux/man-pages/man2/setxattr.2.html, a0 is
> the file path's starting memory address, a1 is the extended attribute
> name's starting memory address, a2 is the extended attribute
> value's starting memory address and a3 is the size in bytes of the extended
> attribute value.
> 
> Is it safe to access those memory addresses in order to get their values? I
> guess not because their content may have been overwritten between the time
> the syscall log entry was generated by the kernel and the time it's
> consumed by a Linux Audit client. If indeed it's unsafe to access these
> memory addresses, is there any other way to get the extended attribute
> name/value in the setxattr syscall using the Linux Audit framework?

They would not be safe to access from userspace after the syscall has
finished.  audit records the values of a number of specific syscall
parameters in special records so this would most likely need a new
special record to add to the audit syscall event to record those pointer
contents.

> My specific use case: I'm using Auditbeat/Linux Audit to track permission
> changes done to a disk partition which is mounted by Samba on a Windows
> Server box. When a Windows user changes permissions of a file in the Samba
> mount, Linux Audit records a setxattr event and Auditbeat (connected to the
> kernel's Audit framework via netlink) notifies me of the event. I need to
> know what permission changes the user has done in the file and AFAIK
> parsing the ext attrib name/value is the only way to do that.

This use case adds and additional challenge.  Since this is a filesystem
that is changed remotely, you may not have a record of the remote user
who made the change, but only the server daemon locally that brokered
the change unless that information is in those pointers.

> Thanks in advance.

- RGB

--
Richard Guy Briggs <rgb at redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635